signifies the passing of any body from a fluid to a solid state: so that the term is thus applicable to metals when they resume their solid form after being heated, to water when it freezes, to wax, spermaceti, &c., when they become solid after having been rendered fluid by heat; and in general to all processes, where the whole substance of the fluid is converted into a solid: but it differs from crystallization; because in the latter process, though the salt passes from a fluid to a solid state, a considerable quantity of liquid is always left, so that the term congelation is never applied in this case.

The process of congelation in all cases depends upon, or at least is accompanied with, the emission of heat, as has been evinced by experiments made not only on water, but on spermaceti, wax, &c.: for in all of these, though the thermometer immersed in them while fluid continued to descend gradually till a certain period, yet it was as constantly observed to remain stationary, or even to ascend while the congelation went on. See Chemistry.

It is not known whether all kinds of fluids are naturally capable of congelation or not; though we are certain that there are very great differences among them in this respect. The most difficult of all those of which the congelation has been actually ascertained is quicksilver. This was long thought capable of resisting any degree of cold whatever; and it is only within a few years that its congelation by artificial means was known, and still more lately that some climates were found to be so severe as to congeal this fluid by the cold of the atmosphere.

The congelation of quicksilver was first ascertained experimentally by M. Joseph Adam Braun, professor of philosophy at Petersburg. He had been employed in making thermometrical experiments; not with a view to make the discovery he actually did, but to see how many degrees of cold he could produce. An excellent opportunity for this occurred on the 14th of December 1759, when the mercury stood naturally at —34°, which is now known to be only five or six degrees above its point of congelation. M. Braun, having determined to avail himself of this great degree of natural cold, prepared a freezing mixture of nitric acid and pounded ice, by means of which his thermometer was reduced to —69°. Part of the quicksilver had now really congealed; yet so far was M. Braun from entertaining any suspicion of the truth, that he had almost desisted from farther attempts, being satisfied with having so far exceeded all the philosophers who went before him. Animated, however, by the hopes of producing a still greater degree of cold, he renewed the experiment; but having expended all his pounded ice, he was obliged to substitute snow in its place. With this fresh mixture the mercury sunk to —100°, 240°, and 352°. He then supposed that the thermometer was broken; but on taking it out to observe whether it was so or not, he found the quicksilver fixed, and continuing so for 12 minutes. On repeating the same experiment with another thermometer which had been graduated no lower than —120°, all the mercury sunk into the ball, and became solid as before, not beginning to reascend till after a still longer interval of time. Hence the professor concluded that the quicksilver was really frozen, and prepared for making a decisive experiment. This was accomplished on the 25th of the same month, and the bulb of the thermometer broken as soon as the metal was congealed. The mercury was now converted into a solid and shining metallic mass, which extended under the stroke of a pellucid, in hardness rather inferior to lead, and yielding a dull sound like that metal. Professor Æpinus made similar experiments at the same time, employing both thermometers and tubes of a larger bore; in which last he remarked, that the quicksilver fell sensibly on being frozen, affuming a concave surface, and likewise that the congealed pieces sunk in fluid mercury.

The fact being thus established, and fluidity no longer to be considered as an essential property of quicksilver, M. Braun communicated an account of his experiments to the Edinburgh Academy, on the 6th of September 1760; of which a large extract was inserted in the Philosophical Transactions, vol. iii. p. 156. Five years afterwards he published another treatise on the same subject, under the title of Supplement to his former dissertation. In this he declared, that, since his former publication, he had never suffered any winter to elapse without repeating the experiment of congealing quicksilver, and never failed of success when the natural cold was of sufficient strength for the purpose. This degree of natural cold he supposes to be —10° of Fahrenheit, though some commencement The commencement of the congelation might be perceived when the temperature of the air was as high as +2°. The results of all his experiments were, that with the above-mentioned frigorific mixtures, and once with rectified spirits and snow, when the natural cold was at 28°, he congealed the quicksilver, and discovered that it is a real metal which melts with a very small degree of heat. Not perceiving, however, the necessary consequence of its great contraction in freezing, he, in this work, as well as in the former, confounded its point of congelation with that of its greatest contraction in freezing, and thus marked the former a great deal too low; though the point of congelation was very uncertain according to him, various difficulties having occurred to his attempts of finding the greatest point of contraction while freezing.

The experiments of M. Braun were not repeated by any person till the year 1774, when Mr John Frederic Blumenbach, then a student of physic at Göttingen, performed them to more advantage than it appears M. Braun had ever done. He was encouraged to make the attempt by the excessive cold of the winter that year. "I put (says he), at five in the evening of January 11th, three drachms of quicksilver into a small sugar-glass, and covered it with a mixture of snow and Egyptian sal-ammoniac. This mixture was put loose into the glass, so that the quicksilver lay perfectly free, being only covered with it as by pieces of ice; the whole, together with the glass, weighed somewhat above an ounce. It was hung out at a window in such a position as to expose it freely to the north-west; and two drachms more of sal-ammoniac mixed with the snow on which it stood. The snow and sal-ammoniac, in the open air, soon froze into a mass like ice; no sensible change, however, appeared in the quicksilver that evening; but at one in the morning it was found frozen solid. It had divided into two large and four smaller pieces; one of the former was hemispherical, the other cylindrical, each seemingly rather above a drachm in weight; the four small bits might amount to half a scruple. They were all with their flat side frozen hard to the glass, and nowhere immediately touched by the mixture; their colour was a dull pale white with a bluish cast, like zinc, very different from the natural appearance of quicksilver. Next morning, about eleven o'clock, I found that the larger hemisphere began to melt, perhaps because it was most exposed to the air, and not so near as the others to the sal-ammoniac mixture which lay beneath. In this state it resembled an amalgam, sinking on that side to which the glass was inclined; but without quitting the surface of the glass, to which it was yet firmly congealed: the five other pieces had not yet undergone any alteration, but remained frozen hard. Toward eight o'clock the cylindrical piece began to soften in the same manner, and the other four soon followed. About eight they fell from the surface of the glass, and divided into many fluid shining globules, which were soon lost in the interstices of the frozen mixture, and reunited in part at the bottom, being now exactly like common quicksilver." At the time this experiment was made, the thermometer stood at —10° in the open air.

The circumstances attending this experiment are still unaccountable; for, in the first place, the natural cold was scarcely sufficient, along with that of the artificial mixture, which produces 32° more, to have congealed the quicksilver; which yet appears to have been very effectually done, by the length of time it continued solid. 2. It is not easy to account for the length of time required for congealing the quicksilver in this experiment, since other frigorific mixtures begin to act almost immediately; and, 3. There was not at last even the appearance of action, which consists in a solution of the snow, and not in its freezing into a mass.

"The whole experiment (says Dr Blagden) remains involved in such obscurity, that some persons have supposed the quicksilver itself was not frozen, but only covered over with ice; to which opinion, however, there are great objections. It is worthy of remark, that Gottingen, though situated in the same latitude as London, and enjoying a temperate climate in general, becomes subject at times to a great severity of cold. This of 11th of January 1774 is one instance: I find others where the thermometer sunk to —12°, —16°, or —19°; and at Cattlenburg, a small town about two German miles distant, to —30°. By watching such extraordinary occasions, experiments on the freezing of quicksilver might easily be performed in many places, where the possibility of them is at present little suspected. The cold observed at Glasgow in 1780 would have been fully sufficient for that purpose."

In consequence of the publication of M. Braun's Experiments, the Royal Society desired their late secretary Dr Maty to make the necessary application to the Hudson's Bay Company, in order to repeat the experiment in that country. Mr Hutchins, who was then at London, and going out with a commission as governor of Albany fort, offered to undertake the experiments, and executed them very completely, freezing quicksilver twice in the months of January and February 1775. The account of his success was read before the Royal Society at the commencement of the severest winter that had been known for many years in Europe; and at this time the experiment was repeated by two gentlemen of different countries. One was Dr Lambert Bicker, secretary to the Batavian society at Rotterdam; who, on the 28th of January 1776, at eight in the morning, made an experiment to try how low he could bring the thermometer by artificial cold, the temperature of the atmosphere being then +2°. He could not, however, bring it lower than —94°, at which point it stood immovable; and on breaking the thermometer, part of the quicksilver was found to have lost its fluidity, and was thickened to the consistence of an amalgam. It fell out of the tube in little bits, which bore to be flattened by pressure, without running into globules like the inner fluid part. The experiment was repeated next day, when the thermometer stood at +8°, but the mercury would not then descend below —8°; and as the thermometer was not broken, it could not be known whether the mercury had congealed or not. All that could be inferred from these experiments therefore was, that the congealing point of mercury was not below —94° of Fahrenheit's thermometer. The other who attempted the congelation of this fluid was the late Dr Anthony Fothergill; but it could not be determined whether whether he succeeded or not. An account of his experiments is inserted in the Philosophical Transactions, vol. lxvi.

No other attempts were made to congeal quicksilver until the year 1781, when Mr Hutchins resumed the subject with great success, in such a manner that from his experiments the freezing point of mercury is now almost as well settled as that of water. Preceding philosophers, indeed, had not been altogether inattentive to this subject. Professor Braun himself had taken great pains to investigate it; but for want of paying the requisite attention to the difference between the contraction of the fluid mercury by cold, and that of the congealing metal by freezing, he could determine nothing certain concerning it. Others declared it as their opinion, that nothing certain could be determined by merely freezing mercury in a thermometer filled with that fluid. Mr Cavendish and Dr Black first suggested the proper method of obviating the difficulties on this subject. Dr Black, in a letter to Mr Hutchins, dated October 5, 1779, gave the following directions for making the experiment with accuracy:

"Provide a few wide and short tubes of thin glass, sealed at one end and open at the other: the wideness of these tubes may be from half to three quarters of an inch, and the length of them about three inches. Put an inch or an inch and a half depth of mercury into one of these tubes, and plunging the bulb of the thermometer into the mercury, set the tube with the mercury and the thermometer in it into a freezing mixture, which should be made for this purpose in a common tumbler or water glass; and, N.B. in making a freezing mixture with snow and nitric acid, the quantity of the acid should never be so great as to dissolve the whole of the snow, and only enough to reduce it to the consistence of panada. When the mercury in the wide tube is thus set in the freezing mixture, it must be stirred gently and frequently with the bulb of the thermometer; and if the cold be sufficiently strong, it will congeal by becoming thick like an amalgam. As soon as this is observed, the thermometer should be examined without lifting it out of the congealing mercury; and I have no doubt that in every experiment thus made, with the same mercury, the instrument will always point to the same degree, provided it has been made and graduated with accuracy."

The apparatus recommended by Mr Cavendish, and which Mr Hutchins made use of, consisted of a small mercurial thermometer, the bulb of which reaches about 2½ inches below the scale, and was inclosed in a glass cylinder swelled at the bottom into a ball, which when used was filled with quicksilver, so that the bulb of the thermometer was entirely covered with it. If this cylinder be immersed in a freezing mixture till great part of the quicksilver in it is frozen, it is evident that the degree shown at that time by the inclosed thermometer is the precise point at which mercury freezes; for as in this case the ball of the thermometer must be surrounded for some time with quicksilver, part of which is actually frozen, it seems impossible that the thermometer should be sensibly above that point; and while any of the quicksilver in the cylinder remains fluid, it is impossible that it should sink sensibly below it. The diameter of the bulb of the thermometer was rather less than a quarter of an inch, that of the swelled part of the cylinder two-thirds; and as it was easy to keep the thermometer constantly in the middle of the cylinder, the thickness of quicksilver between it and the glass could never be much less than the sixth part of an inch. The bulb of the thermometer was purposely made as small as it conveniently could, in order to leave a sufficient space between it and the cylinder, without making the swelled part larger than necessary, which would have caused more difficulty in freezing the mercury in it.

The first experiment with this apparatus was made on the 15th of December 1781; the thermometer had stood the evening before at −18°. A bottle of spiritus nitri fortis was put on the house-top, in order to cool it to the same temperature. The thermometers made use of had been hung up in the open air for three weeks to compare their scales. On the morning of the experiment they were about 23° below 0. In making it, the thermometer of the apparatus was suspended in the bulb of the cylinder by means of some red worsted wound about the upper part of its stem, to a sufficient thickness to fill the upper part of its orifice; and a space of near half an inch was left empty between the quicksilver and worsted.

The apparatus was placed in the open air, on the top of the fort, with only a few deer skins sewed together for a shelter; the snow lay 18 inches deep on the works, and the apparatus was stuck into the snow, in order to bring it sooner to the temperature of the air. The instruments were afterwards placed in three fresh freezing mixtures, in hopes of being able by their means to produce a greater degree of cold, but without effect; nor was any greater cold produced by adding more nitric acid. The mercury, however, was very completely frozen, that in the thermometer descending to 448°. On plunging the mercury into the freezing mixture, it descended in less than one minute to 450° below 0.

The second experiment was made the day following; and the same quantity of quicksilver employed that had been used in the former. As too small a quantity of the freezing mixture, however, had been originally made, it was necessary to add more during the operation of congelation; by which means the spirit of nitre, in pouring it upon the snow, sometimes touched the bulb of the thermometer, and instantly raised it much higher; nor did the mercury ever descend below 200°, which was not half as far as it had done the day before, though the temperature of the atmosphere had been this day at −34° before the commencement of the operation. That in the apparatus, however, sunk to −91°. The apparatus was taken out of the mixture for half a minute, in order to examine whether the mercury was perfectly congealed or not, and, during that time, it showed no sign of liquefaction.

The third experiment was made the same day, and with the freezing mixture used in the last. By it the point of congelation was determined to be not below 40°.

The fourth experiment was made January 7th 1782; and in it he observed, that the mercury in the apparatus thermometer, after standing at 42 and 41° for a considerable time, fell to 77°, not gradually, but at once as a weight falls.

In the fifth experiment the weather was excessively severe, so that it ought to have frozen the metal in the open air; but this did not then happen.

At the time of making the sixth experiment, the quicksilver in the open air stood at 44 below 0°; and Mr Hutchins resolved to make use of this opportunity to observe how far it was possible to make it descend by means of cold, observing the degrees at the same time with a spirit thermometer made by Nairne and Blount, with which he had been furnished by the Royal Society in 1774. In this, however, he did not succeed; for the mercury never fell below 43°8, nor the standard 48°. It stood at 27°½ at the beginning of the experiment. The reason of this was supposed to be, that the atmosphere was too cold for making this kind of experiment, by reason of its freezing the thread of quicksilver in the stem of the thermometer, so that it became incapable of contraction along with that in the bulb. In other experiments, though the metal in the bulb became solid, yet that in the stem always remained fluid; and thus was enabled to subside to a great degree by the diminution of bulk in the solid mercury. That this was really the case, appeared from the quicksilver falling at once from —86° to —43°4, when the cold of the freezing mixture diminished, and the temperature of the air becoming about the same time somewhat milder, melted the congealed part in the stem, which thus had liberty to descend to that point.

In this experiment, also, the mixtures were made in double quantity to those of the former; these being only in common tumblers, but the mixtures for this experiment in pint basins. It was observed that they liquefied faster than in other experiments. He had usually made them of the consistence of pap; but though he added snow at different times, it had very little effect in augmenting the cold, but rather decreased it. The congealed piece of the metal fell to the bottom, as might naturally have been expected from its great contraction in becoming solid.

From this experiment Mr Hutchins concluded, that the nearer the temperature of the atmosphere approached to the congealing point of mercury (so that a great degree of cold might be communicated to the bulb of a thermometer, and yet the quicksilver in the tube remain fluid), he might make the experiment of ascertaining the greatest contraction of mercury to more advantage. With this view he made another experiment, when the temperature of some of his thermometers stood as low as —37°; and after an hour's attendance, he perceived the mercury had fallen to 136°7; but the thermometer unluckily was broken, and its bulb thrown away with the mixture. Professor Braun had likewise observed, that his thermometers were always broken when the mercury descended below 60°.

The eighth experiment was made with a view to try whether quicksilver would congeal when in contact with the freezing mixture. For this purpose, he did not use the apparatus provided for other experiments, but filled a gallipot made of flint stone (as being thinner than the common sort), containing about an ounce, half full of quicksilver, into which he inserted a mercurial thermometer, employing another as an index. Thus he hoped to determine exactly when the quicksilver was congealed, as he had free access to it at all times, which was not the case when it was inclosed in the cylindrical glass, the worsted wound round the tube of the thermometer to exclude the air being equally effectual in excluding any instrument from being introduced to touch the quicksilver. He then made a kind of skewer, with a flat blunt point, of dried cedar-wood, on account of its lightness, which he found would remain in the gelatinous freezing mixture at any depth he chose; but, when inserted into the quicksilver, the great difference betwixt the specific gravity of it and that ponderous fluid, made it always rebound upward; and by the degree of resistance, he could always know whether it proceeded from fluid or solid metal. At this time, however, the experiment did not succeed; but, at another trial, having employed about ¾ths of a pound of metal, and let it remain a considerable time immersed in the same mixture which had just now been supposed to fail, he found that part of it was congealed; and, on pouring off the fluid part, no less than two-thirds remained fixed at the bottom.

The last experiment which has been published concerning the congelation of quicksilver by means of dilute snow, is that of Mr Cavendish, and of which he gives an account in the Phil. Transact. vol. Ixxiii. p. 325. Here, speaking of the cold of freezing mixtures, he says, "There is the utmost reason to think that Mr Hutchins would have obtained a greater degree of cold by using a weaker nitrous acid than he did. I found (says he) by adding snow gradually to some of this acid, that the addition of a small quantity produced heat instead of cold; and it was not until so much was added as to increase the heat from 28° to 51°, that the addition of more snow began to produce cold; the quantity of snow required for this purpose being pretty exactly one quarter of the weight of the spirit of nitre; and the heat of the snow, and air of the room, as well as of the acid, being 28°. The reason of this is, that a great deal of heat is produced by mixing water with spirit of nitre; and the stronger the spirit is, the greater is the heat produced. Now it appears, from this experiment, that before the acid was diluted, the heat produced by its union with the water formed from the melting snow, was greater than the cold produced by the same; and it was not until it was diluted by the addition of one quarter of its weight of that substance, that the cold, generated by the latter cause, began to exceed the heat generated by the former. From what has been said, it is evident, that a freezing mixture made with undiluted acid will not begin to generate cold until so much snow is dissolved as to increase its heat from 28° to 51°; so that no greater cold will be produced than would be obtained by mixing the diluted acid heated to 51° with snow of the heat of 28°. This method of adding snow gradually is much the best way I know of finding what strength it ought to be of, in order to produce the greatest effect possible. By means of this acid... acid diluted in the above-mentioned proportion, I froze quicksilver in the thermometer called G (A) by Mr Hutchins, on the 26th of February 1782. I did not indeed break the thermometer to examine the state of the quicksilver therein; for, as it sunk to $-110^\circ$, it certainly must have been in part frozen; but immediately took it out, and put the spirit thermometer in its room, in order to find the cold of the mixture. It sunk only to $-35^\circ$; but by making allowance of the spirit in the tube being not so cold as that in the ball, it appears, that if it had not been for this cause, it would have sunk to $-35^\circ$ (B); which is $6^\circ$ below the point of freezing, and is within one degree of as great a cold as that produced by Mr Hutchins.

"In this experiment the thermometer G sunk very rapidly; and, as far as I could perceive, without stopping at any intermediate point till it came to the above-mentioned degree of $-110^\circ$, where it sunk. The materials used in making the mixture were previously cooled, by means of salt and snow, to near $0^\circ$; and the temperature of the air was between $20^\circ$ and $25^\circ$; the quantity of acid used was $4\frac{1}{2}$ oz.; and the glass in which the mixture was made, was surrounded with wool, and placed in a wooden box, to prevent its losing its cold so fast as it would otherwise have done. Some weeks before this I made a freezing mixture with some spirit of nitre much stronger than that used in the foregoing experiment, though not quite so strong as the undiluted acid, in which the cold was less intense by $42^\circ$. It is true the temper of the air was much less cold, namely $35^\circ$, but the spirit of nitre was at least as cold, and the snow not much less so.

"The cold produced by mixing sulphuric acid, properly diluted with snow, is not so great as that produced by spirit of nitre, though it does not differ from it by so much as $80^\circ$; for a freezing mixture, prepared with diluted sulphuric acid, whose specific gravity, at $60^\circ$ of heat, was $1.5642$, sunk in the thermometer G to $-37^\circ$; the experiment being tried at the same time, and with the same precautions, as the foregoing. It was previously found, by adding snow gradually to some of this acid, as was done by the nitrous acid, that it was a little, but not much stronger, than it ought to be, in order to produce the greatest effect."

The experiment made by Mr Walker, in which he congealed quicksilver by means of nitric acid and Glauber's salt, without any snow, concludes the history of the artificial congelation of mercury. It now remains that we say something of the congelation of it by the natural cold of the atmosphere.

Dr Blagden, from whose paper in the Philosophical Transactions, vol. lxxiii., this account is taken, observes, that it was not till near the year 1790 that thermometers were made with any degree of accuracy; and in four or five years after this, the first observations were made which prove the freezing of quicksilver.

On the accession of the empress Anne Ivanovna to the throne of Russia, three professors of the Imperial academy were chosen to explore and describe the different parts of her Asiatic dominions, and to inquire into the communication betwixt Asia and America. These were Dr John George Gmelin, in the department of natural history and chemistry; M. Gerard Frederic Muller, as general historiographer; and M. Louis de l'Isle de la Croyere, for the department of astronomy; draughtsmen and other proper assistants being appointed to attend them. They departed from Peterburgh in 1733; and such as survived did not return till ten years after. The thermometrical observations were communicated by Professor Gmelin, who first published them in his Flora Sibirica, and afterwards more fully in the Journal of his Travels. An abstract of them was likewise inserted in the Peterburgh Commentaries for the years 1756 and 1765, taken, after the professor's death, from his original dispatches in possession of the Imperial academy.

In the winter of 1734 and 1735, Mr Gmelin being at Yeneseisk in $58^\circ$ N. Lat. and $92^\circ$ E. Long, from Greenwich, first observed such a descent of the mercury, as we know must have been attended with congelation. "Here (says he) we first experienced the Executive truth of what various travellers have related with regard to the extreme cold of Siberia; for, about the middle of December, such severe weather set in, as we were sure had never been known in our time at Peterburgh. The air seemed as if it were frozen, with the appearance of a fog, which did not suffer the smoke to ascend as it issued from the chimneys. Birds fell down out of the air as dead, and froze immediately, unless they were brought into a warm room. Whenever the door was opened, a fog suddenly formed round it. During the day, short as it was, parhelion and haloes round the sun were frequently seen; and in the night mock-moons, and haloes about the moon. Finally, our thermometer, not subject to the same deception as the senses, left us no doubt of the excessive cold; for the quicksilver in it was reduced on the 13th of January O.S. to $-120^\circ$ of Fahrenheit's scale, lower than it had ever hitherto been observed in nature."

The next instance of congelation happened at Yakutsk, in N. Lat. $62.$ and E. Long. $130.$ The weather there was unusually mild for the climate, yet the thermometer fell to $-72^\circ$; and one person informed the professor by a note, that the mercury in his barometer was frozen. He hastened immediately to his house to behold such a surprising phenomenon; but though he was witness to the fact, the prejudice he entertained against the possibility of the congelation, would not allow him to believe it. "Not feeling (says he), by the way, the same effects of cold as I had experienced at other times in less distances, I began, before my arrival, to entertain suspicions about the congelation of his quicksilver. In fact, I saw that it did not continue in one column, but was divided in different places as into little cylinders, which appeared frozen; and, in some of these divisions between the quicksilver, I perceived like the appearance of frozen moisture.

(a) This was a small mercurial thermometer, made by Nairne and Blount, on an ivory scale, divided at every five degrees, and reaching from $215^\circ$ above to $250^\circ$ below the cipher.

(b) This is to be understood of a spirit thermometer, whose $-29^\circ=40^\circ$ of Fahrenheit's mercurial. moisture. It immediately occurred to me, that the mercury might have been cleaned with vinegar and salt, and not sufficiently dried. The person acknowledged it had been purified in that manner. This same quicksilver, taken out of the barometer, and well dried, would not freeze again, though exposed to a much greater degree of cold, as shown by the thermometer.

Another set of observations, in the course of which the mercury frequently congealed, were made by Professor Gmelin at Kirenga fort in 57° N. Lat. 108° E. Long.; his thermometer, at different times, standing at —168°, —86°, —100°, —113°, and many other intermediate degrees. This happened in the winter of 1737 and 1738. On the 27th of November, after the thermometer had been standing for two days at —46°, he found it sunk at noon to 108°. Suspecting some mistake, after he had noted down the observation, he instantly ran back, and found it at 102°; but ascending with such rapidity, that in the space of half an hour it had risen to —19°. This phenomenon, which appeared so surprising, undoubtedly depended on the expansion of the mercury frozen in the bulb of the thermometer, and which now melting, forced upwards the small thread in the stem.

A similar appearance was observed at the same fort a few days after; and on the 29th of December, O.S., he found the mercury, which had been standing at —40° in the morning, sunk to —100° at four in the afternoon. At this time, he says, "I saw some air in the thermometer separating the quicksilver for the space of about six degrees." He had taken notice of a similar appearance the preceding evening, excepting that the air, as he supposed it to be, was not then collected into one place, but lay scattered in several.

These appearances undoubtedly proceeded from a congelation of the mercury, though the prejudice entertained against the possibility of this phenomenon would not allow the professor even to inquire into it at all. Several other observations were made; some of which were lost, and the rest contain no farther information.

The second instance where a natural congelation of mercury has certainly been observed, is recorded in the Transactions of the Royal Academy of Sciences at Stockholm. The weather, in January 1760, was remarkably cold in Lapland; so that on the 5th of that month, the thermometers fell to —76°, —128°, or lower; on the 23rd and following days they fell to —58°, —70°, —92°, and below —238° entirely into the ball. This was observed at Tornea, Sombio, Jakasjärvi, and Utsjoki, four places in Lapland, situated between the 65th and 78th degrees of N. Lat. and the 21st and 28th of E. Long. The person who observed them was M. Andrew Hellant, who makes the following remarks, of themselves sufficient to show that the quicksilver was frozen. "During the cold weather at Sombio (says he), as it was clear sunshine, though scarcely the whole body of the sun appeared above the low woods that covered our horizon, I took a thermometer which was hanging before in the shade and exposed it to the rising sun about eleven in the forenoon, to see whether when that luminary was so low, it would have any effect upon the instrument. But to my great surprise, upon looking at it about noon, I found that the mercury had entirely subsided into the ball, though it was standing as high as —61° at 11 o'clock, and the scale reached down to 238° below 0°." On bringing the instrument near a fire, it presently rose to its usual height; and the reason of its subsiding before was its being somewhat warmed by the rays of the sun; which, feeble as they were, had yet sufficient power to melt the small thread of congealed mercury in the stem of the thermometer, and allow it to subside along with the rest. Mr Hellant, however, so little understood the nature of this phenomenon, that he frequently attempted to repeat it by bringing the thermometer near a fire, when the cold was only a few degrees below the freezing point of water, but could never succeed until it fell to —58°, or lower, that is, until the cold was sufficiently intense to congeal the metal. The only seeming difficulty in his whole account is, that when the mercury had subsided entirely into the ball of the thermometer, a vacuum or empty spot appeared, which run round the cavity like an air bubble, on turning the instrument; but this proceeded from a partial liquefaction of the mercury, which must necessarily melt first on the outside, and thus exhibit the appearance just mentioned.

The most remarkable congelation of mercury, which remains ever yet been observed, was that related by Drable and Peter Simon Pallas, who had been sent by the emperors of Russia, with some other gentlemen, on an expedition similar to that of Dr Gmelin. He did not, however, spend the winters in which he was in Siberia in the coldest parts of that country; that is, about the middle of the northern part. Twice indeed he resided at Krafnoyarfik, in N. Lat. 64°, E. Long. 93°; where, in the year 1772, he had an opportunity of observing the phenomenon we speak of. "The winter (says he) fell in early this year, and was felt with uncommon severity in December. On the 6th and 7th of that month happened the greatest cold I have ever experienced in Siberia; the air was calm at the time, and seemingly thickened; so that, though the sky was in other respects clear, the sun appeared as through a fog. I had only one small thermometer left, in which the scale went no lower than —7°; and on the 6th in the morning, I remarked that the quicksilver in it sunk into the ball, except some small columns which stuck fast in the tube. When the ball of the thermometer, as it hung in the open air, was warmed by being touched with the finger, the quicksilver rose; and it could plainly be seen that the solid columns stuck and resisted a good while, and were at length pushed upward with a sort of violence. In the mean time I placed upon the gallery, on the north side of my house, about a quarter of a pound of clean and dry quicksilver in an open bowl. Within an hour I found the edges and surface of it frozen solid, and some minutes afterwards the whole was condensed by the natural cold into a soft mass very much like tin. While the inner part was still fluid, the frozen surface exhibited a great variety of branched wrinkles; but in general it remained pretty smooth in freezing, as did also a larger quantity which I afterwards exposed to the cold. The congealed mercury was more flexible than lead; but on being bent short, it was found more brittle than tin; and when hammered out thin, it seemed somewhat granulated. If the hammer had not been been perfectly cooled, the quicksilver melted away under it in drops; and the same thing happened when the metal was touched with the finger, by which also the finger was immediately benumbed. In our warm room it thawed on its surface gradually, by drops, like wax on the fire, and did not melt all at once. When the frozen mass was broken to pieces in the cold, the fragments adhered to each other and to the bowl on which they lay. Although the frost seemed to abate a little towards night, yet the congealed quicksilver remained unaltered, and the experiment with the thermometer could still be repeated. On the 7th of December, I had an opportunity of making the same observations all day; but some hours after sunset, a north-west wind sprung up, which raised the thermometer to —46°, when the mass of quicksilver began to melt."

In the beginning of the year 1780 Van Elterlein, of Vytegra, a town of Russia, in N. Lat. 61° E., Long. 36°, froze quicksilver by natural cold; of which he gives the following account. "On the 4th of January 1780, the cold having increased to —34° that evening at Vytegra, I exposed to the open air three ounces of very pure quicksilver in a china tea-cup, covered with paper pierced full of holes. Next day, at eight in the morning, I found it solid, and looking like a piece of cast lead, with a considerable depression in the middle. On attempting to loosen it in the cup, my knife raised shavings from it as if it had been lead, which remained sticking up; and at length the metal separated from the bottom of the cup in one mass. I then took it in my hand to try if it would bend: it was stiff like glue, and broke into two pieces; but my fingers immediately lost all feeling, and could scarcely be restored in an hour and a half by rubbing with snow. At eight o'clock a thermometer, made by Mr Lexmann of the academy, stood at —57°; by half after nine it was risen to —42°; and then the two pieces of mercury which lay in the cup had lost so much of their hardness, that they could no longer be broken, or cut into shavings, but resembled a thick amalgam, which, though it became fluid when pressed by the fingers, immediately afterwards resumed the consistence of pap. With the thermometer at —39°, the quicksilver became fluid. The cold was never less on the 5th than —28°, and by nine in the evening it had increased again to —33°."

An instance of the natural congelation of quicksilver also occurred in Jemtland, one of the provinces of Sweden, on the 1st of January 1782; and lastly, on the 26th of the same month, Mr Hutchins observed the same effect of the cold at Hudson's bay. "The subject of this curious phenomenon (says he), was quicksilver put into a common two-ounce phial, and corked. The phial was about a third part full, and had constantly been standing by the thermometer for a month past. At eight o'clock this morning I observed it was frozen rather more than a quarter of an inch thick round the sides and bottom of the phial, the middle part continuing fluid. As this was a certain method of finding the point of congelation, I introduced a mercurial and a spirit thermometer into the fluid part, after breaking off the top of the phial, and they rose directly and became stationary; the former at 40° or 40°, the latter at 29°, both below the cipher. Having taken these out, I put in two others, G a mercurial one formerly described, and a spirit thermometer; the former of which became stationary at 40° and the latter at 30°. I then decanted the fluid quicksilver, to examine the internal surface of the frozen metal, which proved very uneven, with many radii going across, some of which resembled pin-heads. Urgent business called me away an hour. On my return I found a small portion only had liquefied in my absence. I then broke the phial entirely, and with a hammer repeatedly struck the quicksilver. It beat out flat, yielded a deathly sound, and became fluid in less than a minute afterwards. It may be worth remarking, that the quicksilver in one of the thermometers, which had sunk to very near 50°, and was then at 44°, very readily run up and down the tube by elevating either end of the instrument."

These are all the well authenticated accounts of the congelation of mercury by the natural cold of the atmosphere. Some others have been published; but being either less important, or not so well authenticated, we forbear to mention them. A very considerable confirmation is obtained from the above history, of the theory of congelation delivered by Dr Black, and which is fully explained under the article Chemistry. On Mr Hutchins's experiments, and on congelation in general, Mr Cavendish makes many valuable remarks; the substance of which is as follows:

"If a vessel of water, with a thermometer in it, be exposed to the cold, the thermometer will sink several degrees below the freezing point, especially if the water be covered up so as to be defended from the wind, and care taken not to agitate it; and then on dropping in a bit of ice, or on mere agitation, spiculae of ice float suddenly through the water, and the inclosed thermometer rises quickly to the freezing point, where it remains stationary." In a note he says, that though in conformity to the common opinion he has allowed that "mere agitation may set the water a freezing, yet some experiments lately made by Dr Blagden seem to show, that it has not much, if any, effect of that kind, otherwise than by bringing the water in contact with some substance colder than itself. Though in general also the ice floats rapidly, and the inclosed thermometer rises very quick; yet he once observed it to rise very slowly, taking up not less than half a minute, before it ascended to the freezing point; but in this experiment the water was cooled not more than one or two degrees below freezing, and it should seem, that the more the water is cooled below the freezing point, the more rapidly the ice floats and the inclosed thermometer rises."

Mr Cavendish then observes, that from the foregoing experiments we learn that water is capable of being cooled considerably below the freezing point, without any congelation taking place; and that, as soon as any means a small part of it is made to freeze, the ice spreads rapidly through the whole of the water. The cause of this rise of the thermometer is, that all, or almost all bodies by changing from a fluid to a solid state, or from the state of an elastic to that of an unelastic fluid, generate heat; and that cold is produced by the contrary process. Thus all the circumstances of the phenomenon may be perfectly well explained; for, as soon as any part of the water freezes, heat will be generated thereby in consequence of the above-mentioned law, so that the new formed ice and remaining water will be warmed, and must continue to receive heat by the freezing of fresh portions of water, till it is heated exactly to the freezing point, unless the water could become quite solid before a sufficient quantity of heat was generated to raise it to that point; which is not the case: and it is evident, that it cannot be heated above the freezing point: for as soon as it comes thereto, no more water will freeze, and consequently no more heat will be generated.—The reason why the ice spreads all over the water, instead of forming a solid lump in one part, is, that, as soon as any small portion of ice is formed, the water in contact with it will be so much warmed as to be prevented from freezing, but the water at a little distance from it will still be below the freezing point, and will consequently begin to freeze.

"Were it not for this generation of heat, the whole of any quantity of water would freeze as soon as the process of congelation began; and in like manner the cold is generated by the melting of ice; which is the cause of the long time required to thaw ice and snow. It was formerly found that, by adding snow to warm water, and stirring it about until all was melted, the water was as much cooled as it would have been by the addition of the same quantity of water rather more than 150° degrees colder than the snow; or, in other words, somewhat more than 150° of cold are generated by the thawing of the snow; and there is great reason to believe that just as much heat is produced by the freezing of water. The cold generated in the experiment just mentioned was the same whether ice or snow was used.

"A thermometer kept in melted tin or lead till they become solid, remains perfectly stationary from the time the metal begins to harden round the sides of the pot till it is entirely solid; but it cannot be perceived, at all, to sink below that point, and rise up to it, when the metal begins to harden. It is not unlikely, however, that the great difference of heat between the air and melted metal might prevent this effect from taking place; so that though it was not perceived in these experiments, it is not unlikely that those metals, as well as water and quicksilver, may bear being cooled a little below the freezing or hardening point (for the hardening of melted metals, and freezing of water, seems exactly the same process), without beginning to lose their fluidity.

"The experiments of Mr Hutchins prove, that quicksilver contracts or diminishes in bulk by freezing, and that the very low degrees to which the thermometers have been made to sink, is owing to this contraction, and not to the cold having been in any degree equal to that shown by the thermometer. In the fourth experiment, one of the thermometers sunk to 45°, though it appeared, by the spirit thermometers, that the cold of the mixture was not more than five or six degrees below the point of freezing quicksilver. In the first experiment also, it sunk to 44° at a time when the cold of the mixture was only 2° below that point; so that it appears that the contraction of quicksilver by freezing must be at least equal to its expansion by 40° degrees of heat (c). This, however, is not the whole contraction that it suffers; for it appears by an extract from a meteorological journal kept by Mr Hutchins at Albany fort, that his thermometer once sunk to 49° below 0°; though it was known by a spirit thermometer that the cold scarcely exceeded the point of freezing quicksilver. There are two experiments also of Professor Braun, in which the thermometer sunk to 544 and 536° below 0°; which is the greatest defect he ever observed without the ball being cracked. It is not indeed known how cold his mixtures were; but from Mr Hutchins's experiments, there is great reason to think they could not be many degrees below 40°. If so, the contraction which quicksilver suffers in freezing, is not much less than its expansion by 50° or 51° of heat, that is, almost ⅓ of its whole bulk; and in all probability is never much more than that, though it is probable that this contraction is not always determinate; for a considerable variation may frequently be observed in the specific gravity of the same piece of metal cast different times over; and almost all cast metals become heavier by hammering. Mr Cavendish observed, that on casting the same piece of tin three times over, its density varied from 7.252 to 7.294, although there was great reason to think that no hollows were left in it, and that only a small part of this apparent difference could proceed from the error of the experiment. This variation of density is as much as is produced in quicksilver by an alteration of 66° of heat; and it is not unlikely, that the defect of a thermometer, on account of the contraction of the quicksilver in its ball by freezing, may vary as much in different trials, though the whole mass of quicksilver is frozen without any vacuities.

"The cold produced by mixing spirit of nitre with snow is entirely owing to the melting of the mixture. Now, in all probability, there is a certain degree of cold, in which the spirit of nitre, so far from diffusing snow, will yield part of its own water, and suffer that to freeze, as is the case with solutions of common salt; so that if the cold of the materials before mixing is equal to this, no additional cold can be produced. If the cold of the materials is less, some increase of cold will be produced; but the total cold will be less than in the former case, since the additional cold cannot be generated without some of the snow being dissolved, and thereby weakening the acid, and making it less able to dissolve more snow; but yet the less the cold of the materials is, the greater will be the additional cold produced. This is conformable to Mr Hutchins's experiments; for in the fifth experiment,

(c) The numbers here given are those shown by the thermometer without any correction; but if a proper allowance is made for the error of that instrument, it will appear, that the true contraction was 2° less than here set down; and from the manner in which thermometers have been usually adjusted, it is likely that in the fifth experiment of Mr Hutchins, as well as in those of Professor Braun, the true contraction might equally fall short of that by observation. in which the cold of the materials was —49°, the additional cold produced was only 5°. In the first experiment, in which the cold of the materials was only —23°, an addition of at least 10° of cold was obtained; and by mixing some of the same spirit of nitre with snow in this climate, when the heat of the materials was +26°, Mr Cavendish was able to sink the thermometer to —29°, so that an addition of 55 degrees of cold was produced.

"It is remarkable that in none of Mr Hutchins's experiments the cold of the mixture was more than 6° of the spirit thermometer below the freezing point of quicksilver; which is so little, that it might incline one to think that the spirit of nitre used by him was weak. This, however, was not the case; as its specific gravity at 58° of heat was 1.4923. It was able to dissolve $\frac{1}{1.42}$ its weight of marble, and contained very little mixture of sulphuric or muriatic acid; as well as could be judged from an examination of it, it was as little phlogisticated as acid of that strength usually is."

Acids, especially those of the mineral kind, powerfully resist congelation. There is, however, a peculiarity with regard to that of vitriol. M. Chaptal, a foreign chemist, observed, that it condensed by the cold of the atmosphere, and the crystals began to melt only at —75° of his thermometer; which, if Reaumur's, corresponds to about 47° of Fahrenheit. The crystals were transparent from the melting acid, and they felt warmer than the neighbouring bodies: the form was that of a prism of six sides, flattened and terminated by a pyramid of six sides; but the pyramid appeared on one end only; on the other, the crystal was lost in the general mass. The pyramid resulted from an assemblage of six isosceles triangles; the oil, when the crystal was melted was of a yellowish black; on redistilling it in a proper apparatus, no peculiar gas came over. M. Chaptal repeated his experiments with the highly concentrated acid, but found that it did not freeze; that the density of the acid which he thought froze most easily was to the oil, of the usual strength for sale, as from 63 and 65 to 66; and the necessary degree of cold about 16° of Fahrenheit. Sulphuric acid once melted will not crystallize again with the same degree of cold.

In the experiments which had been made on the freezing of sulphuric acid, Mr Cavendish found some uncertainty in determining the point at which it freezes most readily; neither could he determine whether the cold necessary for congelation does not increase without any limitation in proportion to the strength of the acid. A new set of experiments were therefore made by Mr Keir to determine this point. He had observed, after a severe frost at the end of the year 1784 and beginning of 1785, that some sulphuric acid, contained in a casked phial, had congealed, while other bottles containing the same, some stronger and some weaker, retained their fluidity. As the congelation was naturally imputed to the extremity of the cold, he was afterwards surprised to find, when the frost ceased, that the acid remained congealed for many days, when the temperature of the atmosphere was sometimes above 40° of Fahrenheit; and when the congealed acid was brought into a warm room on purpose to thaw it, a thermometer placed in contact with it during its thawing continued stationary at 45°. Hence he concluded, that the freezing and thawing point of this acid was nearly at 45°; and accordingly, on exposing the liquor which had been thawed to the air at the temperature of 32°, the congelation again took place in a few hours. From the circumstance of other parcels of the same acid, but of different strengths, remaining fluid, though they had been exposed to a much greater degree of cold, he was led to believe that there must be some certain strength at which the acid is more disposed to congeal than at any other. The specific gravity of the acid which had frozen was to that of water nearly as 1800 to 1000, and that of the stronger acid which had not frozen was as 1846 to 1000, which is the common density of that usually sold in England; and there was not the least difference, excepting in point of strength, between the acid which had frozen and that which had not; Mr Keir having taken the acid some weeks before with his own hands from the bottle which contained the latter, and diluted it with water, till it became of the specific gravity of 1800.

To render the experiment complete, Mr Keir immersed several acids of different strengths in melting snow, instead of exposing them to the air; the temperature of which was variable, whereas that of melting snow was certain and invariable. Those which would not freeze in melting snow were afterwards immersed in a mixture of common salt, snow, and water; the temperature of which, though not so constant and determinate as that of melting snow, generally remained for several hours at 18°, and was sometimes several degrees lower. The intention of adding water to the snow and salt was to lessen the intensity of the cold of this mixture, and to render it more permanent than if the snow and salt alone were mixed. The acids which had frozen in melting snow were five in number; which being thawed and brought to the temperature of 60°, were found on examination to have the following specific gravities, viz. 1786, 1784, 1780, 1778, 1775. Those which had not congealed with the melting snow, but which did so with the mixture of snow, salt, and water, were found, when brought to the temperature of 60°, to be of the following specific gravities, viz. 1814, 1815, 1804, 1794, 1792, 1772, 1759, 1750. Those which remained, and would freeze neither in melting snow nor in the mixture of snow, salt, and water, were of the gravities 1864, 1839, 1815, 1745, 1720, 1700, 1610, 1551. From the first of these it appears, that the medium density of the acids which froze with the natural cold was 1780; and from the second, that at the densities of 1790 and 1770 the acid had been incapable of freezing with that degree of cold. Hence it follows, that 1780 is nearly the degree of strength of easiest freezing, and that an increase or diminution of that density equal to $\frac{1}{18}$th of the whole, renders the acid incapable of freezing with the cold of melting snow, though this cold is something above the freezing point of the most congealable acid. From the second it appears, that by applying a more intense cold, viz. that produced by a mixture of snow, salt, and water, the limits of the densities of acids capable of congelation were extended to about $\frac{1}{18}$th above or below the point point of easiest freezing; and there seems little reason to doubt, that, by greater augmentations of cold, these limits may be further extended; but in what ratio these augmentations and extensions proceed, cannot be determined, without many observations made in different temperatures.

"But (says Mr Keir) though it is probable that the most concentrated acids may be frozen, provided the cold be sufficiently intense, yet there seems reason to believe, that some of the conglomerations which have been observed in highly concentrated acids, have been effected in consequence of the density of these acids being reduced nearly to the point of easy freezing by their having absorbed moisture from the air: for the Duke d'Ayen and M. de Morveau exposed their acids to the air in cups or open vessels; and the latter even acquaints us, that on examining the specific gravity of the acid which had frozen, he found it to that of water as 129 to 74; which density being less than that of easiest freezing, proves that the acid he employed, and which he had previously concentrated, had been actually weakened during the experiment. I have several times exposed concentrated sulphuric acid in open vessels in frosty weather; and I have sometimes, but not always, observed a congelation to take place. Upon separating the congealed part, and on examining the specific gravity of the latter after it had thawed, I found that it had been reduced to the point of easiest freezing. When the congealed acid was kept longer exposed it gradually thawed, even when the cold of the air increased; the reason of which is not to be imputed to the heat produced by the moisture of the air mixing with the acid, but principally to the diminution below the point of easiest freezing, which was occasioned by the continued absorption of moisture from the air, and which rendered the acid incapable of continuing frozen without a great increase of cold.

"It appears, then, that the concentration of M. de Morveau's acid, at the time of its congelation, from which circumstance Mr Cavendish infers generally that sulphuric acid freezes more easily as it is more dense, is not a true premise; and that therefore the inference, though justly deduced, is invalid. On the contrary, there seems every reason to believe, that as the density of the acids increases beyond the point of easiest freezing, the facility of the congelation diminishes; at least to as great density as we have ever been able to obtain sulphuric acid: for if it were possible to divest it entirely of water, it would probably assume a solid form in any temperature of the air.

"The crystallization of sulphuric acid is more or less distinct, according to the flowness of the formation of the crystals and other favourable circumstances. Sometimes they are very large, distinctly shaped, and hard. Their shape is like those of the common mineral alkali and selenite spar, but with angles different in dimensions from either of these. They are solid, consisting of ten faces; of which the two largest are equal, parallel, and opposite to each other; and are oblique-angled parallelograms or rhomboids, whose angles are, as near as could be measured, of 105 and 75 degrees. Between these rhomboidal faces are placed eight of the form of trapeziums; and thus each crystal may be supposed to be compounded of two congruent and similar frustums of pyramids joined together by their rhomboidal bases. They always funk in the fluid acid to the bottom of the vessel, which showed that their density was increased by congelation. It was attempted to determine their specific gravity by adding to this fluid some concentrated acid, which should make them float in the liquor, the examination of whose specific gravity should ascertain that of the floating crystals; but they were found to sink even in the most concentrated acid, and were consequently denser. Some of the congealable acid previously brought to the freezing temperature was then poured into a graduated narrow cylindrical glass, up to a certain mark, which indicated a space equal to that occupied by 200 grains of water. The glass was placed in a mixture of snow, salt, and water; and when the acid was frozen, a mark was made on the part of the glass to which it had sunk. Having thawed the acid and emptied the glass, it was filled with water to the mark to which it had sunk by freezing; and it was then found that 15 grains more of water were required to raise it to the mark expressing 200 grains; which shows, that the diminution of bulk sustained by the acid in freezing had been equal to $\frac{1}{13}$ of the whole. Computing from this datum, we should estimate the specific gravity of the congealed acid to have been 1924; but as it evidently contained a great number of bubbles, its real specific gravity must have been considerably greater than the above calculation, and cannot easily be determined on account of these bubbles. By way of comparison, Mr Keir observed the alteration of bulk which water contained in the same cylindrical vessel would suffer by freezing; and found that its expansion was equal to about $\frac{1}{75}$th of its bulk. The water had been previously boiled, but nevertheless contained a great number of air bubbles; so that in this respect there is a considerable difference between the conglomerations of water and sulphuric acid; though perhaps it may arise principally from the bubbles of elastic fluid being in greater proportion in the one than the other.

"Greater cold is produced by mixing snow or pounded ice with the congealed than with the fluid sulphuric acid, though the quantity is not yet determined. The greatest cold produced by Mr McNab at Hudston's Bay, was effected by mixing snow with a sulphuric acid which had been previously congealed; and to this circumstance Mr Cavendish imputes the intensity of the cold, as the liquefaction both of the acid and the snow had concurred in producing the same effect; while in mixing fluid acids with snow, the thawing of the snow is probably the only productive cause.

"To compare the times requisite for the liquefactions of ice and of congealed sulphuric acid, two equal and similar glasses were filled, one with the congealable sulphuric acid, the other with water; and after having immersed them in a freezing mixture, till both were congealed and reduced to the temperature of 28°, the glasses were withdrawn, wiped dry, and placed in a room where the thermometer stood at 62°. The ice thawed in 40 minutes, and the acid in 95; at the end..." end of which time the thermometer, which stood near the glass, had risen to 64°. Hence it appears that the congealed acid requires more than twice the time for its liquefaction that ice does, though it cannot thence be fairly inferred, that the cold generated by the liquefaction of the ice and of congealed acid are in the above proportions of the times, from the following considerations, viz. that as, during the liquefaction of the ice, its temperature remains stationary at 32°, and during the liquefaction of the acid, its temperature remains about 44 or 45°, it appears, that the ice being considerably colder than the acid, will take the heat from the contiguous air much faster. By this experiment, however, we know that a considerable quantity of cold is generated by the liquefaction of the acid; and hence it appears probable, that in producing cold artificially, by mixing snow with acids in very cold temperatures, it would probably be useful to employ a sulphuric acid of the proper density for congelation, and to freeze it previously to its mixture with snow. It must not, however, be imagined, that the cold generated by the mixture of these two frozen substances is nearly equal to the fums of the colds generated by the separate liquefactions of the congealed acid and ice, when singly exposed to a thawing temperature; for the mixture resulting from the liquefaction, consisting of sulphuric acid and the water of the snow, appears from the generation of heat which occurs from the mixture of these ingredients in a fluid state, to be subject to different laws than those which rule either of the ingredients separately.

"The sulphuric acid, like water and other fluids, is capable of retaining its fluidity when cooled considerably below its freezing point. A phial containing some congealable sulphuric acid being placed in a mixture of salt, snow, and water, a thermometer was soon afterwards immersed in it while the acid was yet fluid, on which it quickly sunk from 50 to 29°. On moving the thermometer in the fluid, to make it acquire the exact temperature, the mercury was observed suddenly to rise; and on looking at the acid, numberless small crystals were observed floating in it, which had been suddenly formed. The degree to which the mercury then rose was 46½°; and at another time, while the acid was freezing, it stood at 45°."

From these experiments our author infers, "1. That sulphuric acid has a point of easiest freezing, and that this is when the specific gravity is to that of water as 1785 to 1000. 2. That the greater or less disposition to congelation does not depend on any other circumstance than the strength of the acid. 3. That the freezing and thawing degree of the most congealable acid is about 45° of Fahrenheit's scale. It is, however, to be observed, that this degree is inferred from the temperature indicated by the thermometers immersed in the freezing and thawing acids; but the congelation of the fluid acid could never be accomplished without exposing it to a greater degree of cold, either by exposing it to the air in frosty weather or to the cold of melting snow. 4. Like water, this acid possesses the property of retaining its fluidity when cooled several degrees below the freezing point; and of rising suddenly to it when its congelation is promoted by agitation, or by contact even with a warmer thermometer. 5. That, like water and other congealable fluids, sulphuric acid generates cold by its liquefaction, and heat during its congelation, though the quantity of this heat and cold remains to be determined by future experiments. 6. That the acid, by congelation, when the circumstances for distinct crystallization are favourable, assumes a regular crystalline form, a considerable solidity and hardness, and a density much greater than it possessed in its fluid state."

Besides this species of congelation, sulphuric acid is subject to another, probably the same described by Basil Valentine and some of the older chemists. This is effected in the ordinary temperature of the air, even in summer; and according to Mr Keir*, is peculiar to that species of sulphuric acid which is distilled from green vitriol, and which is possessed of a smoking quality in a high degree; "for not only the authors (says Mr Keir) by whom this congelation has been observed, have given this description of the acid employed, but also the late experiments of Mr Dolfs, seem to show that this smoking quality is essential to the phenomenon: for neither the acid obtained from vitriol, when deprived of rectification of its smoking quality, nor the English sulphuric acid, which is known to be obtained by burning sulphur, and which does not smoke, were found by his trials to be susceptible of this species of congelation. It may, however, be worth the attention of those chemists who have an opportunity of seeing this icy sulphuric acid, as it is called, to observe more accurately than has yet been done, the freezing temperature and the density of the congealable acids; and to examine whether the density of this smoking acid also is connected with the glacial property. It seems also further deserving of investigation, whether there be not some analogy between the congelation of the smoking sulphuric acid and the very curious crystallization which Dr Priestley observed in a concentrated sulphuric acid saturated with nitrous acid vapours; and whether this smoking quality does not proceed from some marine or other volatile acid, which may be contained in the martial vitriol whence the sulphuric acid is obtained."

Mr Keir also observes, that M. Cornetier has effected the crystallization of sulphuric acid, by distilling it with nitrous acid and charcoal; and we can add from our own experience, that a crystallization instantly takes place on allowing the fumes of the nitrous and sulphuric acids to mix together; and this, whether the former be procured from martial vitriol or sulphur, and whether it be in a phlogisticated state or not, concentration in both acids is here the only requisite.