Chapter 2

_Experiments and Observations made in the Year 1773, and the Beginning of 1774._

Sect. I. _Observations on ALKALINE AIR_ 163

Sect. II. _Of COMMON AIR diminished, and made noxious by various processes_ 177

Sect. III. _Of NITROUS AIR_ 203

Sect. IV. _Of MARINE ACID AIR_ 229

Sect. V. _Of INFLAMMABLE AIR_ 242

Sect. VI. _Of FIXED AIR_ 248

Sect. VII. MISCELLANEOUS EXPERIMENTS 252

Sect. VIII. _QUERIES, SPECULATIONS, and HINTS_ 258

THE APPENDIX.

Number I. _EXPERIMENTS made by Mr. Hey to prove that there is no OIL of VITRIOL in water impregnated with FIXED AIR_ 288

Number II. _A Letter from Mr. HEY to Dr. PRIESTLEY, concerning the effects of fixed Air applied by way of Clyster_ 292

Number III. _Observations on the MEDICINAL USES of FIXED AIR. By THOMAS PERCIVAL, M. D. Fellow of the ROYAL SOCIETY, and of the SOCIETY of ANTIQUARIES in LONDON_ 300

Number IV. _Extract of a Letter from WILLIAM FALCONER, M. D. of BATH_ 314

Number V. _Extract of a Letter from Mr. WILLIAM BEWLEY, of GREAT MASSINGHAM, NORFOLK_ 317

Num. VI. _A Letter from Dr. FRANKLIN_ 321

Number VII. _Extract of Letter from Mr. HENRY of MANCHESTER_ 323

THE INTRODUCTION.

SECTION I.

_A general view of PRECEDING DISCOVERIES relating to air._

For the better understanding of the experiments and observations on different kinds of air contained in this treatise, it will be useful to those who are not acquainted with the history of this branch of natural philosophy, to be informed of those facts which had been discovered by others, before I turned my thoughts to the subject; which suggested, and by the help of which I was enabled to pursue, my inquiries. Let it be observed, however, that I do not profess to recite in this place _all_ that had been discovered concerning air, but only those discoveries the knowledge of which is necessary, in order to understand what I have done myself; so that any person who is only acquainted with the general principles of natural philosophy, may be able to read this treatise, and, with proper attention, to understand every part of it.

That the air which constitutes the atmosphere in which we live has _weight_, and that it is _elastic_, or consists of a compressible and dilatable fluid, were some of the earliest discoveries that were made after the dawning of philosophy in this western part of the world.

That elastic fluids, differing essentially from the air of the atmosphere, but agreeing with it in the properties of weight, elasticity, and transparency, might be generated from solid substances, was discovered by Mr. Boyle, though two remarkable kinds of factitious air, at least the effects of them, had been known long before to all miners. One of these is heavier than common air. It lies at the bottom of pits, extinguishes candles, and kills animals that breathe it, on which account it had obtained the name of the _choke damp_. The other is lighter than common air, taking its place near the roofs of subterraneous places, and because it is liable to take fire, and explode, like gunpowder, it had been called the _fire damp_. The word _damp_ signifies _vapour_ or _exhalation_ in the German and Saxon language.

Though the former of these kinds of air had been known to be noxious, the latter I believe had not been discovered to be so, having always been found in its natural state, so much diluted with common air, as to be breathed with safety. Air of the former kind, besides having been discovered in various caverns, particularly the _grotta del Cane_ in Italy, had also been observed on the surface of fermenting liquors, and had been called _gas_ (which is the same with _geist_, or _spirit_) by Van Helmont, and other German chymists; but afterwards it obtained the name of _fixed air_, especially after it had been discovered by Dr. Black of Edinburgh to exist, in a fixed state, in alkaline salts, chalk, and other calcareous substances.

This excellent philosopher discovered that it is the presence of the fixed air in these substances that renders them _mild_, and that when they are deprived of it, by the force of fire, or any other process, they are in that state which had been called _caustic_, from their corroding or burning animal and vegetable substances.

Fixed air had been discovered by Dr. Macbride of Dublin, after an observation of Sir John Pringle's, which led to it, to be in a considerable degree antiseptic; and since it is extracted in great plenty from fermenting vegetables, he had recommended the use of _wort_ (that is an infusion of malt in water) as what would probably give relief in the sea-scurvy, which is said to be a putrid disease.

Dr. Brownrigg had also discovered that the same species of air is contained in great quantities in the water of the Pyrmont spring at Spa in Germany, and in other mineral waters, which have what is called an _acidulous_ taste, and that their peculiar flavour, briskness, and medicinal virtues, are derived from this ingredient.

Dr. Hales, without seeming to imagine that there was any material difference between these kinds of air and common air, observed that certain substances and operations _generate_ air, and others _absorb_ it; imagining that the diminution of air was simply a taking away from the common mass, without any alteration in the properties of what remained. His experiments, however, are so numerous, and various, that they are justly esteemed to be the solid foundation of all our knowledge of this subject.

Mr. Cavendish had exactly ascertained the specific gravities of fixed and inflammable air, shewing the former of them to be 1-1/2 heavier than common air, and the latter ten times lighter. He also shewed that water would imbibe more than its own bulk of fixed air.

Lastly, Mr. Lane discovered that water thus impregnated with fixed air will dissolve a considerable quantity of iron, and thereby become a strong chalybeate.

These, I would observe, are by no means all the discoveries concerning air that have been made by the gentlemen whose names I have mentioned, and still less are they all that have been made by others; but they comprise all the previous knowledge of this subject that is necessary to the understanding of this treatise; except a few particulars, which will be mentioned in the course of the work, and which it is, therefore, unnecessary to recite in this place.

SECTION II.

_An account of the APPARATUS with which the following experiments were made._

Rather than describe at large the manner in which every particular experiment that I shall have occasion to recite was made, which would both be very tedious, and require an unnecessary multiplicity of drawings, I think it more adviseable to give, at one view, an account of all my apparatus and instruments, or at least of every thing that can require a description, and of all the different operations and processes in which I employ them.

It will be seen that my apparatus for experiments on air is, in fact, nothing more than the apparatus of Dr. Hales, Dr. Brownrigg, and Mr. Cavendish, diversified, and made a little more simple. Yet notwithstanding the simplicity of this apparatus, and the ease with which all the operations are conducted, I would not have any person, who is altogether without experience, to imagine that he shall be able to select any of the following experiments, and immediately perform it, without difficulty or blundering. It is known to all persons who are conversant in experimental philosophy, that there are many little attentions and precautions necessary to be observed in the conducting of experiments, which cannot well be described in words, but which it is needless to describe, since practice will necessarily suggest them; though, like all other arts in which the hands and fingers are made use of, it is only _much practice_ that can enable a person to go through complex experiments, of this or any other kind, with ease and readiness.

For experiments in which air will bear to be confined by water, I first used an oblong trough made of earthen ware, as _a_ fig. 1. about eight inches deep, at one end of which I put thin flat stones, _b. b._ about an inch, or half an inch, under the water, using more or fewer of them according to the quantity of water in the trough. But I have since found it more convenient to use a larger wooden trough, of the same general shape, eleven inches deep, two feet long, and 1-1/2 wide, with a shelf about an inch lower than the top, instead of the flat stones above-mentioned. This trough being larger than the former, I have no occasion to make provision for the water being higher or lower, the bulk of a jar or two not making so great a difference as did before.

The several kinds of air I usually keep in _cylindrical jars_, as _c_, _c_, fig. 1, about ten inches long, and 2-1/2 wide, being such as I have generally used for electrical batteries, but I have likewise vessels of very different forms and sizes, adapted to particular experiments.

When I want to remove vessels of air from the large trough, I place them in _pots_ or _dishes_, of various sizes, to hold more or less water, according to the time that I have occasion to keep the air, as fig. 2. These I plunge in water, and slide the jars into them; after which they may be taken out together, and be set wherever it shall be most convenient. For the purpose of merely removing a jar of air from one place to another, where it is not to stand longer than a few days, I make use of common _tea-dishes_, which will hold water enough for that time, unless the air be in a state of diminution, by means of any process that is going on in it.

If I want to try whether an animal will live in any kind of air, I first put the air into a small vessel, just large enough to give it room to stretch itself; and as I generally make use of _mice_ for this purpose, I have found it very convenient to use the hollow part of a tall beer-glass, _d_ fig. 1, which contains between two and three ounce measures of air. In this vessel a mouse will live twenty minutes, or half an hour.

For the purpose of these experiments it is most convenient to catch the mice in small wire traps, out of which it is easy to take them, and holding them by the back of the neck, to pass them through the water into the vessel which contains the air. If I expect that the mouse will live a considerable time, I take care to put into the vessel something on which it may conveniently sit, out of the reach of the water. If the air be good, the mouse will soon be perfectly at its ease, having suffered nothing by its passing through the water. If the air be supposed to be noxious, it will be proper (if the operator be desirous of preserving the mice for farther use) to keep hold of their tails, that they may be withdrawn as soon as they begin to shew signs of uneasiness; but if the air be thoroughly noxious, and the mouse happens to get a full inspiration, it will be impossible to do this before it be absolutely irrecoverable.

In order to _keep_ the mice, I put them into receivers open at the top and bottom, standing upon plates of tin perforated with many holes, and covered with other plates of the same kind, held down by sufficient weights, as fig. 3. These receivers stand upon _a frame of wood_, that the fresh air may have an opportunity of getting to the bottoms of them, and circulating through them. In the inside I put a quantity of paper or tow, which must be changed, and the vessel washed and dried, every two or three days. This is most conveniently done by having another receiver, ready cleaned and prepared, into which the mice may be transferred till the other shall be cleaned.

Mice must be kept in a pretty exact temperature, for either much heat or much cold kills them presently. The place in which I have generally kept them is a shelf over the kitchen fire-place where, as it is usual in Yorkshire, the fire never goes out; so that the heat varies very little, and I find it to be, at a medium, about 70 degrees of Fahrenheit's thermometer. When they had been made to pass through the water, as they necessarily must be in order to a change of air, they require, and will bear a very considerable degree of heat, to warm and dry them.

I found, to my great surprize, in the course of these experiments, that mice will live intirely without water; for though I have kept them for three or four months, and have offered them water several times, they would never taste it; and yet they continued in perfect health and vigour. Two or three of them will live very peaceably together in the same vessel; though I had one instance of a mouse tearing another almost in pieces, and when there was plenty of provisions for both of them.

In the same manner in which a mouse is put into a vessel of any kind of air, a _plant_, or any thing else, may be put into it, viz. by passing it through the water; and if the plant be of a kind that will grow in water only, there will be no occasion to set it in a pot of earth, which will otherwise be necessary.

There may appear, at first sight, some difficulty in opening the mouth of a phial, containing any substance, solid or liquid, to which water must not be admitted, in a jar of any kind of air, which is an operation that I have sometimes had recourse to; but this I easily effect by means of _a cork cut tapering_, and a strong, wire thrust through it, as in fig. 4, for in this form it will sufficiently fit the mouth of any phial, and by holding the phial in one hand, and the wire in the other, and plunging both my hands into the trough of water, I can easily convey the phial 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 may also be put into it again at pleasure, and conveyed the same way out again.

When any thing, as a gallipot, &c. is to be supported at a considerable height within a jar, it is convenient to have such _wire stands_ as are represented fig. 5. They answer better than any other, because they take up but little room, and may be easily bended to any shape or height.

If I have occasion to pour air from a vessel with a wide mouth into another with a very narrow one, I am obliged to make use of a funnel, fig. 6, but by this means the operation is exceedingly easy; first filling the vessel into 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. These funnels are best made of glass, because the air being visible through them, the quantity of it may be more easily estimated by the eye. It will be convenient to have several of these funnels of different sizes.

In order to expel air from solid substances by means of heat, I sometimes put them into a _gun-barrel_, fig. 7, and filling it up with dry sand, that has been well burned, so that no air can come from it, I lute to the open end the stem of a tobacco pipe, or a small glass tube. Then having put the closed end of the barrel, which contains the materials, into the fire, the generated air, issuing through the tube, may be received in a vessel of quicksilver, with its mouth immersed in a bason of the same, suspended all together in wires, in the manner described in the figure: or any other fluid substance may be used instead of quicksilver.

But the most accurate method of procuring air from several substances, by means of heat, is to put them, if they will bear it, into phials full of quicksilver, with the mouths immersed in the same, and then throw the focus of a burning mirror upon them. For this purpose the phials should be made with their bottoms round, and very thin, that they may not be liable to break with a pretty sudden application of heat.

If I want to expel air from any liquid, I nearly fill a phial with it, and having a cork perforated, I put through it, and secure with cement, a glass tube, bended in the manner represented at _e_ fig. 1. I then put the phial into a kettle of water, which I set upon the fire and make to boil. The air expelled by the heat, from the liquor contained in the phial, issues through the tube, and is received in the bason of quicksilver, fig. 7. Instead of this suspended bason, I sometimes content myself with tying a flaccid bladder to the end of the tube, in both these processes, that it may receive the newly generated air.

In experiments on those kinds of air which are readily imbibed by water, I always make use of quicksilver, in the manner represented fig. 8, in which _a_ is the bason of quicksilver, _b_ a glass vessel containing quicksilver, with its mouth immersed in it, _c_ a phial containing the ingredients from which the air is to be produced; and _d_ is a small recipient, or glass vessel designed to receive and intercept any liquor that may be discharged along with the air, which is to be transmitted free from any moisture into the vessel _b_. If there be no apprehension of moisture, I make use of the glass tube only, without any recipient, in the manner represented _e_ fig. 1. In order to invert the vessel _b_, I first fill it with quicksilver, and then carefully cover the mouth of it 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 in the quicksilver.

In order to generate air by the solution of metals, or any process of a similar nature, I put the materials into a phial, prepared in the manner represented at _e_ fig. 1, and put the end of the glass tube under the mouth of any vessel into which I want to convey the air. If heat be necessary I can easily apply to it a candle, or a red hot poker while it hangs in this position.

When I have occasion to transfer air from a jar standing in the trough of water to a vessel standing in quicksilver, or in any other situation whatever, I make use of the contrivance represented fig. 9, which consists of a bladder, furnished at one end with a small glass tube bended, and at the other with a cork, perforated so as just to admit the small end of a funnel. When the common air is carefully pressed out of this bladder, and the funnel is thrust tightly into the cork, it may be filled with any kind of air as easily as a glass jar; and then a string being 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 to any place, 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. A little practice will make this very useful manoeuvre perfectly easy and accurate.

In order to impregnate fluids with any kind of air, as water with fixed air, I fill a phial with the fluid larger or less as I have occasion (as _a_ fig. 10;) and then inverting it, place it with its mouth downwards, in a bowl _b_, containing a quantity of the same fluid; and having filled the bladder, fig. 9, with the air, I throw as much of it as I think proper into the phial, in the manner described above. To accelerate the impregnation, I lay my hand on the top of the phial, and shake it as much as I think proper.

If, without having any air previously generated, I would convey it into the fluid immediately as it arises from the proper materials, I keep the same bladder in connection with a phial _c_ fig. 10, containing the same materials (as chalk, salt of tartar, or pearl ashes in diluted oil of vitriol, for the generation of fixed air) and taking care, lest, in the act of effervescence, any of the materials in the phial _c_ should get into the vessel _a_, to place this phial on a stand lower than that on which the bason was placed, I press out the newly generated air, and make it ascend directly into the fluid. For this purpose, and that I may more conveniently shake the phial _c_, which is necessary in some processes, especially with chalk and oil of vitriol, I sometimes make use of a flexible leathern tube _d_, and sometimes only a glass tube. For if the bladder be of a sufficient length, it will give room for the agitation of the phial; or if not, it is easy to connect two bladders together by means of a perforated cork, to which they may both be fastened.

When I want to try whether any kind of air will admit a candle to burn in it, I make use of a cylindrical glass vessel, fig. 11. and a bit of wax candle _a_ fig. 12, fastened 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 that the candle is admitted. In this manner I have frequently extinguished a candle more than twenty times successively, in a vessel of this kind, though it is impossible to dip the candle into it without giving the external air an opportunity of mixing with the air in the inside more or less. The candle _c_, at the other end of the wire 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.

In order to draw air out of a vessel which has its mouth immersed in water, and thereby to raise the water to whatever height may be necessary, it is very convenient to make use of a glass _syphon_, fig. 13, putting one of the legs up into the vessel, and drawing the air out at the other end by the mouth. If the air be of a noxious quality, it may be necessary to have a syringe fastened to the syphon, the manner of which needs no explanation. I have not thought it safe to depend upon a valve at the top of the vessel, which Dr. Hales sometimes made use of.

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 may then be closed with wax or cement.

If the generated air will neither be absorbed by water, nor diminish common air, it may be convenient to put part of the materials into a cup, supported by a stand, and the other part into a small glass vessel, placed on the edge of it, as at _f_, fig. 1. Then having, by means of a syphon, drawn the air to at convenient height, the small glass vessel may be easily pushed into the cup, by a wire introduced through the water; or it may be contrived, in a variety of ways, only to discharge the contents of the small vessel into the larger. The distance between the boundary of air and water, before and after the operation, will shew the quantity of the generated air. The effect of processes that _diminish_ air may also be tried by the same apparatus.

When I want to admit a particular kind of air to any thing that will not bear wetting, and yet cannot be conveniently put into a phial, and especially if it be in the form of a 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) I first exhaust a receiver, in which it is previously placed; and having a glass tube, bended for the purpose, as in fig. 14, I screw it to the stem of a transfer of the air pump on which the receiver had been exhausted, and introducing it through the water into a jar of that kind of air with which I would fill the receiver, I only turn the cock, and I gain my purpose. In this method, however, unless the pump be very good, and several contrivances, too minute to be particularly described, be made use of a good deal of common air will get into the receiver.

When I want to measure the goodness of any kind of air, I put two measures of it into a jar standing in water; and when I have marked upon the glass the exact place of the boundary of air and water, I put to it one measure of nitrous air; and after waiting a proper time, note the quantity of its diminution. If I be comparing two kinds of air that are nearly alike, after mixing them in a large jar, I transfer the mixture into a long glass tube, by which I can lengthen my scale to what degree I please.

If the quantity of the air, the goodness of which I want to ascertain, be exceedingly small, so as to be contained in a part of a glass tube, out of which water will not run spontaneously, as _a_ fig. 15; I first measure with a pair of compasses the length of the column of air in the tube, the remaining part being filled with water, and lay it down upon a scale; and then, thrusting a wire of a proper thickness, _b_, into the tube, I contrive, by means of a thin plate of iron, bent to a sharp angle _c_, to draw it out again, 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. I then measure the length of this column of nitrous air which I have got into the tube, and lay it also down upon the scale, so as to know the exact length of both the columns. After this, holding the tube under water, with a small wire I force the two separate columns of air into contact, and when they have been a sufficient time together, I measure the length of the whole, and compare it with the length of both the columns taken before. A little experience will teach the operator how far to thrust the wire into the tube, in order to admit as much air as he wants and no more.

In order to take the electric spark in a quantity of any kind of air, which must be very small, to produce a sensible effect upon it, in a short time, by means of a common machine, I put a piece of wire into the end of a small tube, and fasten it with hot cement, as in fig. 16; and having got the air I want into the tube by means of the apparatus fig. 15, I place it inverted in a bason containing either quicksilver, or any other fluid substance by which I chuse to have the air confined. I then, by the help of the air pump, drive out as much of the air as I think convenient, admitting the quicksilver, &c. to it, as at _a_, and putting a brass ball on the end of the wire, I take the sparks or shocks upon it, and thereby transmit them through the air to the liquor in the tube.

To take the electric sparks in any kind of fluid, as oil, &c. I use the same apparatus described above, and having poured into the tube as much of the fluid as I conjecture I can make the electric spark pass through, I fill the rest with quicksilver; and placing it inverted in a bason of quicksilver, I take the sparks as before.

If air be generated very fast by this process, I use a tube that is narrow at the top, and grows wider below, as fig. 17, that the quicksilver may not recede too soon beyond the striking distance.

Sometimes I have used a different apparatus for this purpose, represented fig. 18. Taking a pretty wide glass tube, hermetically sealed at the upper-end, and open below, at about an inch, or at what distance I think convenient from the top, I get two holes made in it, opposite to each other. Through these I put two wires, and fastening them with warm cement, I fix them at what distance I please from each other. Between these wires I take the sparks, and the bubbles of air rise, as they are formed, to the top of the tube.