rator may stop when he fees he has got a quantity fufficient for his purpose: whereas, unlefs he has a gage connected with his tranf ferer (which may be inconvenient) he must admit water into his receiver before he can certainly tell whether he has procured 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 fet loose very copiously on its first admiffion into the receiver.

But if the air, difengaged from any fubstance, will be attracted by mercury, as is the cafe with all thofe which contain the nitrous acid, this process cannot be used, and recourse must be had to the vacuum; and for this purpofe it is neceffary that the operator be provided with receivers made very thin, on purpose for these experiments. Such as are commonly used for other experiments are much too thick for this purpofe, being very liable to break with the application of the heat produced by the burning lens. In this process, care should be taken to place the materials on glafs, a piece of crucible, or fome other fubstance that is known to yield no air by heat.

The figure b, represents a common glass phial with a ground ftopple, with many small holes in it, which was a happy contrivance of

my

my ingenious pupil and friend Mr. Benjamin Vaughan. It is of excellent ufe 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 it, without admitting any mixture of the common air, or even of the water; and yet the air generated within it has a fufficient out-let. These phials will be found useful in a great variety of experi

ments.

The figure c, represents a phial of the fame form with a; but the neck is thicker, in order to be fitted with a ground ftopple, perforated, and drawn out into a tube, to be used instead. of the phial e, vol. I. plate 1. Till I hit upon this contrivance, which was executed for me by the direction of Mr. Parker, I had a great deal of trouble in perforating common corks, bending and fitting tubes to them; and, after all, the corks themselves, or the cement, with which I generally found it convenient to cover the ends of the tubes, were apt to give way, and to be the occafion of very disagreeable accidents. Befides, if any hot acid was used, the vapour would corrode the cork, and an allowance was to be made for the effect of that circumftance on the air: whereas, with this apparatus, which is exceedingly convenient and elegant, the opera

tor may be fure that nothing but glass is contiguous to the materials he works upon, as he can perfectly exclude every other foreign influence; and while it remains unbroken, it is never out of repair, or unfit for use.

For many purposes, however, the former method, with corks and tubes, will be found very fufficient, and much less expensive; especially with the fluor acid; which corrodes glafs, and which will presently eat through one of thefe delicate phials. For this purpose, therefore, I would recommend the use of a common and very thick phial, especially as no great degree of heat, and never any fudden application of heat is wanted.

The phial c, will be found fufficient for any purpose that does not require more heat than the flame of a candle held close to the bottom of it, can fupply: but if there be occafion to place the phial in a fand-heat, and confequently if it must be put into a crucible placed on the fire, it will be neceffary to have the tube, in which the ground stopple terminates, made as long as may be, as reprefented by e; otherwise the veffels that receive the air will be too near the fire. Nine or twelve inches,, d however,

however, will be a fufficient length for any purpose.

I have great reason to congratulate myself on this apparatus, having found it to be of moft admirable ufe. For, in experiments with air, where the greatest poffible accuracy is required, lutes are by no means to be trusted, fince a variety of vapours, coming into contact with them, are confiderably affected; whereas these stopples being ground air-tight, the operator may be perfectly at ease, both with refpect to the quantity and the quality of his produce, To exprefs this process as concisely as poffible, I generally allude to it, by saying that the phials have ground stopples and tubes.

In experiments in which it is not worth while to be at the expence of thefe phials with ground-stopples and tubes, and yet where gunbarrels cannot be trufted to, on account of the materials corroding the iron, I have recourse to a kind of long phial, or a tube made narrower at the open end, nine or twelve inches in length, and of an equal thickness throughout, reprefented fig. d. When these phials are put into a crucible with fand, the bottom may be made red-hot, while the top is fo cool, that a common cork (into which a glass-tube is inferted) will not be affected by the heat. In

fact,

fact, this veffel is a kind of gun-barrel made of glass, and is ufed exactly like the gunbarrel, except that it is not exposed to so great a degree of heat.

When the materials are put into this veffel, it must be filled up to the mouth with fine fand, that will give no air by the application of heat, and the cork must be thrust down close upon the fand. The air must be received as in plate 2. fig. 7. vol. I. Could this glassveffel bear as great a degree of heat, and as fuddenly applied as the gun-barrel, it would be an excellent inftrument indeed. I have fometimes thought of getting them made of that kind of clay which is used for making crucibles; but these of glass have been generally fufficient for my purpose.

When a perfon has a great many trials to make of the goodness of air, it is of no small importance to have contrivances by which he may fave time. Having, particularly, had frequent occafion to measure the purity of air by means of nitrous air, in which it is fometimes neceffary to put several measures of one kind to one measure of the other; and being wearied with taking all the measures separately, at length I hit upon the very ufeful expedient of having the measures ready-made, con

fifting