CHEMISTRY AND ITS DEVELOPMENT 197 



nological, or Practical chemistry, which deals with the uses 

 of chemistry in the arts and manufactures. 



THE METHODS OF CHEMICAL PHILOSOPHY. Like any 

 other science, chemistry may use two different ways of dis- 

 covering and demonstrating its general principles. On 

 the one hand and this is the surest way a principle may 

 be induced from a large number of experimental observa- 

 tions; it is then nothing but a general statement of a 

 general fact and is termed an empirical law. Thus the prin- 

 ciple of the conservation of matter is an empirical law. 

 Perhaps this law may suggest itself a priori; but as a law 

 of science it has been induced from facts established by 

 the balance. On the other hand, there are problems which 

 cannot be attacked by experiment. Thus the problem of 

 the ultimate structure of matter lies far beyond our power 

 of direct observation; yet it is intimately connected with 

 the correlation of substances, and therefore chemistry is 

 compelled to consider it for purely practical reasons. In 

 cases of this nature, chemistry, like any other science, and 

 like speculative philosophy, makes some plausible assump- 

 tion, termed an hypothesis. Like speculative philosophy, 

 it developes the hypothesis, conbines it, if necessary, with 

 other assumptions, and thus builds up a theory. But at 

 this point, where speculative research reaches its ne plus 

 ultra, the work of the scientist really begins. The general 

 principles forming part of the theory are busily applied to 

 phenomena capable of direct observation, and then, if 

 their correctness is indicated by actual experiment, they 

 become theoretical laws. A scientific theory has for its 

 object, first to correlate seemingly different facts, and, 

 secondly, to throw light on the road of investigation and 

 lead to the establishment of new facts. Thus, the atomic 

 theory has correlated the various substances with regard 

 to their composition and constitution, and it has revealed 

 the existence of innumerable compounds many of which 

 have since been actually prepared an achievement not 

 unlike the discovery of Neptune by theoretical astronomy. 



