general name for all fluids of the aerial kind, excepting the common air we breathe. It is derived from the German gascht or gasch, signifying an eruption of wind, or the ebullition attending the expulsion of elastic fluids from substances in a state of fermentation or effervescence. It was originally given by Van Helmont to the vapour of charcoal, the same with the fluid now called fixed air, and some other factitious airs; and from him has been employed by modern philosophers as a general term for all the fluids about which aerology is concerned.
Under the article Aerology, the nature and properties of these fluids are explained at large; here, however, for the more easy comprehension of the subject, we shall give a list of these fluids, with a general account of the most remarkable particulars hitherto discovered concerning them. The gases, or permanently elastic fluids, as yet known, are,
1. Common or atmospherical air. 2. Fixed air. 3. Inflammable air. 4. Nitrous air. 5. Dephlogisticated air. 6. Vitriolic-acid air. 7. Marine-acid air. 8. Nitrous-acid air. 9. Fluor-acid air. 10. Vegetable-acid air. 11. Alkaline air. 12. Dephlogisticated nitrous air. 13. Sulphurated inflammable air. 14. Hepatic air. 15. Phlogisticated air.
The most remarkable properties of these are as follows.
1. Atmospheric air supports both animal and vegetable life; and surrounding the whole globe to a considerable height, is one of the great agents employed by nature for executing her most important purposes. It is composed of one part of dephlogisticated air, and three or more of phlogisticated air.
2. Fixed air is produced in great plenty in all kinds of combustion. If the combustible body be set on fire in pure dephlogisticated air, the fixed air is proportionably pure; but if in the common atmosphere the produce is contaminated by the whole quantity of phlogisticated air contained in that portion of the atmosphere by which the combustion was supported, it proves fatal to animals in a very short time, but is favourable to vegetation. It is absorbed in considerable quantities by water, to which it communicates an agreeable acidulous taste, and a power of dissolving iron; and is the principal ingredient in mineral waters. It is taken up in great quantity by pure alkaline salts whether fixed or volatile; by calcined magnesia, lime, or calcareous earth; all of which it naturalizes, and forms with them salts of different kinds. Lime absorbs it more readily either when quite dry or entirely dissolved in water than when exposed to it in a moist mass, and lime-water is readily precipitated by it; but magnesia attracts it more readily when in a moist mass; and the fixed alkalies equally well in either case, unless when violently
air is contained in great quantity in fermented liquors, to which it gives their agreeable taste and briskness; and by impregnating them with it, they may be recovered from a vapid state, and rendered brisk and agreeable as before. It has a considerable antiseptic power; and notwithstanding its pernicious qualities when taken into the lungs, has been found serviceable in putrid diseases when swallowed, or when injected by way of glycerine. Being a natural product of combustion, it is met with in great quantities in the neighbourhood of volcanoes, or mountains which have formerly been volcanoes, where it often produces mischievous effects. It is also met with in mines, where it often proves fatal to the workmen. In the artificial way it is procured from fermenting liquors; from the calcination of magnesia and calcareous earths by heat; and from a mixture of these earths with acids, chiefly the vitriolic. When procured from large quantities of fermenting liquors, it lies in a large body on the surface of the liquor, generally nine inches or a foot thick, and affords an amusing appearance on extinguishing lighted candles or chips of wood in it. In these experiments the smoke readily unites with the gas, so that little or none of it can disperse itself into the atmosphere; and it is remarkable, that the upper surface of this smoke which floats in the fixed air is smooth and well defined, but the under part is ragged, and sometimes even collecting itself into balls connected with the upper surface by slender threads. Sometimes the smoke will form itself into broad flakes parallel to the surface of the liquor, and at different distances from it, exactly like clouds; and these appearances will continue for upwards of an hour with very little variation.
Dr Priestley tried the smoke of gun-powder, rosin, sulphur, and other electric substances; and found them all retained equally by the fixed air, as well as the smoke of vitriolic acid raised by putting a burning coal in it.
Fixed air does not very readily unite with common air. It is near twice as heavy as atmospheric air, and acquires a proportionably greater elasticity by heat. It is composed of dephlogisticated air and phlogiston; which two ingredients are partly separated by the electric spark. It may be kept for any length of time in vessels inverted into quicksilver, or even into water, with a coat of oil about half an inch thick on its surface.
3. Inflammable air is composed of phlogiston and water rashed by heat. It is the lightest fluid known in nature, being 10 or 12 times specifically lighter than the common atmosphere. It is pernicious to animals, but supports vegetable life. By itself it extinguishes flame; but when mixed with a certain quantity of common air, the whole explodes violently, and a small portion of nitrous acid is produced. If dephlogisticated air be used instead of common air, the explosion is much more violent; Dr Priestley says 50 times. It is very readily absorbed by the calces of some metals, particularly lead, to which it restores its metallic form; and is even decomposed by keeping it long in a tube made of white glass, to which it communicates a black colour by its attraction for the lead in its composition. It is produced naturally in coal-pits and other mines; where, being mixed with common air, and frequently inflamed by the lights which the miners have along with them, it explodes with prodigious violence, and often produces much mischief. Sometimes it rises out of marshes, or from the mud at the bottom of springs and rivers; in which case the water will seem to take fire on holding a lighted candle to its surface. It is produced from the vapours of rancid oil; whence it has sometimes been collected in the large bellows used in foundries, and burnt them with explosion. It seems also to be a natural product of putrefaction of every kind, being sometimes met with in jakes and privies, where it has exploded as usual on the approach of a candle. When mixed with common air it may be fired by an electric spark, but less readily by one from flint and steel; though there are instances of its taking fire in this manner also. In the artificial way, it is most usually procured from iron-filings and diluted vitriolic acid; and lately from the fleam of water conveyed over iron-wires through a red-hot tube. It is likewise obtained by distillation of wood, coal, &c., and by dissolving charcoal with the heat of a burning glass in vacuo. Dr Pennington of Philadelphia informs us, that it refutes putrefaction; but its virtues in this way have not been particularly examined.
4. Nitrous air is produced from inflammable substances combined with the nitrous acid; and, according to that class of philosophers styled Phlogistians, consists of nitrous acid supersaturated with phlogiston; according to the Antiphlogistians, it consists of the same acid, deprived of a part of its oxygenous principle, the same with what the other party call the basis of dephlogisticated air. This is the most noxious of all the kinds of air hitherto discovered; being not only instantly fatal to animal life, but to vegetables also, as well as extinguishing flame in the most perfect manner. It has a strong offensive smell; and when mixed with a quantity of dephlogisticated or common atmospheric air, a very considerable diminution of the latter takes place, attended with heat, red fumes, and the production of nitrous acid. The diminution is greatest of all when pure dephlogisticated air is made use of; and with atmospheric air is, greater or less, according to its degree of purity or the quantity of dephlogisticated air it contains. This kind: kind of air is a most powerful antiseptic; and has been tried, but without success, to preserve meat fresh for a long time; though for a week or two it might perhaps be useful. It is heavier than common air.
5. Dephlogisticated air supports animal life and flame in the most perfect manner, but is less friendly to vegetables; though it seems not to possess any property absolutely detrimental to them, further than as it contains none, or only a small quantity, of that phlogistic matter, which is now found to be the proper food of plants. The heat produced by it in burning bodies seems to be very little if at all inferior to that of a large burning mirror. It unites with water but in small quantity; but attaches itself strongly to iron when fired in it, causing the metal to burn with a bright flame, and being diminished by this combustion to a surprising degree. With other inflammable matters it produces fixed air. It is naturally found in sea-water, and rises in some waters through the earth. It is produced in the daytime by the leaves of many plants; but not in the night. It is also produced from water exposed to the sun's light, especially if certain substances be put into it, of which a particular account is given under the article AEREOLOGY. It is also produced by the distillation of nitre, manganese, and other substances.
6. Vitriolic acid air is not essentially different from the fumes of burning sulphur, only that the latter are always mixed with common air. It is very heavy, and destructive both to animal and vegetable life; extinguishing flame also; and uniting in large quantity with water, from which, however, it is easily expelled by heat. By virtue of its properties as an acid, it readily unites with alkaline salts. It dissolves also camphor, reducing it to a transparent liquor; from which, however, the camphor separates on the addition of water. It is produced by distilling oil of vitriol mixed with inflammable substances with a gentle heat.
7. Marine acid air is no other than the vapour of marine acid, which may be procured either by distilling the marine acid with a very gentle heat, or the mixture of vitriolic acid, and common salt usually made use of for procuring that acid. It is absorbed largely by water; so that a very strong and smoking acid liquor can thus be obtained. It is pernicious to animal and vegetable life, but less so than the two former. It likewise extinguishes flame: but a candle will burn in common air, though mixed with a large proportion of it; in which case the flame appears of a most beautiful blue or green colour.
8. Nitrous acid air has been but little examined on account of its extreme corrosive property, by which it destroys all kinds of liquids wherewith we attempt to confine it. By the addition of a certain quantity of phlogiston it is converted into phlogisticated air, as Dr Priestley found on attempting to confine it by means of whale oil. It is absorbed by water, and forms nitrous acid; being the vapour of that, as the marine acid air is the vapour of the concentrated marine acid.
9. The fluor-acid air is not distinct from the vapour of that acid loaded with siliceous earth, which it plentifully dissolves. A full account of its properties is given under the article CHEMISTRY.
10. Vegetable acid air was procured from an exceedingly concentrated acetic acid, but with more difficulty than the others on account of the volatility of the liquid. It extinguishes flame, unites with alkalies, and performs every other thing that could be expected from the acetic acid, manifesting its inferior acid power even in its aerial state. It is very readily imbibed by water and by charcoal. It is likewise absorbed pretty readily by olive oil, on which it has a remarkable effect. The oil takes up about ten times its bulk of the air; and, from being of a yellowish colour, turns almost as clear as water, losing also somewhat of its viscidity, and approaching to the consistence of an essential oil.
A singular appearance was observed by Dr Priestley on attempting to determine how much of this air a certain quantity of water would imbibe. Having put the liquid into a glass tube closed at one end, and introduced it to the vegetable acid air, a small bubble of common air which appeared at the bottom began to expand, and continued to do so till all the water was thrown out of the tube. The same effect took place when the tube was hermetically sealed at the end. With spirit of wine it was the same, and with oil of turpentine the effect took place more quickly; but with olive oil it was more slow.
11. Alkaline air is lighter than that of the common atmosphere, and much more expansible by heat. It is now found to be composed of inflammable and phlogisticated air. It has all the properties of caustic volatile spirits, uniting with acids and forming neutral fats. It is obtained from a mixture of sal ammoniac and lime by distillation with a very gentle heat; or it may be had in great quantity by distilling spirit of wine with volatile alkaline spirit. It is inflammable when mixed with common air, but burns without any explosion.
12. Dephlogisticated nitrous air is produced from the nitrous kind exposed to liver of sulphur, or for a longer time to iron. It may also be produced directly in the operation for making nitrous air from the acid and iron. After a great quantity of nitrous air has been extricated from this mixture without heat, the dephlogisticated nitrous air will come over by the application of heat. It is remarkable for being able to sustain flame, without supporting animal life. In this kind of air a candle burns sometimes naturally, and sometimes with an enlarged flame of a blue or green colour. It is less proper as a test for the purity of the atmosphere than common nitrous air.
13. Sulphurated inflammable air is a late discovery of Dr Priestley's, and seems to be composed of inflammable and hepatic air mixed together.
14. Hepatic air is the steam which arises from liver of sulphur on the addition of water, but still more copiously on the addition of an acid. It is fatal to animal life, and burns without explosion. Its effect on vegetables is not well known; it is remarkable for the property of giving a black colour to some metallic calces, whence it has the property of rendering sympathetic ink visible.
15. Phlogisticated air is one of the component parts of the atmosphere; and is produced in great plenty by all the processes of putrefaction, calcination of metals, and many cases of combustion. It is very destructive to animal life, and likewise extinguishes flame; but it is exceedingly proper for the support of vegetables, which thrive much better in it than in the common air. There are disputes concerning its composition; the Antiphlogitians supposing it to be a primary element, and their antagonists that it is composed in great part of phlogiston; though they have not been able to prove this part of their doctrine either by reviving the calces of metals, or purifying it in such a manner as to make it respirable. It is somewhat lighter than atmospheric air. Mixed with dephlogisticated and inflammable air, it produces nitrous acid by detonation. Inflammable, nitrous, and alkaline air, may be partly converted into it.
Having thus briefly recapitulated the properties of the different gases hitherto discovered, we shall next proceed to consider the apparatus necessary for making the various experiments with these gases, which have been for some time in so much repute among modern philosophers. These experiments may be reduced to the following classes. 1. The production and preservation of the gases themselves. 2. The impregnating water or other fluids with them. 3. Trying their effects upon animals and vegetables. 4. The effects produced on them by electricity. 5. Their capacities for conducting heat.
Where one can have access to large quantities of fermenting liquor, fixed air may be easily procured of tolerable purity, by filling a vial or tumbler with water, and then emptying it below the surface of the mephitic atmosphere which floats above the surface of the liquor, the fixed air occupying the place of the water as it is discharged from the vessel. It may then be preserved by stopping the mouth of the vial with a cork; or, if it is a wide-mouthed vessel, by inverting it in quicksilver, or in water covered with oil. In most cases, however, the different kinds of air may be for a short time preserved in vessels inverted in water alone without any oil. For experiments, therefore, on those kinds of airs which may be preserved in water, Dr Priestley made use of an oblong trough of wood, such as is represented, Plate CCVII. fig. 1. The dimensions were generally two feet in length, 18 inches in breadth, and 11 inches in depth. About an inch below the top is a wooden shelf all round, for the purpose of setting the inverted vials or jars of air upon it. The vessels he commonly made use of were such jars as he had been accustomed to use in his electrical batteries, about 10 inches long and 2½ wide; though for different experiments he had them of various shapes and sizes. When he had occasion to remove vessels of air from the large trough, he put them into pots or dishes of the form represented fig. 2; these dishes being first put under water, and the jars then slid off the shelf upon them. For the mere removal of jars of air from one place to another, where they are to stand only for a few days, he makes use of common tea-dishes, which hold water enough, unless the air be in a state of diminution by any process going on in the vessel. When anything, as a gallipot, is to be supported in a jar full of air, wire-stands, such as are represented fig. 3, may be conveniently made use of. They answer better than any others, on account of their taking up less room, and being easily bent into any shape. When there is occasion to pour a quantity of air from one vessel to another, a funnel, fig. 4, must be made use of. Thus the operation is rendered exceedingly easy, by first filling the vessel in which the air is to be conveyed with water, and holding the mouth of it together with the funnel both under water with one hand, while the other is employed in pouring the air; which, ascending through the funnel up into the vessel, makes the water descend, and takes its place. It will be convenient to have several of these funnels of different sizes. They are best made of glass. An improvement on this part of the apparatus was made by the Duke de Chaulnes, and consists in having the under part of the shelf hollowed out into the shape of a funnel, with a hole over the middle, on which the vial is to be placed, and the air ascends without any trouble. When there is occasion to transfer air from a jar standing in the trough of water to a vessel standing in quicksilver, or any other situation whatever, the apparatus represented fig. 5, may be made use of. It consists of a bladder furnished at one end with a small glass-tube bent, and at the other with a cork perforated in such a manner as just to admit the small end of the funnel. When the common air is carefully pressed out of this bladder, and the funnel thrust tightly into the cork, it may be filled with any kind of air as easily as a glass jar. A string being then tied above the cork in which the funnel is inserted, and the orifice in the other cork closed by pressing the bladder against it, it may be carried anywhere; and if the tube be carefully wiped, the air may be conveyed quite free from moisture through a body of quicksilver or any thing else. When it is wanted to try whether a candle will burn in any kind of air, a cylindrical glass vessel, fig. 6, may be used, with a bit of a wax candle fattened to the end of a wire b, and turned up in such a manner as to be let down into the vessel with the flame upwards. The vessel should be kept carefully covered till the moment the candle is admitted. In this manner, the Doctor tells us, he has frequently extinguished a candle more than 20 times successively; though it is impossible to dip the candle into it without giving the external air an opportunity of mixing more or less with that in the inside. The candle c at the other end is very convenient for holding under a jar standing in water, in order to burn as long as the inclosed air can supply it; for the moment that it is extinguished, it may be drawn through the water before any smoke can have mixed with the air. To draw the air out of a vessel which has its mouth immersed in water, and thereby to raise the liquid to any height that may be required, a glass syphon is very convenient, such as is represented, fig. 8. Putting one of the legs up into the vessel, and drawing the air out at the other end by the mouth, or rather, as most of the gases have a noxious quality, by a syringe properly fastened to it. Dr Hales sometimes made use of a valve at the top of the vessel; but Dr Priestley does not think this can be altogether depended upon. If, however, a very small hole be made at the top of a glass vessel, it may be filled to any height by holding it under water while the air is issuing out at the hole, which is then to be closed with wax or cement. When the gas employed in the experiment is of such a nature that it will neither be absorbed by water, nor diminish common air, it may be convenient to put part of the materials which generate the gas into a cup, as at f, fig. 1. Then having, by means of a syphon, drawn drawn the air to a convenient height, the small glass vessel may be easily pushed into the cup by a wire introduced through the water. The contents of the small vessel may be discharged into the larger by a variety of contrivances; and the distance between the boundary of air and water, before and after the operation, will show the quantity of the generated air. The effect of such lubricants as diminish air may also be tried by this apparatus. When air is to be admitted to any thing that will not bear wetting, and yet cannot be conveniently put into a vial, and especially if it be in the form of powder, and must be placed upon a stand (as in those experiments in which the focus of a burning mirror is to be thrown upon it), the receiver in which it is to be placed must first be exhausted; then having a glass tube bent for the purpose, as in fig. 9, it is screwed to the transfer of an air-pump on which the receiver had been exhausted; and introducing it through the water into a jar of air of that kind with which the receiver is to be filled, the purpose is gained by only turning the cock: but in this way, unless a great deal of care be taken, some common air is apt to get into the receiver. If it is wished to try the goodness of any particular kind of air, two measures of it must be put into a jar standing in water; and having marked on the glass the exact place of the boundary of the air and water, a measure of nitrous air is put to it; and after waiting a proper time, the quantity of its diminution is to be noted. If two kinds of air nearly alike are to be compared together after mixing them in a large jar, the mixture is transferred into a long glass tube, by which the scale can be lengthened as much as we please. When the quantity of air, the goodness of which we wish to ascertain, happens to be so small that it is contained in a portion of a glass tube from which water will not run out, as fig. 10, the length of the column of air in the tube is first to be measured with a pair of compasses, the remaining part being filled with water. The length of the column is then to be laid down upon a scale; and then thrusting into the tube a wire of a proper thickness, it is drawn out again by means of a thin plate of iron bent to a sharp angle, when the whole of this little apparatus has been introduced through the water into a jar of nitrous air, and the wire being drawn out, the air from the jar must supply its place. The length of this column of nitrous air is then to be measured, and laid down upon the scale, so as to know the exact length of both the columns. After this, holding the tube under water, the two columns of air are to be forced into contact by means of a small wire; and when they have been a sufficient time together, the length of the whole is measured and compared with the length of both columns taken together.
In many experiments, the matters from which air is to be expelled must be subject to a very considerable degree of heat. In these cases Dr Priestley frequently made use of a gun-barrel, fig. 11. Into this he put the substance from which the air was to be extracted; then filling it up with dry sand so that very little air could be lodged in the barrel, and having also previously burned the sand, so that no air could come from it, he fitted to the open end the stem of a tobacco pipe or small glass tube. Then having put the closed end of the tube which contains the materials into the fire, the generated air, issuing through the tube, may be received into a vessel of quicksilver, with its mouth inverted into a basin of the same, suspended all together by wires as represented in the figure. Any other fluid substance may be used instead of quicksilver.
The best method, however, of procuring air from several substances by means of heat, is to put them into vials full of quicksilver, with their mouths immersed in the same, and then throwing the focus of a burning mirror upon them. For this purpose the vials should have round bottoms very thin, that they may not be liable to break on any sudden application of heat. If it is wanted to expel air from any liquid, a vial is to be nearly filled with it; then having a cork perforated, a bent glass tube is put through it and secured with cement, represented at c, fig. 1. The vial is then put into a kettle of water, which is set upon the fire and made to boil. The air expelled by the heat issues through the tube, and is received in the basin of quicksilver, fig. 11. But instead of this suspended basin, the simple apparatus of a flaccid bladder, tied to the end of the tube in order to receive the generated air, may sometimes be advantageously made use of. To produce air by the solution of metals, or any similar process, the materials are to be put into a vial prepared in the manner represented at c, fig. 1, and the end of the glass tube put under the mouth of any vessel into which it is wanted to convey the air. Heat may be easily applied while it hangs in this position, by means of a candle or red-hot poker.
In making experiments on such kinds of air as are readily imbibed by water, quicksilver must always be made use of, as represented fig. 12, where a is the basin of quicksilver, b a glass vessel containing quicksilver with its mouth immersed in it, c a vial containing the ingredients from which the air is to be produced, and d a small recipient or glass vessel to intercept any liquor that may come over along with the air, which must be transmitted as free from moisture as possible into the vessel b. If there be no danger of moisture, however, the glass tube only is made use of in the manner represented at a, fig. 1. To invert the vessel b, fig. 12, it must first be filled with quicksilver, and its mouth then carefully covered with a piece of soft leather; after which it may be turned upside down without any danger of admitting the air; and the leather may be withdrawn when it is plunged into the quicksilver.
The figures aaa, fig. 13, represent a kind of vials much used by Dr Priestley in all his experiments. They are made round and very thin at the bottom, and the mouth is to be ground smooth, so that they may be either used with a cork, or will stand firm when inverted after being filled with quicksilver or any other fluid. When used as common vials with corks, they will bear the application of a sudden heat without breaking, much better than the common vials which are thicker at the bottom. These vessels are particularly useful in extracting air from any substance confined by quicksilver; for, standing with their mouths downwards, and the substances with which the experiment is made lying upon the surface of the metal, just under the thinnest part of the glass, they are easily subjected to the action of a burning glass without any danger of breaking the vial which contains them. Care must be taken, taken, however, not to put them at once into the very focus, lest the glass should give way. In most cases this moderate heat will produce a considerable quantity of air; by which means there will be some space left between the glass and the substance from which the air is to be extracted, so that the greatest heat of the glass may easily be directed upon the substance itself, independent of the glass which contains it; whence the latter is in no danger of being broken or melted. "A skilful operator (says Dr Priestley) will be able to fill his vessel with the newly generated air by this means: but in general, he will do well to content himself with getting it half-full, or less; for as the glass is necessarily thicker towards the mouth, there will be some danger of breaking it when the rays are transmitted near that place, and of losing the air that has been perhaps with great trouble and difficulty procured. If the substance on which the experiment is made be in the form of a powder, such as red-lead, and even many very light substances, it will be most convenient to put them into the vessel first; and the quicksilver may, with care, be poured upon them afterwards, so as to keep the substance at the bottom; and yet when the vessel is inverted it will remain upon the mouth. When the light matter will not be close, it will not be difficult sometimes to intercept it in the first part of the vessel at the neck; but it will often be most convenient to form these light matters into small balls, and put them into the vessel through the quicksilver with which it has been previously filled. I would observe with respect to this process, and every other in which vessels are to be filled with quicksilver, and then to be placed inverted in basins of the same, that no operation is easier (unless the mouth of the vessel be exceedingly wide) when the mouth of it is covered with soft leather, and, if necessary, tied on with a string, before it be turned upside down; and the leather may be drawn from under it when it is plunged in the quicksilver. If the mouths of the vessels be very narrow, it will be sufficient, and most convenient, to cover them with one's finger. In this process there remains less doubt of the generated air coming from the materials on which the experiment is made, than when the focus of the lens is thrown upon them in vacuo; because there will often be room to suspect, that common air may get into the receiver in the course of a long process, at some place not sufficiently guarded; and besides it is a great satisfaction to see the quantity of air that is generated at any particular time, during the course of a process, that the operator may stop when he sees he has got a quantity sufficient for his purpose, whereas unless he has a gage connected with his transferer (which may be inconvenient), he must admit water into his receiver before he can certainly tell whether he has obtained any air or not; and then it will be liable to be affected by the water, or by the air contained in the water, and which will be set loose very copiously on its first admission into the receiver. But in cases where the air is apt to corrode the quicksilver, which always happens when the nitrous acid is concerned, recourse must then be had to the vacuum: and for this purpose it is necessary to have the receivers made as thin as possible, the thick ones being very apt to break by the heat of the lens. Care must be taken in these experiments to place the materials on a crucible, a piece of glass, or some substance of that kind which yields no air."
A vial, with a ground-stopper, having the latter perforated with a number of small holes, will be found of excellent use to convey any liquid, or even any kind of air, contained in it, through the water into a jar standing with its mouth inverted in the liquid, without admitting any mixture of common air or even of the water, and yet the generated air will have a sufficient outlet.
Fig. 14, c, represents a kind of vial of the same form with those shown at a, fig. 13, but fitted with a ground stopper terminating in a tube, and which is occasionally to be used instead of that marked e, fig. 14. In most cases this is preferable to the corks and tubes the Doctor formerly employed; but in some the latter are still to be preferred, particularly where the fluor acid is to be used, which would soon corrode any of those thin vials. For experiments with this acid, therefore, the Doctor recommends the use of common vials made very thick, especially as no great degree of heat or sudden application of it is ever wanted. The vial c will be found sufficient for any purpose that does not require more heat than can be given by the flame of a candle held close to the bottom of it; but if there be occasion to place the vial in a sand-heat, and consequently if it must be put into a crucible placed on the fire, it will be necessary to have the tube in which the ground-stopper terminates made as long as possible, that the vessel which receives the air may not be too near the fire. Nine or twelve inches will be a sufficient length for the purpose. In such experiments, however, as are not worth the expense of these vials and stoppers, and yet where gun-barrels cannot be safely trusted, the Doctor has recourse to a vial made narrower at the open end than the other, of about 9 or 12 inches in length, and of an equal thickness throughout. When these vials are put into a crucible with sand, the bottom may be made red-hot, while the top is so cold that a common cork into which a glass-tube is inserted will not be affected by the heat. When the materials are put into this vessel, it must be filled up to the mouth with fine sand that will give no air by the application of heat; and the cork must be thrust close down upon the sand. The air is to be received in the same manner as directed for the gun-barrel.
For the purpose of making a quantity of air pass through a body of water or any other fluid, it is convenient to have a number of vials of the form represented fig. 15, the tube which conveys the air into the vial going nearly to the under part, and that which delivers it perforating only the upper part. Thus the air is forced to go through the whole body of the water or powder with which the vial may be filled. These vessels may be of various sizes, from a pint down to half an ounce; the larger end may generally be stopped with a cork, though in some cases it will be necessary to have this stopper also of glass, with only two perforations for inserting the tubes. Sometimes he had occasion to use a number of these vessels placed together, as represented fig. 16, that the same air might pass through them all; and sometimes it was found improper to use any kind of lute or cement, so that all the stoppers, large as well as small, were fitted by grinding; which made the apparatus very expensive. The long vial annexed to this apparatus was chiefly made use of where the nitrous acid was concerned. It was made deep in order to admit of a sudden and violent effervescence without any danger of the liquid being thrown over; and the tube proceeding from it ought to be long enough to go to the bottom of any vessel in which the vapour is to be delivered.
Fig. 17 represents a simple apparatus, being no other than a frame of wood so disposed about a vessel containing quicksilver, that the latter may admit of several glass tubes which support themselves against its sides, and thus may be employed in experiments all at the same time.
Fig. 18 shows a cylindrical vessel of tin perforated with holes, and enclosing another of iron wire. A charcoal fire may be made in the outer one, while a vial containing any quantity of air which it wished to heat may be put within the frame of iron wire. Thus an equable heat will be produced on every part of the glass, without heating the bottom more than the rest; which in many cases is greatly to be wished for.
Fig. 19 shows the apparatus by which were made the principal experiments relating to the apparent conversion of water into air, on which Dr Priestley bestowed considerable attention, though he found it at last to be a mistake*. It consists of an earthen vessel; the bulb of which, containing moistened clay, is fixed in the inside of a glass vessel, through which the heat of a burning lens may be thrown upon it; while the inside has a communication with a bason of water or of quicksilver, in which vessels may be placed to receive the air that is forced through the body of the earthen vessel, while the water or mercury in the bason in which the glass stands rises to supply its place.
Fig. 20 shows the apparatus for transmitting steam through a red-hot tube containing iron or other substances. The contrivance is evident from an inspection of the figure; only the vessel which receives the air must be much larger in proportion than is here represented. Instead of the small furnace, one of Argand's lamps may be advantageously used. Fig. 21 shows a method of receiving the air under a funnel, when large balloons are to be filled for the purposes of aerolation.
These are the principal parts of the apparatus used by Dr Priestley in his numerous experiments for the production of airs, of all different kinds, from a vast variety of substances, and for preserving, transferring, or mingling them with one another as occasion required. On this part of his apparatus no improvement of any consequence has ever been made. It has been otherwise, however, with the method he proposed for impregnating water with the various gases, especially fixed air, which for some time engrossed a considerable share of the public attention. In this operation a principal requisite is to expose a large surface of the water as possible to the action of the air; for it is only in proportion to the expanded surface, and not to the depth of the liquid, that the air is taken up. It is also requisite to save the air as much as possible, by stopping every outlet, and at the same time to prevent the vessels from bursting, which they might otherwise be apt to do. The first apparatus invented by Dr Priestley, and which seems to have also been the first ever made use of by any person, is represented fig. 22. It consists of a glass vessel a, with a pretty narrow neck, but so formed that it will stand with its mouth downwards; and having filled it with water, lay a slip of clean paper or thin pasteboard upon it; then if they be pressed close together, the vessel may be turned upside down without danger of admitting common air into it; and when it is thus inverted, it must be placed in another vessel in the form of a bowl or bason b, with a little water in it, so much as to permit the slip of paper or pasteboard to be withdrawn, and the end of the pipe c introduced. The pipe used by the Doctor was at first made of leather, that by means of its flexibility he might be able to shake the vessel d containing the effervescing mixture; but afterwards he found it more convenient to use one of glass, making use of two bladder instead of the one represented in the figure at d. These two are joined together by a perforated cork, and give room enough for shaking the vessel, which one would scarcely admit of. Having put the oil of vitriol to the calcareous earth in the bottle e, the fixed air will very soon distend the bladder or bladders d, which must then be pressed out into the vessel a, but will not suddenly be absorbed by it. A quantity proportionable to the bulk of the air will therefore descend into the bason; and after the bottle a has thus been about half emptied, it will be necessary to shake it briskly; when the agitation will cause the air to be imbibed, and the water will reascend into the bottle. This is to be repeated till the water will not take up any more; after which it ought to be put into a bottle well corked and cemented; the air being very apt to escape after being once taken up.
Though this apparatus must evidently answer the purpose of impregnating water with fixed air very effectually, yet it is troublesome on account of the attendance required; and objections were also made to the use of bladders in it, as being apt to communicate a disagreeable taste to the liquor. The latter objection was particularly insisted upon by Dr Nooth; who from his own experience declared, that the use of them communicated some degree of urinous flavour to the impregnated water. Dr Priestley made light of this objection, but allowed the validity of that from the attendance necessary during the impregnation. Though he reckoned Dr Nooth's apparatus therefore inferior to his own with regard to its power, and the slower in its operation as well as more expensive, he constantly used it himself in his after experiments; and it has now become almost universally employed for the purpose of impregnating small quantities of water with this kind of gas.
Dr Nooth's apparatus, with some improvements in it by Mr Parker, is represented fig. 23, and is all made of crystal glass. The lowermost vessel contains the chalk and diluted oil of vitriol; the middle one the water to be impregnated; and the upper one is designed to give vent to such part of the air as cannot be imbibed. The air is admitted to the water through a number of holes, so small that the water cannot get through them on account of the resistance made by the ascending gas. The uppermost vessel is filled to a certain height with water, which is prevented from descending into the middle vessel by the resistance of the air. air in the empty part. As the gas ascends, that part of it which is not condensed compresses the water, and forces it up into the upper vessel, leaving thereby a greater space for the air to expand in; at the same time that a considerable pressure is made by the weight of the incumbent water, which very much promotes the absorption of this or any other gas. The effervescing materials may be renewed, and the water drawn off, by the cocks represented in the figure. Fig. 24 shows a farther improvement upon this apparatus by Mr Blades of Ludgate-hill. The only difference is in the shape of the vessels, and having a glass cock for letting off the impregnated water instead of a tube closed with a cork.
Another apparatus, capable of answering the purpose at least as well as that of Dr Nooth, and much less expensive, was invented by Dr Withering, and is represented fig. 25. A, is a glass-vessel about 10 inches high in the cylindrical part, and 6½ inches diameter. B, a glass-vessel about 12 inches high in the conical part, 1½ in the neck, and 5 inches diameter at the bottom. C, a copper pipe passing through the stopper of the vessel B, and tied fast into the flexible tube D. This tube is made of strong leather, and kept hollow by means of a spiral wire running through its whole length. E, a conical brass pipe, with a stopcock fastened to the tube D. F, a conical pipe, with a stopcock G; having the end of the tube E accurately ground to it so as to be air tight. G, the stopcock cutting off all communication with the atmosphere when the pipe E is removed. H, two large hog's bladders, each of which ought to contain two quarts. I, a stopcock to prevent the water from rising into the bladders when the vessel A is agitated. K, a bladder tied to the crooked tube with the stopcock L, which occasionally opens or shuts the communication with the vessel B. M, a glass funnel accurately fitted with the glass-stopper N. O, the aperture fitted with a glass-stopper, from which the impregnated water is to be drawn for use; or, instead of the glass-stopper, a silver-pan may be more conveniently applied. P, the tube opening into the vessel A.
To make use of this apparatus, we must, 1. Fill the vessel A with pure water, adding such other ingredients as are necessary along with the gas. The vessel is calculated to hold five quarts conveniently for impregnation. 2. Put into the vessel B as much marble or whiting in small lumps as will cover its bottom to the height of two inches, then pour in water to the height represented by the dotted line. 3. Let all the bladders be tied round their respective tubes, so that they may be perfectly air-tight. 4. Fit the mouth of the vessel A tight with a cork, and through a hole in this pass the tube F, putting on the cork some sealing-wax of the softest kind, or else modelling wax, so that the whole may be made air-tight. 5. Stop the mouth of the vessel B with a piece of mahogany prepared in the following manner. Let the wood be first turned in a lathe of a conical figure, but a little larger than the mouth of the glass will admit. Put this piece of wood into melted bees-wax, and heat the wax until the wood begins to grow black. When cool, turn it again in a lathe until it fits the mouth of the vessel. The tubes C, L, and M, are fitted into holes bored through the wooden stopper previous to its being immersed in the melted bees-wax. 6. Push these tubes through their respective holes in the wooden stopper; press their stoppers into the orifice of the vessel B, and cement the whole with sealing or modelling wax. 7. Shut the stop-cocks I and L, having previously pressed the air out of the bladder K; open the stop-cocks G and E; then squeeze the air out of the bladder H, and afterwards press the conical pipe E into the pipe F. 8. Pour about a large spoonful of oil of vitriol through the funnel M, and stop it with its stopper N; on this the fixed air rising through the tube C passes into the bladders H, and dilutes them. 9. When the bladders are distended, open the stop-cock I, and draw off about a quart of water from the aperture at O. The empty space left by the water will quickly be filled with gas, which the remaining water will begin to absorb, and the absorption will still be supplied by fresh gas from the bladders; for which reason these must be kept pretty fully distended, by adding more oil of vitriol when they begin to grow flaccid. 10. If it be required to impregnate the water quickly, turn the stop-cocks at G and E, and open that at L; then separate the pipe E from the tube F, and agitate the vessel A. During this time the fixable air that is produced passes into the bladder K, from whence it may be afterwards pressed into the other bladders when the parts of the apparatus are again united. 11. During the agitation close the stop-cock at I, opening it only occasionally to replace from the bladders H the air absorbed by the water. 12. If a strong impregnation be desired, the temperature of the room where the operation is carried on ought not to be more than 45° Fahrenheit. 13. That the cocks may continue perfectly air-tight, it will be necessary to supply them once a year with a very small quantity of unaltered lard. Modelling wax, of which mention is made in this description, may be made by adding two ounces of tallow and one of turpentine to half a pound of bees-wax. It may be coloured with red-lead or Spanish-brown; and the mixture must be kept stirring till cold.
These are the principal discoveries which have yet been made concerning the method of impregnating water with fixed air, and they may undoubtedly be applied to the impregnation of that fluid with any other kind of gas which it will take up; only it must be observed, that where any of the acids are concerned, that of flour alone excepted, there is an absolute necessity for having all parts of the apparatus made of glass.
For making experiments on vegetables, the large cylindrical glass, fig. 2, is very proper. When it was wished to try how long a small animal would live in a certain quantity of air, Dr Priestley found a large beer-glass, such as is represented at d, fig. 1, very convenient. The animals he most commonly made experiments upon were mice; and in a beer-glass containing between two and three ounces, he found that one of these creatures would live 20 minutes or half an hour. To obtain mice for such experiments, he directs that they should be caught in wire-traps, from whence they may be easily taken. To get them into the air, they must be passed through the water into the vessel containing it. The wet they sustain on this occasion does them no hurt; but if the air is of such a quality that it is expected the mouse can live any time, it must have something to fit upon out of the reach of the water. Where this is not the case, it will be proper to hold the animal by the tail, in order to withdraw it as soon as possible; but if the air has been thoroughly noxious, it will be irrecoverable by a single inspiration. The mice are kept in receivers open at top and bottom, standing upon plates of tin perforated with many holes, and covered with others of the same kind to admit the air, kept down with weights, as in fig. 26. A quantity of paper or tow must be put into the vessel, and changed every two or three days in order to clean it; for which purpose it is proper to have another receiver of the same kind ready cleaned to hold them in. These animals must be kept in a place of moderate temperature, either too much heat or too much cold being prejudicial to them. The place where Dr Priestley kept his had a temperature of about 70° of Fahrenheit. When they had been made to pass through water, it was necessary to give them a considerable degree of heat in order to dry and warm them. In the course of his experiments he found, that mice will live entirely without water; for though he kept them for three or four months, and several times offered them water, they would never taste it; notwithstanding which they continued in perfect health and vigour. Two or three of them will live very peaceably together in the same vessel; though the Doctor had one instance of a mouse tearing another almost in pieces, when there was plenty of provisions for both.
Some difficulty may occur in opening the mouth of a phial containing any kind of substance to which water must not be admitted in a jar of air; but this will easily be overcome, by having a cork cut tapering with a strong wire thrust through it, as in fig. 27. For thus it will easily fit the mouth of any phial; and by holding the phial in one hand, and plunging both into the trough of water, the phial can easily be conveyed through the water into the jar, which must either be held by an assistant, or be fastened by strings with its mouth projecting over the shelf. When the phial is thus conveyed into the jar, the cork may easily be removed, and put in again at pleasure.
Fig. 28. represents an apparatus for determining the conducting power of air with regard to heat. It consists only of a jar, which may be filled with any kind of air, with a very sensible thermometer inserted in it, as is represented in the figure. The scale of this was so large, that the Doctor could mark upon it 20 divisions, each larger than half an inch, between the mean temperature of the atmosphere and a heat much below that of boiling water. By frequent trials he at last adjusted it in such a manner, that having filled the vessel with any kind of air, he could plunge it first in hot and then in cold water, so that the mercury would rise to the division 20 and fall to that of 6 or 7 in no great time; and thus, by means of a clock which beat seconds, he could not well make a mistake of more than two in noting down any particular division. The hot water was always made to boil, and the cold water was always brought fresh from the same pump. The mouth of the air-vessel was placed in a cup of mercury kept always at the same height; so that he could thus try any kind of air with as much accuracy as one would think were possible. Notwithstanding this, however, he could not thoroughly satisfy himself with the results; at least he has not yet thought proper to publish fully the results of his experiments. The differences, he says, were less striking than he expected. Inflammable air, however, appeared to conduct heat better than any other; the mercury ascending the same space in half the time in it that it took up in common air. Fixed air, and all the acid airs, were considerably worse than common air. Alkaline air was a little better, and dephlogisticated air a little worse, than common air; but the result of this last experiment was uncertain.
The electric spark is easily taken in any kind of air by filling a small tube with the air desired, with two wires approaching within striking-distance of each other in the middle.
We shall close this account of the apparatus for making experiments on gases with an account of an instrument invented by Dr Pearson of London for collecting air of any kind which escapes in bubbles from the surface of springs or rivers. It consists of a funnel inserted into a phial in such a manner that the gas as it issues through the water may come under the funnel, and thus rise into the phial. For the convenient holding it to receive the air from any place where it may issue copiously, it is furnished with a handle and a ring, to which the funnel is tied by springs, as represented fig. 29.