MODERN THEORIES OF HEAT AND LIGHT 



versally distributed fluid in nature ; as entering in some 

 degree into the composition of nearly all other sub- 

 stances; as being sometimes liquid, sometimes con- 

 densed or solid, and as having weight that could be de- 

 tected with the balance. Following Newton, he spoke 

 of light as a "corpuscular emanation" or fluid, com- 

 posed of shining particles which possibly are transmut- 

 able into particles of heat, and which enter into chem- 

 ical combination with the particles of other forms of 

 matter. Electricity he considered a still more sub- 

 tile kind of matter perhaps an attenuated form of 

 light. Magnetism, "vital fluid," and by some even 

 a "gravic fluid," and a fluid of sound were placed 

 in the same scale; and, taken together, all these sup- 

 posed subtile forms of matter were classed as "impon- 

 derables." 



This view of the nature of the " imponderables" was 

 in some measure a retrogression, for many seventeenth- 

 century philosophers, notably Hooke and Huygens and 

 Boyle, had held more correct views ; but the material- 

 istic conception accorded so well with the eighteenth- 

 century tendencies of thought that only here and there 

 a philosopher like Euler called it in question, until well 

 on towards the close of the century. Current speech 

 referred to the materiality of the "imponderables" 

 unquestioningly. Students of meteorology a science 

 that was just dawning explained atmospheric phe- 

 nomena on the supposition that heat, the heaviest im- 

 ponderable, predominated in the lower atmosphere, 

 and that light, electricity, and magnetism prevailed in 

 successively higher strata. And Lavoisier, the most 

 philosophical chemist of the century, retained heat and 



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