PHYSICS OF NINETEENTH CENTURY 167 



Yet, when collected and tested by the balance, the 

 products of the reaction weigh the same as the reagents. 

 This principle is accepted by Lavoisier in all his 

 investigations ; he states it in definite form : " The 

 quantity of matter is the same at the end as at the 

 beginning of every operation." 



Nevertheless, in spite of the disappearance of 

 phlogiston, and the establishment of the principle of 

 Imponderable the conservation of mass, the conception 

 Fluids. O f a weightless fluid had still many 

 years of useful life before it. Although the most 

 acute of the natural philosophers, such men as 

 Newton, Boyle and Cavendish, inclined to the 

 opinion that heat was due to a vibratory agitation 

 of the particles of bodies, in the absence of definite 

 conceptions corresponding to our modern notions 

 of energy, that opinion could not be developed 

 into a useful working hypothesis. The advance 

 which was waiting to be made was the idea of heat 

 as a measurable quantity, unchanged in amount 

 as it passed by contact from one body to another. 

 To experiment with this conception as a guide, men 

 needed a definite and suitable representation of the 

 nature of heat. This conception was at hand in 

 the theory that heat might be a subtle, invisible, 

 weightless fluid, passing between the particles of bodies 

 with perfect freedom. It was in the light of this 

 theory that Black discovered and investigated the 

 phenomena of the latent heat required to produce a 

 change of state from solid to liquid, or liquid to gas, 

 and the specific heat needed to raise the temperature 

 of a substance. He thus established the science of 



