BLACK. 151 



Glasgow for the next ten years, and it was during this period, 

 between the years 1759 and 1763, that he brought to maturity his 

 speculations concerning heat, which had occupied his attention from 

 the very first commencement of his philosophical investigations. 

 His two great discoveries were the doctrines of ' Latent Heat,' and 

 ' Specific Heat.' The theory of ' Latent' Heat, which mainly urged 

 Watt to the adoption of improved arrangements in the steam- 

 engine, may be briefly described as the absorption of heat by bodies 

 passing from the solid to the fluid state, and from the fluid to the 

 aeriform, the heat having no effect on surrounding bodies (being, 

 :herefore, insensible to the hand or thermometer), and only by its 

 absorption maintaining the body in the state which it has assumed, 

 and which it retains until the absorbed heat is given out and has 

 become again sensible, when the state of the body is changed back 

 again from fluid to solid, from aeriform to fluid. 



The doctrine of ' Specific Heat,' or as it was called by Dr. Black 

 the capacity of bodies for heat, is summed up in the facts, that 

 different 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 sensible temperature. Thus 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, because the capacity of water for heat is 19 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 Priestly and Scheele upon oxygen 

 gas, and of Cavendish on hydrogen gas, was founded mainly upon 

 the doctrines of latent and specific heat ; and it was thus the sin- 

 gular felicity of Black to have furnished both the pillars upon which 

 modern chemistry reposes. 



In 1766 Black succeeded Dr. Cullen in the professorship of che- 

 mistry at the University of Edinburgh, and in the new scene on 

 which he entered his talents became more conspicuously and more 

 extensively useful. Dr. Robison thus characterises him as a lecturer 

 " He became one of the principal ornaments of the university, his 

 lectures were attended by an audience which continued increasing 

 from year to year ; his personal appearance and manners were those 

 of a gentleman, and peculiarly pleasing. His voice in lecturing was 

 low but fine, and his articulation so distinct that he was perfectly 

 well heard by an audience consisting of several hundreds. His 

 discourse was so plain and perspicuous, his illustration by experi- 

 ment so apposite, that his sentiments on any subject never could be 

 mistaken even by the most illiterate." Dr. Black continued to 

 lecture at the University of Edinburgh for thirty years ; he then 



