54 On Chemical Philosophy. 
ice into water ; and that this 140° is the difference in the quan- 
tity required in each of these different relative states of existence ; 
the quantity, in short, which is necessary to produce the liquid 
form of water, as well as to support this change of form which 
it has produced. The same principle is seen in the further so- 
lution and conversion of this element into vapour, steam or gas: 
950° of heat disappear and become latent in every portion of 
water passing into this state; so that though water and steam 
are both at the same temperature 212°, yet the steam contains 
950° more of caloric than the same weight of water at that tem- 
perature: the caloric is hence said to become latent, or the 
energies of this quantity necessary to produce the change are 
occupied or suspended in supporting it in this new state of ex- 
istence it has produced. 
This I conceive is a clearer expression of latent heat, free ea- 
loric, caloric of temperature, eapacily, &e., for it not merely 
expresses the fact, but the explanation of that fact at the same 
time. Indeed, in this respeet, Dr. Black stated the fact, as far 
as it was then known to him, in sucha clear and beautiful man- 
ner, that I wonder how this should ever have been a subject of 
controversy, or that Dr. Irvin and Dr. Cleghorn should imagine 
they gave any better explanation of it, by saying that ‘* heat dis- 
appears and becomes latent, mot in effecting the change, but as 
the effect of that change.” This to me conveys a very imper- 
fect conception of the change ; it is at any rate a very confused, 
partial and imperfect expression of the fact. Heat disappears, 
not merely as a consequence, but as a cause. It is necessary to 
apply heat to effect the change, which heat disappears, and 
again reappears on the steam returning to its former state: what 
then can be so plain, as that the quantity necessary to produce 
the change, disappears or becames latent, because it has. to sup- 
port the change produced ? ; 
It thus appears that one quantity is necessary for the solid, 
another for the liquid, and another for the gaseous states ; and 
this quantity differing in every different species of matter, we find 
substances assuming the liquid and gaseous forms at every pos- 
sible point of temperature. It would carry us too much into de- 
tail, to contemplate the beauty and utility of this law. In these 
Essays we shall therefore merely confine ourselves to the illustra- 
tion afforded by the instance already mentioned, Give to ice 
the quantity necessary for the production of water; the pro- 
perties and active energies of this quantity are then employed in 
supporting this new state of existence ; it cannot therefore act 
upon any thing else, or produce the sensation or effects of free 
caloric; it is said to be latent. Similar also is the case with its 
further conversion intg steam, which, like the gases, is a simple 
ee ane bia oe solution 
