coxe’s TRAVELS IN RUSSIA.

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which ought to be carefully diftinguilhed from the contraftion which takes place whilflit preferves its fluidity ; therefore the ideas we have formed of the cold obtaining in thehabited countries near the poles, and the aftonilhing power of animals to refift it, maltbe erroneous # , as they have been taken from the extraordinary defcent of the mercuryin the thermometer, which we now know is derived from the contraction of the mercurywhen frozen, and not from fuch an extraordinary degree of cold, which if it had takenplace, mud have deftroyed the whole fyftem of organized bodies.

“ We cannot, according to our prefent knowledge of the fubjeCt, afiert, that a muchgreater degree of cold exifts than the point of the congelation of mercury ; ho otherinftrument having been employed to afcertain it than the mercurial thermometer, whichis now proved of no authority below 32 degrees of Reaumur.

“ But it appears, that a thermometer filled with highly rectified fpirits of wine pre-ferves its fluidity in a cold of 32 degrees of Reaumur, or 47 of Fahrenheit, and pro-bably in a greater, which may therefore be employed in northern climates with moreadvantage than one filled with mercury.

“ The furprifing coincidence in the freezingof mercury congealed in Siberia by naturalcold, with that effe&ed by means of artificial cold, merits attention, as they both fix thefreezing point of mercury at 32 of Reaumur ; profeffor Laxman, particularly in a latepaper to the Imperial Academy, declares, that he found common mercury conftantlybecome folid at 210 of De Lifle (32 of Reaumur), and that in 1782, it continued folidfor two months together. Dr. Pallas alfo, in the third volume of his Travels, men-tions the fame phenomenon taking place about the fame part of the fcale.”

In addition to Dr. Guthrie’s remarks, I fhall offer a few obfervations.

From a careful review of Mr. Hutchins’s experiments, and a comparifon of the ther-mometers which he employed on that occafion, Mr. Cavendifh f concludes, that thetrue point at which quickfilver froze on Mr. Hutchins’s thermometer, graduated accord-ing to the fcale Fahrenheit, was 40; and a thermometer adjufted in the manner recom-mended by the Committee of the Royal Society, freezes in 38^, or, in whole numbers,39 below freezing point, or 31^ of Reaumur which anfwers to the conclufion drawn byDr. Guthrie from his experiments, eftimating the point of mercurial congelation at 32of Reaumur, or 40 below o of Fahrenheit.

As the degree of artificial cold requifite to congeal quickfilver was greatly mifcon-ceived and exaggerated, a fimilar mifconception alfo prevailed with refpedl to the degreeof natural cold neceffary to the fuccefs of the experiment.

Dr. Guthrie however proves, that the congelation fucceeded in a cold not exceedingo of Fahrenheit; and fubfequent experiments made at Oxford by Mr. Walker {hew,that a very ftnall degree of natural cold is fuflicient to obtain for the frigorific mixturethe degree of cold neceffary to congeal quickfilver. Mr. Walker froze quickfilver ina mixture of equal parts of vitriolic acid and flrong fuming nitrous acid with fnow, thetemperature of the atmofphere being only at 30, or 2 degrees below freezing point. Hehas alfo {hewn, that it may be even frozen in fummer, in the hotted: climates, by a par-ticular combination of the frigorific mixtures, without the ufe of ice J.

* Sir Charles Blagden ingenioufly infers, from a comparifon of natural cold, during a feries of years, atAlbany Fort, meafured by a tpirit thermometer, and of artificial cold produced by freezing mixtures, thatthe extreme artificial cold produced by fnow and nitrous acid correfponds pretty exaftly with the extremeof natural cold in the molt rigorous climates, which can be well inhabited ; and does not exceed 46° of aftandard mercurial thermometer of Fahrenheit. Phil. Tranfac. vol. Ixxiii p.387.

f Phil. Tranfac. vol. Ixxiii. part 2. page 321.

Walker’s Experiments on the Production of Artificial Cold, Phil. Tranfac. vol. Ixxviii. p.395.—AlfoWalker’s Experiments on the Congelation of Quickfilver in England, Phil. Tranfac. for 1789, vol.lxxix.part 2. p. 199.

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I {hall