prior to the Introduction oj the Potentials. 33 



omitted from it ; some of them, such as oxygen and hydrogen, 

 because they were as yet undiscovered, and others, such as the 

 metals, because they were believed to be compounds. 



Among the chemical elements, it became customary after 

 the time of Newton to include light-corpuscles.* That some- 

 thing which is confessedly imponderable should ever have been 

 admitted into this class may at first sight seem surprising. But 

 it must be remembered that questions of ponderability counted 

 for very little with the philosophers of the period. Three- 

 quarters of the eighteenth century had passed before Lavoisier 

 enunciated the fundamental doctrine that the total weight of 

 the substances concerned in a chemical reaction is the same 

 after the reaction as before it. As soon as this principle came 

 to be universally applied, light parted company from the true 

 elements in the scheme of chemistry. 



We must now consider the views which were held at this 

 time regarding the nature of heat. These are of interest for our 

 present purpose, on account of the analogies which were set up 

 between heat and electricity. 



The various conceptions which have been entertained 

 concerning heat fall into one or other of two classes, according as 

 heat is represented as a mere condition producible in bodies, or 

 as a distinct species of matter. The former view, which is that 

 universally held at the present day, was advocated by the great 

 philosophers of the seventeenth century. Bacon maintained it in 

 the Novum Organum : " Calor," he wrote, " est niotus expansivus, 

 cohibitus, et nitens per partes minores."f Boyle+ affirmed that 

 the " Nature of Heat " consists in " a various, vehement, and 

 intestine commotion of the Parts among themselves." Hooke 

 declared that " Heat is a property of a body arising from the 

 motion or agitation of its parts." And Newton|| asked : " Do not 



* Newton himself (Oplicks, p. 349) suspected that light-corpuscles and 

 ponderable matter might be transmuted into each other : much later, Boscovich 

 (Theoria, pp. 215, 217) regarded the matter of light as a principle or element in 

 the constitution of natural bodies. 



t Nov. Org., Lib. n., Aphor. xx. J Mechanical Production of Heat and Cold. 



Micrographia, p. 37. || Opticks. 



D 



