VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 43/ 



in another to — 556°. Hence it would seem, that Mr. Hutchins had always 

 some part of the quicksilver left unfrozen, or some vacuity remaining, either in 

 the stem or the ball of his instruments ; and as no objection appears against those 

 experiments of M. Braun's, we must conclude, that quicksilver, in becoming 

 solid, contracts about a 23d of its whole bulk. Among the numerous improve- 

 ments in natural knowledge which have been made within a short period of years, 

 perhaps none tends to illustrate more phenomena of nature than the late discovery, 

 that a considerable quantity of heat disappears when bodies pass into a state of 

 fluidity or elastic vapour, and re-appears when they are converted back again to 

 their original condition. This remarkable effect of such changes, it seems, was 

 first observed at Glasgow, about 20 years ago, by Dr. Black and Mr. Irwin, who 

 endeavoured to determine its most material circumstances by various experiments. 

 Since that time Dr. Black has constantly taught it in his chemical lectures ; and 

 considering the heat which disappears as still remaining in the fluid or vapour, 

 but deprived for the time of its property of being communicated to other bodies, 

 and thereby becoming sensible, he calls it latent heat, a term sufficiently ex- 

 pressive of his manner of conceiving the fact. 



In the year 1772, Professor Wilcke inserted, in the Transactions of the Royal 

 Acad, of Sciences at Stockholm, a paper professedly on the subject of the cold 

 produced by snow in melting. He seemed not at all acquainted with what Dr. 

 Black had done, but speaks of it as his own discovery, originating in an accidental 

 attempt to melt away a quantity of snow by the affusion of hot water ; when he 

 found the process go on so slowly, and so little effect produced, that he deter- 

 mined to investigate the cause of so unexpected an event. After a series of ex- 

 periments with this view, he came to the following conclusion ; that snow, in 

 melting, constantly absorbs a certain and equal quantity of heat, which is em- 

 ployed entirely in giving it fluidity. Two principal methods have been adopted 

 to prove this loss of heat ; one, by adding ice at the freezing point to a certain 

 proportion of water at a known degree of heat, and observing how much the 

 temperature of the mixture comes out below that which should have resulted ac- 

 cording to the common laws of the distribution of heat among bodies ; the other, 

 by observing how much faster water near the freezing point acquires sensible heat, 

 than an equal quantity of ice melting under similar circumstances. It is obvious, 

 that both these methods tend not only to prove the fact, but likewise to discover 

 the quantity of heat so absorbed ; and that the latter also, if the operation be re- 

 versed, will show the quantity of heat evolved, when a fluid congeals or becomes 

 solid. In this way Dr. Black estimates the heat in question to be equal to 140 

 degrees on Fahrenheit's scale ; M. Wilcke, by a great variety of experiments 

 with different proportions of snow and water, brought it out pretty uniformly 



