IDEAS OF PHYSICAL CHEMISTRY 9 



thing more tangible and material than energy, but this is be- 

 cause we confound substance with matter. In fact, we are 

 no more acquainted with matter than with energy. We know 

 matter by its reaction with energy, and energy by its re- 

 action with matter. Since one thus defines and differentiates 

 the other, we are moving in a vicious circle, and must pretty 

 well despair of reaching, upon physical grounds, a perfect 

 view of the real essence of either. Matter must, however, be 

 defined for our purposes, and let us therefore frankly regard 

 it as Aristotle did, the v\r} 9 or stuff, from which the Universe 

 is shaped as is the house from timber, or pottery from clay. 



Whether this ultimate matter is homogeneous or not, we 

 have no means of knowing, but our instinctive love of sim- 

 plification leads us to suppose that it is homogeneous and one. 

 This, however, is a purely hypothetical assumption. When 

 we make an impartial study of fact, we find that by com- 

 bining certain portions of matter with certain portions of 

 energy, we obtain what is known as substance, and this may 

 appear in infinitely different forms. It will seem odd to you 

 that I should define substance as a combination of matter 

 and energy, but a little reflection will show you that this view 

 is quite admissible. All substances show certain qualities 

 which are referable to energy, and may be considered as a 

 combined phenomenon of matter and energy. Thus we are 

 almost unable to imagine substance without the energy of 

 gravitation, of motion, of chemical affinity. 



Modern physics teaches us that energy is one, inasmuch as 

 each form of energy is convertible into every other form, in 

 certain definite quantitative ratios. The instinct of the al- 

 chemists told them that all matter was one, and each form 

 was convertible into every other form. If this were the 

 modern aim of chemistry, we should be obliged to confess 

 that very little has been accomplished as yet. We have 



