CONGELATION

CONGELATION, or FREEZING, is a term used to denote the transition of certain substances from a fluid to a solid form. Though equally applicable to the hardening of melted metals, it is seldom used in this case; as their consolidation, though attended with a certain

Congelation. certain degree of cold, yet leaves them possessed of an intense heat; and therefore, the term which is used to express the hardening of water, &c. which is attended with a very great degree of sensible cold, seems not so proper in common language.

1 Different degrees of cold required to freeze different liquors. Various degrees of cold are required for the conge-lation of different fluids. Water congeals when Fahrenheit's thermometer stands at $32^\{\backslash circ\}$ above 0; vinegar, when it sinks to $27^\{\backslash circ\}$; wine, when it sinks to $20^\{\backslash circ\}$; but brandy cannot be frozen till it has sunk to $7^\{\backslash circ\}$ below 0, and weak spirit of wine does not consolidate till it has sunk to $33^\{\backslash circ\}$. Mercury requires a still greater degree of cold, such as in this country we can have no conception of. In fact, this substance having been found to retain its fluidity under the greatest degrees of cold, known not only in this country, but in every other, it was till very lately considered as essentially fluid. Mr Brawn, however, at Peterburg, where very severe frosts are often felt, thought of making an experiment how far the quicksilver in the thermometer might be made to sink, by plunging it into a freezing mixture, while the cold was at its most extreme degree in that rigorous climate. Accordingly, one day while the frost was so intense that the thermometer stood at $40^\{\backslash circ\}$ below 0, he plunged it into a mixture of snow and spirit of nitre; upon which the quicksilver descended to $352^\{\backslash circ\}$, and then became stationary. Upon removing the thermometer into a warm room, he observed, that it remained there 12 minutes before it began to rise; which made him suspect that the mercury had actually been frozen. To be convinced whether or not this was the case, he repeated the experiment, and when the quicksilver had again become stationary, he broke the glass, and found that it had really assumed a solid form, was perfectly flexible, and bore several strokes of a hammer.

In making this experiment he remarked, that the quicksilver descended regularly to $148^\{\backslash circ\}$, after which it remained stationary for a little, and then would suddenly descend $100^\{\backslash circ\}$ at once. From this Dr Black conjectures, that $148^\{\backslash circ\}$ below 0 is the freezing point of quicksilver. The subsequent irregular descent of the mercury he attributes to the contraction of the solid metal in the act of conge-lation; for after it had descended this length, it would suddenly descend by starts, a great number of degrees at once. In this he is the more confirmed, that Mr Brawn had one thermometer, the quicksilver of which did not become solid; notwithstanding which, it would not descend below $148^\{\backslash circ\}$.

2 Quicksilver frozen. Though every known substance, and water as well as others, suffers a remarkable diminution in bulk on being exposed to a cold air; yet after water is arrived at the freezing point, the consolidation which then instantly takes place in some part of it, makes it suddenly expand about $\backslash frac\{1\}\{3\}$ of its original bulk; which expansion is found to be owing to an innumerable quantity of small bubbles with which the ice is filled. On thawing a piece of ice under an exhausted receiver, these bubbles rise to the top of the liquid, and then break; but do not affect the mercurial gage of the air-pump in the least. What is more remarkable, though water will freeze when the thermometer sinks as low as $32^\{\backslash circ\}$, yet if it is suffered to remain absolutely at rest,

it may be cooled to a considerable degree below this: but the moment it is touched or shaken, part of it is converted into ice; and what is still more strange, it now becomes warmer than before, the thermometer rising immediately to $32^\{\backslash circ\}$ degrees.

The action of conge-lation is always instantaneous, and appears very similar to the crystallization of salts. It is greatly promoted by plunging the liquor to be frozen into a mixture of snow and salt, spirit of nitre, or marine acid, and salt, &c.; and no substances have been found to retain their fluidity when exposed to the intense cold produced by these mixtures, except spirit of wine in its strongest state, ether, and the bituminous oil called naphtha. (See COLD.)

Concerning the manner in which the conge-lation of water is produced, there have been various suppositions; some reckoning it to be occasioned merely by the deprivation of a certain quantity of sensible heat; others by the introduction of some extraneous matter, which they distinguished by the name of frigorific particles, and supposed them to be of a saline, acid, or nitrous nature.

4 Accounted for by Mr Muschenbroek. This last hypothesis hath been adopted by Mr Muschenbroek; who is of opinion that these particles insinuate themselves between the parts of the water, and thus stop their motion, by which means the water loses its fluidity. His reasons are as follow.

congelation. vessel; but if it is disturbed in that state, it freezes instantly.

10. A wet cloth sometimes freezes when hung in the air, when there is no ice elsewhere. This does not always succeed; and when it does, Mr Muschenbroek thinks it depends on the presence, or the attraction, of congelation or frigorific particles.

11. If a mixture of salt and snow is put over a fire, and a bowl with some water placed in the middle of it; while the salt and snow are melted into a liquid, the water will be frozen into a mass of ice. This he considers as a plain proof that the fire drives the congelation particles from the mixture into the bowl that contains the water: and he thinks it very absurd that the congelation should depend merely on the diminution of heat; for how can the heat of the fire, by entering the snow, produce any diminution of heat in the water?

All these reasons are answered, and most of them very satisfactorily, by Dr Black. Indeed, if we except the 1^st, the 9^th, and the 11^th, there is none of them from which any conclusion can reasonably be formed concerning the cause of congelation either by frigorific particles, or any thing else. Mr Muschenbroek's last reason is shewn by the doctor to be a mere juggle: for placing the apparatus on the fire is by no means conducive to the success of the experiment, but the contrary. The salt and snow would congeal the water, though kept at a distance from the fire; and the power of this mixture to produce cold is so great, that even the setting it over the fire does not hinder the water from being frozen.

The most puzzling circumstance attending the congelation of water, is its expansion. This, however, is now explained by Dr Black in a satisfactory manner, who proves that it is owing to an extrication of air from the water. His experiment in proof of this is, to freeze water in Florence flasks. The thinness of the glass of which those vessels are formed, gives them a degree of flexibility, by which they sometimes yield so much to the expansive power of water in the act of congelation, that a Florence flask full of water may be completely frozen without being broke. When this is the case, and the vessel is inverted, and set in a warm place, the bubbles, to which the water owed its expansion when congealed, will ascend to the top, where they manifestly discover themselves to be air by forming one large bubble, which after some time is absorbed by the water. Their not affecting the mercurial gage of the air-pump, he thinks is owing to the small quantities of liquid that have been made use of in these experiments.

The doctor's theory, by which he accounts for the congelation of water, as well as the conversion of it into steam, is very ingenious and satisfactory. See EVAPORATION. He is of opinion, that all fluidity depends, not upon that quantity of heat contained in any substance which makes it sensibly warm to the touch, or by which it affects a thermometer, but on a certain quantity taken into the inmost essence or composition of the body, so as to become an essential part of itself.

The easiest method of conceiving these two different states of the element of fire or heat, is by mixing

a quantity of powdered salts with water. For some time the salt will remain undissolved, and render the fluid opaque when shaken. In that state it represents sensible heat, or the element of fire combined with the fluid in such a manner as to affect a thermometer, or to make it sensibly warm. When the salt is dissolved, the fluid then becomes transparent, and the salt invisible; so that it cannot be known by the eye, whether any salt exists there or not. In like manner, when the element of fire enters into the solid ice, so as to render it fluid, it becomes imperceptible to the touch, and only discovers itself by communicating a part of its own subtlety and fluidity to the ice.

That this actually is the case, he proves from its being possible to cool water below the freezing point, provided it is suffered to remain at rest; and from its raising the thermometer to 32°, immediately upon the congelation taking place. Indeed the sudden increase of heat cannot be accounted for on any other supposition whatever. The water having more of its sensible heat carried off than is sufficient to keep the latent heat from breaking out, it instantly leaves some part of the water wherewith it was conjoined, and heats the whole to a certain degree.

From some experiments made by Dr Black, mentioned in a letter from him to Sir John Pringle, and published in the Philosophical Transactions, water is shewn to freeze sooner after it has been boiled than when it has not. The difference, however, he supposes to consist in this, that the unboiled water may be cooled down to a few degrees below the freezing point without losing its fluidity, provided it is suffered to remain perfectly at rest; but the unboiled water begins to freeze as soon as it is cooled down to 32°. If unboiled water, however, is kept continually stirring, he thinks it would begin to freeze full as soon: for though a considerable quantity of ice was formed on the boiled water, while the other remained perfectly fluid; yet, upon stirring the latter, it immediately froze, and the ice was formed on it in as great quantity as the former.

The reason of this difference appears to the doctor to be as follows. The congelation of common water is hastened by disturbing it; and as in boiling water the air is expelled from it, (see AIR), it begins to imbibe the air which it has lost, as soon as it is exposed to it. By this absorption the boiled water is continually exposed to the action of a disturbing cause, viz. the entrance of the air into it, to which the other is not; and, therefore, it is impossible to make boiled water in the least colder than 32°, without some part of it being formed into ice. In this opinion he is confirmed by some experiments of Fahrenheit's; who having put some water purged of its air into little glass globes, and sealed them up, he was surprized to find them continue much longer unfrozen than he expected; but the moment they were opened, or the glass shaken in the least, the congelation took place.

The force with which ice expands in the act of congelation is prodigious. Huygens, in order to try it, filled a cannon, whose sides were an inch thick, with water; and then closed the mouth and touch-hole, so that none could escape. The instrument thus filled

filled was then exposed to a strong freezing air. In less than 12 hours the water was frozen, and expanded with such violence as to burst the piece in two different places. At Peterburgh, in the winter of 1749, an iron bomb was burst by water turned into ice. Various other attempts have been made to calculate the expansive force of ice. A strong cylindrical box of metal was filled with water, well purged of its air by boiling and the air-pump. Being then fitted exactly with a metalline cover, between which and the water a wet leather was interposed, it was strongly pressed down with four iron-screws. The box being then placed in a freezing mixture, the water was frozen into a solid mass in less than half an hour; and as its bulk increased, three of the screws were forced off by the violence of the pressure, and the cover was raised up on one side about a quarter of an inch above the rim. This experiment having proved ineffectual to determine the power of expansion by frost, it was repeated in a different form. The cover of the box, instead of being screwed down, was prefaced upon by the shorter arm of a very strong lever, which was compounded with two others, in such a manner that when a weight of 28 lb. was hung on the extremity of the most remote lever, the short arm of the first pressed upon the cover of the box with a force equal to 2296 lb. The water in the box being then made to freeze, it overcame the whole strength of the machine. The Florentine academicians have been the only persons who succeeded in their endeavours to overcome this expansive power. Being desirous of knowing what degree of strength was requisite to overcome the expansive power of a spherule of ice one inch in diameter, they procured a hollow sphere of brass, the sides of which were an inch thick, which was exactly the diameter of the hollow within. Having filled this with water, and folded up the small hole by which it was introduced, they exposed it to the cold of a freezing mixture, but the globe did not burst. They then gave it to a turner, who pared off something from the thickness. Having again exposed it to the cold, it still continued whole; but, on paring it a second time, and then exposing it to the severe cold of a freezing mixture, it burst, notwithstanding it still continued so thick as to require a force (according to Mr Mitchenbroek's calculation) of 27,720 pounds, near 14 tons, to tear it asunder: and, even here, a doubt may be suggested, whether the great thickness of the sides did not prevent the action of the freezing mixture from congealing the water contained in the globe, till it was pared down sufficiently to let the cold have an effect. It is also to be considered, that cold diminishes the toughness of metallic substances, rendering them much more brittle; and consequently the brass globe could not, when exposed to the cold of a freezing mixture, require so much force to burst it, as it would have done in a moderate warmth.

Freezing is much more expeditiously carried on where the liquor is little disturbed, than if it is greatly so; though an absolute tranquillity, as we have already seen, is prejudicial to this operation. In such a case, if the water is disturbed, though by a warm air, it will be congealed in an instant.

In general, the ice of northern countries is much harder than that of the more southern climates. The reason of this probably is, that though a great quantity of the latent heat is exhaled by such a degree of cold as is necessary to produce ice, yet it is not wholly so; in consequence of which, the ice still shows, by its softness, some disposition to fluidity, which gradually lessens as the cold expels more of the latent heat; and the ice grows still harder by being more and more exposed to severe frost.

During the severe frost of 1740, a palace of ice was built at Peterburgh after the most elegant model, and the justest proportions of Augustan architecture. It was 52 feet long, and 20 feet high. The materials were quarried from the surface of the river Neva; and the whole stood glittering against the sun with a brilliancy almost equal to his own. Six cannons of ice, two bombs and mortars, all of the same materials, were planted before this extraordinary edifice. The cannon were three pounders; they were charged with gun-powder, and fired off. The ball of one of them pierced an oak plank two inches thick, at 60 paces distance, and the piece did not burst with the explosion.

In melting of ice, if it be laid upon some substances it melts faster than upon others. It melts sooner in a silver plate than on the palm of the hand, and sooner upon copper than any other metal whatever. It melts sooner in water than in air of an equal temperature; sooner in water a little warm, than in air near the fire where it is hotter. It is sooner dissolved in vacuo than in the atmosphere. If it takes 20 minutes to dissolve in the open air, only four will be requisite to melt it in vacuo.

Where the summer heats are great, ice becomes an article of luxury; and is indeed exceedingly salutary, by preventing, in some measure, the bad effects of these heats on the human body. A ready way of making ice where none is naturally formed, or of preserving it in summer where it is formed in small quantity in the winter time, is, in some countries, eagerly sought after. The snows with which ice melts furnishes an easy method of preserving it. Water, congealed in six minutes, will not resume its fluidity in some hours: (See FLUIDITY.) Nothing more then is requisite to preserve ice, than to bury it to a small depth in the ground, and cover it with a sufficient quantity of thatch to keep out the external heat. Though water placed in such a situation would always retain its fluidity, yet the great quantity of heat absorbed by ice in thawing prevents the dissolution of any considerable quantity of it, where large masses are put together.

If snow or pounded ice (but snow is preferable) be mixed with any salt, such as copperas, alum, salt-petre, or common salt, in the proportion of four pounds of snow to one of salt, the mixture will soon melt; but at the same time grow so much colder, that if a small basin of water is set into it, it will soon be frozen. In this case the congealing begins from the bottom upwards, which is contrary to what happens when water is frozen in the open air. Of all the different kinds of salts, sal-ammoniac is found to be most efficacious in producing ice; but it is said that water may be frozen without any salt whatever. For this purpose

Congelation. pose we need only to fill a small deep pewter-dish with water; and upon that place a common pewter-plate filled, but not heaped, with snow, so that the bottom of the pewter-plate may be in contact with the water. Bring this small apparatus near the fire, and as the snow melts on being stirred, the water will be frozen on the back of the plate. This experiment deserves well to be considered, as it would strongly indicate an actual expulsion of cold as a substance from snow, upon its melting by the heat of the fire. It is similar to one of Geoffroy's, which shows that cold is produced by throwing burning coals into water. See COLD, n° 5.

Though any kind of salt produces a great degree of cold on being mixed with snow, yet the pure acids of nitre and sea-salt have been found more efficacious in this respect than any known substances. Of these, spirit of nitre produces the most violent degree of cold, and spirit of sea-salt the most lasting.

But by methods of this kind ice can only be made from being in possession of some quantity of it, or of snow, already. In those countries, therefore, where no ice is ever formed naturally, it could never be procured, and in those it is most desirable. Boerhaave gives a method of making ice without either ice or snow. He directs, at whatever season of the year this is attempted, to procure the coldest water that can be got. This is to be mixed with salt in the proportion of three ounces to a quart. Of all saline substances, sal-ammoniac is found to answer best. Another quart of water is to be prepared in the same manner. The solution of the salt will make the water in each much colder than before. They are then to be mixed together. Other two quarts of water prepared and mixed in the same manner are to be added to these. The whole of this operation is to be carried on in a cold cellar; and a glass of common water is then to be placed in the vessel of liquor thus artificially cooled, which will be turned into ice in the space of 12 hours.

12 Boerhaave's method of making ice. A process for making ice in the East Indies, without the assistance of snow or salt of any kind, has been communicated to the Royal Society by Sir Robert Barker, F. R. S. The places where this method is followed lie between 25° and 23° degrees of north latitude, where natural ice is seldom or never seen. The following method is used in freezing the water. On a large open plain three or four excavations are made, each about 30 feet square and two feet deep. The bottoms are firewalled about eight inches or a foot deep with sugar-cane, or the stems of the large Indian corn dried. Upon this bed are placed in rows, near to each other, a number of small shallow earthen pans, for containing the water to be frozen. These are unglazed; scarce a quarter of an inch thick; and so porous, that the water visibly penetrates them. Towards the dusk of the evening they are filled with soft water which has been boiled, and then left in the aforesaid situation. The ice-makers usually attend the pits before the sun is above the horizon. They collect in baskets what is frozen by pouring the whole contents of the pans into them, and thereby retaining the ice, which is daily conveyed to the grand receptacle or place of preservation. This is generally prepared on some high dry situation, by sinking a pit of 14 or 15 feet deep, lined first with straw, and then

with a coarse kind of blanketing, where it is beat down with rammers, till at length its own accumulated cold again freezes and forms it into one solid mass. The mouth of the pit is well secured with straw and blankets from the external air, and a thatched roof is thrown over the whole.

The quantity of ice depends materially on the weather; and consequently it sometimes happens that no congelation takes place; at other times perhaps half the quantity will be frozen, and often the whole contents will be formed into a solid cake. The more clear and serene the weather, and the lighter the atmosphere, the more favourable it is for congelation. A frequent change of winds and clouds are certain preventatives; so that in a very sharp cold night, scarce any ice will often be formed, while the whole contents will be frozen through when the night has been sensibly warmer.

The author of this account is of opinion, that water by being placed in such a situation, free from receiving any heat from other bodies, may be made to freeze in a pretty warm air, by having large surfaces exposed to it. The reason of this seems to be, that some part of its sensible heat is perpetually flying off by its insensible evaporation. The spongy substances on which the pans are set are but ill adapted to furnish it with a new supply; for, of all others, either heat or cold is most difficultly transmitted through bodies of this kind. The evaporation of the liquid which transudes through the pores of the earthen vessels, undoubtedly serves also to increase the cold; and this the more, on account of the extreme thinness with which it is spread over them.

That ice is capable of multiplying itself, seems very probable from an observation of Sir Robert Barker's; namely, that when it is collected by the East Indians, in the manner above described, it becomes capable, by being mixed with salt, of freezing more water, or other kinds of fluids, and that during the severe heats of the summer season. The sherbets, creams, or whatever other fluids are intended to be frozen in this manner, are inclosed in silver cups containing about a pint; having their covers well luted on with paste, and placed in a large vessel filled with ice, salt-petre, and common salt; of the two last an equal quantity, and a little water is added to dissolve the whole. By this composition the contents of the cups are immediately frozen to the consistency of the ice creams in Europe; but plain water will become so hard as to require a mallet and knife to break it. Upon applying a thermometer to one of these lumps of ice, it has been known to sink two or three degrees below the freezing point; which shows, that even in the warmest climates a very great degree of cold may be produced: for the hardness of ice is always in proportion to the degree of cold to which it is exposed; and if it is internally cooled to 2 or 3 degrees below the freezing point, we are sure that a violent degree of cold has been exerted upon it externally; for the usual temperature of ice when first frozen, is 32 degrees.