THE STORY OF NINETEENTH-CENTURY SCIENCE 



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. 

 He did not speak of heat, light, electricity, as forms of 

 energy or "force"; he conceived them as subtile forms 

 of matter as highly attenuated yet tangible fluids, sub- 

 ject to gravitation and chemical attraction ; though he 

 had learned to measure none of them but heat with ac- 

 curacy, 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 tem- 

 peratures with the clay pyrometer. 



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

 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, composed 

 of shining particles which possibly are transmutable 

 into particles of heat, and which enter into chemical 

 combination with the particles of other forms of matter. 

 Electricity he considered a still more subtile kind of mat- 

 ter 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 supposed subtile forms of matter were 

 classed as " imponderables." 



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 materi- 

 alistic conception accorded so well with the eighteen th- 



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