‘ 
864 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, differ in their internal 
constitution. The fluid contains, beyond what the solid 
body does, 79° of an imponderable body called calorie. 
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- 
