THE ATOMIC VIEW OF NATURE. 389 



more correctly when he says : " To investigate the essence 

 of a natural phenomenon, three conditions are necessary: 

 We must first study and know the phenomenon itself, 

 from all sides ; we must then determine in what relation 

 it stands to other natural phenomena ; and lastly, when 

 we have ascertained all these relations, we have to solve 

 the problem of measuring these relations and the laws of 

 mutual dependence that is, of expressing them in num- 

 bers. In the first period of chemistry, all the powers 

 of men's minds were devoted to acquiring a knowledge of 

 the properties of bodies ; it was necessary to discover, 

 observe, and ascertain their peculiarities. This is the 

 alchemistical period. The second period embraces the 

 determination of the mutual relations or connections of 

 these properties ; this is the period of phlogistic chemistry. 

 In the third period, in which we now are, we ascertain by 

 weight and measure and express in numbers the degree 

 in which the properties of bodies are mutually dependent. 

 The inductive sciences begin with the substance itself, 

 then come just ideas, and lastly, mathematics are called 

 in, and, with the aid of numbers, complete the work." l 



As Galileo, Huygens, and Newton, by a series of bril- 

 liant investigations and theories, such as those of the pen- 

 dulum, the fall of bodies, finally of universal gravitation, 

 established the usefulness of the mathematical treatment 

 of physical phenomena, so Lavoisier and his school proved 

 the correctness and usefulness of their views by the new 5. 



Theory of 



theory of combustion, as consisting in the combination of combustion 

 a special body or element called oxygen with other bodies 



1 ' Familiar Letters on Chemistry,' translated by Blyth, 4th ed., London, 

 1859, p. 60. 



