56 Henry Riddell on 



have written with the copy of his book on respiration which he 

 sent to Dr. Black — " It is but just that you should be one of the 

 first to receive information of the progress made in a career which 

 you yourself had opened and in which we consider ourselves your 

 disciples." A very human touch is added by the fact that Black 

 sent some cojnes of his thesis to his father who forwarded one to 

 Montesquieu. After a few days the great president called on 

 him and said — Mr. Black, your son will be the honour of your 

 name and family." 



Robison, who edited Black's lectures in 1803, writes : — 

 " What could be more singular than to find so subtle a substance 

 as air existing in a hard stone, and its presence accompanied by 

 such a change in the jiroperties of that stone ] AVhat bounds 

 could be reasonably set to the imagination in supposing that other 

 aerial fluids, as remarkable in their properties, might exist in a 

 solid form in many other bodies which at present attract no 

 notice, because of our ignorance of their nature and their com- 

 position 1 " 



The other great discoveries to which I direct your attention 

 are those known as "latent heat " and specific heat. In modern 

 teaching "latent heat " is known rather as the heat upon which 

 the state of a body depends. Perhaps the most important 

 example is found in water, which exists in the three distinct 

 forms of solid, liquid and gas. Before Black's time it was 

 believed, of course, that upon the temperature of the body 

 depended its state, that as soon as ice reached the temperature of 

 32°F. it melted. It was assumed that the smallest addition of 

 heat to the ice at 32° was sufficient, by raising its temperature 

 slightly above 32°, to cause the ice to melt. It was also implicitly 

 assumed that whatever quantity of heat was required to raise the 

 water through one degree, the same quantity, added to ice at 32°, 

 resulted in water at 33°. Similarly it was believed that the 

 smallest addition of heat to water at 212° resulted in its conver- 

 sion into steam. To Black the mere fact that ice took a long 

 time to melt, and that the kettle on the fire was long in boiling 



