ENERGY IN GENERAL. 63 



there are any explanations other than mechanical. 

 The illustrious English physicist, Lord Kelvin, does 

 not seem willing to admit this. "I am never satisfied," 

 he said, in his Molecular Mechanics, " until I have 

 made a mechanical model of the object. If I can 

 make this model, I understand ; if I cannot, I do not 

 understand." 



This tendency of so vigorous a mind to be con- 

 tent only with mechanical explanations, has been 

 that of the majority of scientific men up to the 

 present day, and from it has arisen the scientific idea 

 of matter. 



What is matter, in fact, to the student of mechanics? 

 It is mass. All mechanics is constructed of masses 

 and forces. Laplace said: " The mass of a body is the 

 sum of its material points." To Poisson, mass is the 

 quantity of matter of which a body is composed. 

 Matter is therefore confused with mass. Now, mass 

 is the characteristic of the motion of a body under the 

 action of a given force ; it defines obedience or resist- 

 ance to the causes of motion ; it is the mechanical 

 parameter; it is the co-efficient proper to every mobile 

 body; it is the first invariant of which a conception 

 has been established by science. 



In fact, the word matter appears to be used in 

 other senses by physicists, but this is only apparently 

 so. They have but broadened the idea of the 

 mechanicians. They have characterized matter by 

 the whole series of phenomenal manifestations which 

 are proportional to mass, such as weight, volume, 

 chemical properties — so that we may say that the 

 notion of matter does not intervene scientifically with 

 a different signification from that of mass. 



Two kinds of Matter. Ponderable and Imponder- 



