JOSEPH BLACK 127 



But a quite different question remained, that of stating the 

 quantity of heat - the unknown something - which is needed 

 in order to raise the temperature of a given body by a given 

 number of degrees. It had never been doubted that double 

 the quantity of the same material requires twice as much heat; 

 but what about the heat required for equal quantities of 

 different substances? This question appeared very diffi- 

 cult, and even insoluble. At that time only a few superficial 

 ideas were in existence. Black it was who created the neces- 

 sary new foundation for dealing with all questions of heat 

 quantity, by an extensive series of observations founded 

 upon a new point of view. When he wished to compare a 

 body as regards the heat needed for warming it, or the heat 

 released from it on cooling, with water, he allowed the heated 

 body to give up its heat to water, and determined the two 

 initial temperatures and the common final temperature. 

 Starting from the idea that the one body must have given up 

 the same amount of heat as that received by the other, 

 if transference of heat to or from the surroimdings has been 

 avoided, he was able to compare the capacity for heat of dif- 

 ferent substances with that of water, and hence also with one 

 another, without inconsistencies. This is the complete 

 fundamental idea of the mixture calorimeter, by means of 

 which, even to-day, capacities for heat (later called 'specific' 

 heats), are measured. Another calorimetric method, the 

 method of cooling, was also applied extensively by Black for 

 the first time; and he was likewise responsible for the funda- 

 mental notion of the ice calorimeter, which was later refined 

 in the highest degree by Bunsen, over a hundred years 

 afterwards, and became one of the most exact methods of 

 measuring heat. 



In the latter case, the foundation was given by a particularly 

 important general truth discovered by Black, and entirely 

 opposed to opinion at the time. He found that the change of 

 state - melting or boiling - of a given amount of a substance 



