prefence, it was abfolutely impoffible that I fhould have done otherwife, without being very tedious, and even appearing ridiculously trifling, to those who were at all versed in things of this nature. And though I am willling to facrifice a great deal to the defire that I have to facilitate thefe inquiries to beginners; yet as I do not, in these volumes, pretend to compose an elementary treatife, for the ufe of those who have no previous knowledge of the fubject (but, beginning where others have left it, to refume the inquiry, and extend the bounds of our knowledge relating to it) propriety requires that I do not facrifice too much to fo foreign an object. Besides, that readiness and certainty in the use of inftruments, which is acquired by experience, cannot be communicated by any verbal inftruction, but must be the refult of much practice, with refpect to others, as it was with myself; and a variety of subsidiary helps, which contribute much to the facility and elegance of operating, will fo certainly occur to any pèrfon who fhall actually go to work in this bunefs, that it is altogether unneceffary to enter into a detail of them.

Befides, every man will, in many things, have a method of his own; fo that two perfons, who fhould do the very fame things,

would

would fall into different methods of doing them, and it is probable that each of them would fancy that there was a peculiar advantage in his own. Leaning, however, as I profess I always do, to an inclination to gratify beginners, and to give them all the affiftance in my power, I fhall be as particular, as with propriety I can be, in the description of the principal inftruments, and mode of operating, which I have made use of in my late experiments,

The figures a, a, a, reprefent phials, of which I have made very great ufe in the whole course of my experiments. They are made round, and very thin at the bottom, and the mouth is ground finooth; fo that they may be either used with a cork, or, being filled with quickfilver, or any other fluid, will stand firm when inverted, and placed upright, in bafons containing the fame fluid. When they are used with corks, like common phials, they will bear the application of a pretty sudden heat from the flame of a candle, or other, wife, which the common phials, being generally thickest at the bottom, will not bear; and therefore, before I got these phials, I used to grind the bottoms of the common phials very thin: but I have found a very great convenience in having thefe made thin on purpofe;

pofe; and befides, their being round at the bottom, is, on many accounts, a great advantage.

Without veffels of this form, it is hardly poffible to extract air from any fubftance confined by quickfilver, which is an operation that the reader will find, in the course of this volume, I have made very great use of; but nothing is easier in fuch veffels as these for ftanding with their mouths downwards, and the fubftances on which the experiment is made lying on the furface of the quickfilver, just under the thinneft part of the glass, it is very easy to prefent them to the focus of the burning lens, in fuch a manner that they shall be exposed to all the power of it, without breaking the glafs. Care, however, must be taken, to place them fhort of the focus at firft, that the greatest degree of heat may not be communicated at once. In most cafes this moderate heat will be fufficient to produce a ,confiderable quantity of air; and as there will then be a space void of every thing but air between the glafs and the fubftance on which the heat is to be thrown, the greatest heat that the lens can produce may be directed upon it; fince the glafs through which the rays are tranfmitted, being at some distance

1

from

from the focus, is in no danger of being broken or melted,

A skilful operator will be able to fill his veffel with the newly generated air by this means; but, in general, he will do well to content himself with getting it half-full, or lefs; for as the glass is neceffarily thicker towards the mouth, there will be some danger of breaking it when the rays are tranfmitted near that place, and of lofing the air that has been, perhaps, with great trouble and difficulty, procured. This has frequently happened to myself, and does so still every now and then, long accustomed as I have been to the operation.

If the substance on which the experiment is made be in the form of a powder, as red lead, and even many very light fubftances, it will be most convenient to put them into the vesfel firft; and the quickfilver may, with care, be poured upon them afterwards, so as to keep the fubftance at the bottom; and yet, when the veffel is inverted, it will remain at the top. When the light matter will not lie close, it will not be difficult, fometimes, to intercept it in the ftrait part of the phial, at the neck; but it will often be most convenient to

form

form these light matters into fmall balls, and put them into the veffel, through the quickfilver, with which it has been previously filled.

I would obferve, with refpect to this process, and every other in which veffels are to be filled with quickfilver, and then to be placed inverted in bafons of the fame, that no operation is eafier (unless the mouth of the veffel be exceedingly wide) when the mouth of it is covered with foft leather, and, if neceffary, tied on with a ftring, before it be turned upfide down; and the leather may be drawn from under it when it is plunged in the quickfilver. If the mouths of the veffels be very narrow, it will be fufficient, and moft convenient, to cover them with the end of one's finger.

In this procefs, there remains lefs 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 fufpect that common air may get into the receiver, in the course of a long process, at some place not fufficiently guarded; and befides, it is a great fatisfaction to fee the quantity of air that is generated at any particular time, during the courfe of a procefs; that the ope

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