VI 

 MODERN THEORIES OF HEAT AND LIGHT 



r "PHE eighteenth - century philosopher made great 

 1 strides in his studies of the physical properties of 

 matter and the application of these properties in 

 mechanics, as the steam-engine, the balloon, the optic 

 telegraph, the spinning - jenny, the cotton - gin, the 

 chronometer, the perfected compass, the Leyden jar, 

 the lightning-rod, and a host of minor inventions testi- 

 fy. In a speculative way he had thought out more or 

 less tenable conceptions as to the ultimate nature of 

 matter, as witness the theories of Leibnitz and Bos- 

 covich and Davy, to which we may recur. But he had 

 not as yet conceived the notion of a distinction between 

 matter and energy, which is so fundamental to the 

 physics of a later epoch. lie did not speak of heat, 

 light, electricity, as forms of energy or " force" ; he con- 

 ceived them as subtile forms of matter as highly at- 

 tenuated yet tangible fluids, subject to gravitation and 

 chemical attraction ; though he had learned to measure 

 none of them but heat with accuracy, and this one he 

 could test only within narrow limits until late in the 

 century, when Josiah Wedgwood, the famous potter, 

 taught him to gauge the highest temperatures with the 

 clay pyrometer. 



He spoke of the matter of heat as being the most uni- 



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