the inflammable air of vegetable or animal bodies than that of metals, and it is eafily decompofed in water.

This property of inflammable air of metals which I have discovered, throws great light upon the analysis of the decompofition of that air which I have made in two different ways. The first is to fire it together with common or dephlogisticated air, in veffels filled with very pure quickfilver, and alfo in veffels filled with diftilled water. The fecond method is to decompofe it by shaking it in pure diftilled water. In the first process a great

number of experiments are required in order to obtain a fenfible refiduum; befides, the igneous part is loft. The fecond method requires an exceedingly long time, but it is the most complete; for which reafon I have used it for the decompofition of other kinds of air.

XXV. On the Variation of the Temperature of boiling Water. By Sir George Shuckburgh, Baronet, F. R. and A. S. and Member of the Academy of Sciences and Belles Lettres at Lyon.

THE

Read March 11, 1779.

HE heat of boiling water having for fome years been used as one of the terms for graduating the scale of thermometers; together with the particular attention the Society has lately given (vide the Report of the Committee, Phil. Tranf. vol. LXVII.) to this branch of inquiry, and I may add the fingular fuccefs with which this age and nation has introduced a mathematical precision, hitherto unheard of, into the conftruction of philosophical inftruments, will render it unneceffary for me to say more in respect of the following experiments, than fimply to lay them before the Royal Society.

That the heat of boiling water was variable, according to the preffure of the atmosphere, feems to have been known to FAHRENHEIT as early as the year 1724′

(a).

(a) Vide Phil. Tranf. N° 385. wherein is propofed a curious project of determining the weight of the atmosphere by means of a thermometer alone, under the title of Barometri novi defcriptio."

A few

A few years after this, Meffieurs LE MONNIER and CASSINI (Mem. de l'Acad. des Sc. pour 1740) made fome decifive observations, to fhew that this quantity was very confiderable. It was left, however, for Mr. DE LUC to make a much more compleat feries of experiments, which he has defcribed and reduced into fyftem in his Recherches fur la Variation de la Chaleur de l'Eau bouillante. It remained only that these should be verified. Towards the latter end of the year 1775 I had an opportunity of repeating thefe obfervations with a small pocket thermometer of about fix inches long, made by Mr. NAIRNE; an instrument, it must be confeffed, not very accurate for fuch an examination, but with which I thought, however, I could obferve to within a quarter of a degree; my object at that time, amidst a variety of other philofophical pursuits, being to affure myself that the variation took place, rather than critically to examine the quantity of it. I fhall relate these observations, as the refult of them upon my return to England led me to fome more accurate.

Table:

Table of obfervations of the boiling-point, made in a

The fecond column gives the height of the barometer at the time of obfervation; the fourth, the heat of boiling water deduced from Mr. DE LUC's rules, compared with the lowermoft obfervation, or that under the greatest preffure; the fixth gives the difference between the theory and the experiment in the motion of the boiling point in hundredth parts of the whole fspace defcribed: from whence it might be concluded, that the motion or variation of the boiling point with a given variation in the preffure of the atmosphere was

or

greater

greater than by the theory alluded to). But these were but grofs experiments, and perhaps unworthy of fuch a competition. They induced me, however, to make the following. In the beginning of last year (1778) with the affistance of Mr. RAMSDEN, I procured a most excellent thermometer, every way adapted for this purpose. It was about fourteen inches long, but the interval between freezing and boiling only 8 inches, and though every degree was fomething less than the

th of an inch, yet, by means of a semi-transparent piece of ivory, which applied itself close behind the glass tube, fliding up and down in a groove cut in the brafs scale for that purpose, carrying a hair-line division, at the extremity of which was a vernier dividing each de

(b) The fame inftrument immersed in snow just melting at the top of Mount Cenis fell to 32°, the point of freezing obferved at the level of the fea.

(c) It may poffibly be fuggefted, that if this interval had been greater, viz. 20, 30, or 40.inches, I should have had a much larger fcale and more convenient inftrument; but in this, as in most other mechanical contrivances, our progress beyond certain limits is prevented; for if the perpendicular height of the column of quickfilver be much increafed, the weight of it will be fuch as to diftend the ball, and the inftrument may differ from itself in a vertical and horizontal pofition by half a degree, as I have seen in a tube only fifteen inches long; and if this circumftance be endeavoured to be corrected by making the bulb of the thermometer thicker, its fenfibility will be proportionably diminished. If my experience were to lead me to conclude any thing, I should confider a tube of a foot long as a maximum, and the bore of such a diameter as to admit a ball of a quarter or one fifth of an inch.

VOL. LXIX.

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