146 PROGRESS OF SCIENCE IN THE CENTURY. 



quantity invariable in nature; it is, correctly speak- 

 ing, never either produced or destroyed." 



Joule and Colding. Prof. Tait note8 that one 

 small chemical experiment would have enabled Hum- 

 ford in 1798 to prove that heat is not matter, just as 

 a little more conclusive reasoning would have brought 

 Davy in 1799 securely to the same conclusion, 

 which he eventually deduced in 1812. 



What Seguin and Mayer approached, but, by de- 

 parting from the scientific method, failed to attain, 

 was achieved by Colding of Copenhagen and Joule 

 of Manchester, " the true modern originators and ex- 

 perimental demonstrators of the conservation of 

 energy in its generality." * 



To Joule in particular, for his experiments were 

 more extensive, his measurements more exact, his con- 

 clusions more generalised than those of Colding, we 

 owe a difficult proof of what Kumford and Davy had 

 foreseen the First Law of Thermodynamics. In 

 Tait's statement this reads: " When equal quantities 

 of mechanical effect nre produced by any means what- 

 ever, from purely thermal sources, or lost in pure 

 thermal effects, then equal quantities of heat are put 

 out of existence or are generated ; and for every unit 

 of heat measured by the raising of a pound of water 

 1 degree Fahrenheit in temperature, you have to ex- 

 pend 772 foot-pounds of work." f 



SUMMARY. The idea that heat is not material but 

 a mode of motion, a form of energy, is older even than 

 Newton's Principia, yet the foundation of the theory 

 may be fairly dated from the experiments of Joule. 

 But many others contributed to the great conclusion, 

 and still more have furthered its development and ap- 

 plication. 

 Tait, op cit., p. 547. f Approximate!/. 



