28 A Century of Science 



the manifestations of force which we group to- 

 gether under the name of electricity, are various 

 modes of undulatory motion transformable one 

 into another ; and that, in the operations of na- 

 ture, energy is never annihilated, but only changed 

 from one form into another. This generalization 

 includes the indestructibility of matter, and thus 

 lies at the bottom of all chemistry and physics and 

 of all science. 



Returning to that chemistry with which we 

 started, we may recall two laws that were pro- 

 pounded early in the century, one of which was 

 instantly adopted, while the other had to wait for 

 its day. Dalton's law of definite and multiple pro- 

 portions has been ever since 1808 the corner stone 

 of chemical science, and the atomic theory by 

 which he sought to explain the law has exercised 

 a profound influence upon all modern speculation. 

 The other law, announced by Avogadro in 1811, 

 that, " under the same conditions of pressure and 

 temperature, equal volumes of all gaseous sub- 

 stances, whether elementary or compound, contain 

 the same number of molecules," was neglected for 

 nearly fifty years, and then, when it was taken up 

 and applied, it remodelled the whole science of 

 chemistry, and threw a flood of light upon the in- 

 ternal constitution of matter. In this direction a 



