BLACK. 339 



latent, had kept them in the state of elastic and in- 

 visible fluids.* 



The third great discovery of Black was that which 

 has since been called the doctrine of specif c 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 tbat 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, 

 because the capacity of water for heat is nineteen times 

 that of gold. So twice as much heat is required to 

 raise water to any given point of sensible heat as to 

 raise mercury, the volumes of the two fluids compared 

 being equal. 



The true doctrine of combustion, calcination of 

 metals, and respiration of animals, which Lavoisier 

 deduced from the experiments of Priestley and Scheele 

 upon oxygen gas, and of Cavendish on hydrogen gas, 

 and which has changed the whole aspect of chemical 

 science, was founded mainly upon the doctrines of 

 latent and specific heat. It was thus the singular 

 felicity of Black to have furnished both the pillars 

 upon which modern chemistry reposes, and to have 



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

 reduce the phenomenon 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 through diaphanous substances and 

 remaining unfixed, its heat is not sensible. 



z 2 



