364 JAMES WATT. 



I do not hesitate to represent this experiment of Black's 

 as one of the most remarkable in modern physics. In- 

 deed, see the consequences to which it leads. 



Water at zero, and ice at zero, difi'er in their internal 

 constitution. The fluid contains, beyond what the solid 

 body does, 79° of an imponderable body called caloric. 

 Those 79° are so well hidden in the composition, — I was 

 almost going to say in the aqueous amalgam, that the 

 most sensitive thermometer does not reveal their exist- 

 ence. Heat then, imperceptible to our senses, impercep- 

 tible to the most delicate instruments, latent heat, for 

 that is the name given to it, is one of the constituent prin- 

 ciples of those bodies. 



The comparison of boiling water, of water at 100° 

 with the vapour that rises from it, and the temperature 

 of which is also 100°, leads, though on a much grander 

 scale, to analogous results. At the moment of the forma- 

 tion of steam of the temperature of 100°, the water at 

 the same time imbibes an enormous quantity of heat in a 

 latent form, in a form not sensible to the thermometer. 

 When the steam resumes its fluid form, this latent heat is 

 disengaged, and heats all bodies it meets with capable of 

 absorbing it. If, for example, we occasion 5-35 kilograms 

 of water at zero to be traversed by a single kilogram of 

 steam at 100°, the steam will be entirely liquified. The 

 6"35 kilograms resulting from the mixture will be of the 

 temperature of 100°. In the composition of one kilogram 

 of steam then, a quantity of latent heat is absorbed that 

 would raise a kilogram of water, provided the evapora- 

 tion was prevented, from zero to 535° of the centigrade 

 scale. This result will undoubtedly appear enormous, but 

 it is certain ; the steam of water is created only on these 

 conditions. Wherever a kilogram of water at 100° evap- 



