JOSEPH BLACK: HIS LIFE AND WORK 83 



termed ' latent ' or hidden heat. But his research was not 

 made with this object; the connection of the two was 

 fortuitous, although of a fundamental nature. 



Between the years 1759 and 1763, he formed opinions 

 regarding the quantity of heat necessary to raise equally 

 the temperatures of different substances. Boerhaave 

 imagined that all equal portions of space contain equal 

 amounts of heat, irrespective of the nature of the matter 

 with which they are filled ; and his reason for this state- 

 ment was that the thermometer stands at the same 

 height if placed in contact with objects near each other. 

 Here we have a confusion between heat and temperature ; 

 and this was perceived by Black, for he pointed out that 

 a distinction must be drawn between quantity and inten- 

 sity of heat : the latter being what we now call tempera- 

 ture. He quotes Fahrenheit to show that while equal 

 measures of water at different temperatures acquire a 

 mean temperature when mixed, it requires three measures 

 of quicksilver at a high temperature to convert two 

 measures of water at a low temperature to the mean of 

 the two temperatures; and this corresponds to twenty 

 times the weight of the water. Black expressed this by 

 the statement that the capacity for heat of quicksilver is 

 much less than that of water. 



But before this, in 1757, Black had made experiments 

 leading up to these views. He had noticed that when ice 

 or any solid substance is changing into a fluid, it receives 

 a much greater amount of heat than what is perceptible 

 in it immediately afterwards by the thermometer. A 

 great quantity of heat enters into it without making it 

 perceptibly warmer. Conversely, in freezing water or any 

 liquid, a large amount of heat comes out of it, which again 

 is not revealed by a thermometer. 



He then proceeded to estimate the quantity of heat 

 which had to be absorbed by a known weight of ice in 



