M. Arago's Biographical Memoir of James Watt. 231 



of water at the freezing point with two pounds at 167° Fahr., 

 the four pounds of the mixture will be found to be at 99°, 

 that is to say, at the mean temperature of the commixed li- 

 quids. The hot water is thus found to have preserved 67° of 

 its previous temperature, and to have yielded 67 other degrees 

 to the cold water. All this is what would readily be expected, 

 and could easily be foreseen. And, now, let us repeat the ex- 

 periment with a single modification. Instead of the two 

 pounds of water at the freezing point, let us take two pounds 

 of ice at precisely the same temperature. From the mixture 

 of this two pounds of ice with the two pounds of water at the 

 temperature of 167°, there will result four pounds of liquid 

 water, since the ice, plunged into the hot water, must needs 

 be dissolved, and will yet retain its former weight ; but you 

 must not conclude that, from this second mixture, there will 

 result as from the former a temperature of 99°. Very far from 

 it ; in this latter experiment, the water will not be above the 

 freezing point, and there will not remain a single trace of the 

 135° of the heat of the two pounds of water ; these 135° will 

 have dissolved all the particles of the ice, and have combined 

 with them, but without having heated them in the slightest 

 degree. 



I have no hesitation in adducing this experiment of Dr 

 Black's as one of the most remarkable in modern physics. 

 Observe its consequences. Ice, at its habitual temperature 

 32°, and water at the same temperature, differ in their essen- 

 tial composition. The liquid, in addition to what is contain- 

 ed in the solid, includes 135° of an imponderable body which 

 is called caloric. These 135° are so thoroughly concealed in 

 the compound, I was about to say the watery alloy, that the 

 most delicate thermometer cannot detect its existence. Hence 

 then, caloric, which is not discoverable by our senses, and 

 which cannot be detected by the most dehcate instruments, — 

 in short, latent heat, for that is the name which has been be- 

 stowed upon it, forms one of the constituent principles of 

 bodies. 



The comparison of boiling water, that is to say of water at 

 212°, with the steam which issues from it, and whose tem- 

 perature is also 212°, leads to analogoxis results, but upon a 



