58 Henry Riddell on 



that of the water in the other globe had risen considerably. 

 Further experiments were made by the method of mixing a 

 known^quantity of warmed water with a weighed mass of ice, 

 and observing the resultant temperature, making due allowances 

 for losses by radiation diu'ing the time occupied in mixing. 

 Considering the difficulty of an unknown operation and the 

 roughness of the apparatus available his results are extraordinarily 

 accurate. Eead in modern terms the latent heat of melting ice, 

 as measured by him, is equal to 143 b.t.u. or a quantity of heat 

 equal to raising the same mass of water, at say 40° to 183°, or 

 through 143° approximately. The agreement is as exact, possibly, 

 as the modern measure of 142-65 B.T.u. In estimating the 

 same quantity for transformation of water into steam Black was 

 assisted by James Watt, whose best results are given as between 

 900° and 950°. At 950° he was nearer than Lavoisier with 

 intricate apparatus, who estimated the figure at 1,000° against 

 the now accepted value of approximately 966°. I need not 

 follow the matter further, but quote Robison — ''Fahrenheit, 

 Boerhave, Mavian, De Luc, and all the inquisitive meteorologist 

 of the two preceding centuries, though incessantly contemplating 

 and employing the same facts in their disquisitions, never mention 

 having such a thought." "it is the undivided property of my 

 ingenious and acute preceptor." 



This perception of the latent heat of water and of steam 

 was one of great importance in science and pregnant with great 

 possibilities. 



An ingenious explanation, no doubt partially true, was offered 

 by Dr. Irvine who had been a pupil of Black's, and was seeking 

 some method of finding absolute zero of temperature, now assumed 

 as about 492° F. below freezing point. Irvine assumed that the 

 total heat in any substance was proportional to the specific heat. 

 That of ice is not much more than half that of water, so that the 

 total heat of the ice was only about half that of the water, and 

 the heat disappearing into the water when the ice melted was 

 that required to bring up the total heat of the water to that 



