The Great Chemist, Joseph Black. 57 



away all the water, seemed to indicate something very different 

 from the common view. 



It had also been assumed that equal quantities of heat were 

 required to raise equal masses of every substance through the 

 same range of temperature, and when it became clear from 

 certain experiments that this was a mistake, it was suggested 

 that "volume" should be substituted for "mass." Dr. Black 

 showed that every substance had its own capacity for heat, 

 dependent on its material, and laid the foundation of the doctrine 

 of Specific Heat, which afterwards developed into the so-called 

 law of Dulong and Petit, that the atoms of all elements in a 

 gaseous state had the same heat capacity. He also showed that 

 there were numerous cases in which large quantities of heat were 

 added to bodies without any change in the temperature, such as 

 that already mentioned in melting ice and in evaporating water. 

 Thus for the first time the scientific world was provided with the 

 idea now known as " heat consumed in, or arising out of, change 

 of state." For perhaps all his life Dr. Black held the opinion 

 that heat was some kind of substance, though he recognized the 

 difficulties, and that the liquid state of water, as of other fluids, 

 was due to the actual combination of this substance of heat with 

 the material which became liquid, and in the combination ceasing 

 to be recognizable by the thermometer, just as the gas or fixed 

 air could not be recognized as forming part of the calcareous 

 eaith by any of its visible properties. It is really curious that 

 when, as a schoolboy, nearly sixty years ago I began to study 

 natural philosophy, the doctrine of latent heat was still taught in 

 terms of combination of caloric, although the substantial 

 character of heat was then rejected by almost every authority. 



Black's first experiment was made by hanging in a large 

 room, very little subject to change of temperature, two globes of 

 thin glass, one containing water and the other ice. He assumed 

 that heat was being constantly supplied by the air of the room to 

 both globes. When the ice was almost all melted the temperature 

 of the water and ice contained in the globe was still 32°, while 



