BLACK. 13 



body in the state which it has assumed, and which it 

 retains until, the absorbed heat being given out, and 

 becoming again sensible, the state of the body is 

 changed back again from fluid to solid, from aeriform 

 to fluid. He never published any account of this dis- 

 covery, but he explained it fully in his Lectures, both 

 at Glasgow and Edinburgh, and he referred to it in the 

 paper already mentioned, which was printed in the 

 4 Philosophical Transactions' for 1775. Well, then, 

 may we marvel that no mention whatever of latent 

 heat is made in the celebrated * Encyclopedic,' which 

 owed its chemical contributions to no less a writer and 

 experimentalist than Morveau. The doctrine of latent 

 heat, however, was immediately applied by all philo- 

 sophers to the production of the different airs which 

 were successively discovered. They were found to 

 owe their permanently elastic state to the heat absorbed 

 in their production from solid or fluid substances, and to 

 regain their fluid or solid state by combining either 

 together or with those substances, and in the act of 

 union giving out in a sensible form the heat which 

 while absorbed and latent, had kept them in the state 

 of elastic and invisible fluids.* 



The third great discovery of Black was that which 

 has since been called the doctrine of specific heat, but 

 which he called the capacity of bodies for heat. Dif- 

 ferent bodies contain different quantities of heat in the 

 same bulk or weight ; and different quantities of heat 

 are required to raise different bodies to the same sen- 

 sible temperature. Thus, by Black's experiment, it 

 was found that a pound of gold being heated to 150, 

 and added to a pound of water at 50, the temperature 

 of both became not 100, the mean between the two, 

 but 55, the gold losing 95, and the water gaining 5, 



* It is by no means impossible that one day we may be able to reduce 

 the phenomena of light within the theory of latent heat. It may be that 

 this body when absorbed, that is, fixed in substances, gives out heat ; as, 

 while passing 1 through diaphanous bodies and remaining unfixed, its heat is 

 not sensible. 



