EMPIRICAL KNOWLEDGE, EXPLANATION, Ac. 181 



Chemistry, although to a great extent an empirical 

 science, has not been without prophetical triumphs. The 

 existence of the metals potassium and sodium was fore- 

 seen by Lavoisier, and their elimination by Davy was one 

 of the chief experimenta crucis which established Lavoi- 

 sier's system. The existence of many other metals which 

 eye had never seen was almost a necessary inference, and 

 theory has not been found at fault. No sooner, too, had 

 a theory of organic compounds been conceived by Pro- 

 fessor A. W. Williamson than he foretold the formation of 

 a complex substance consisting of water in which both 

 atoms of hydrogen are replaced by atoms of acetyle. This 

 substance, known as the acetic anhydride, was afterwards 

 produced by Gerhardt. In the subsequent progress of 

 organic chemistry occurrences of this kind have been mul- 

 tiplied almost indefinitely. The theoretical chemist by 

 the classification of his specimens and the manipulation 

 of his formulas can plan out as it were the creation of 

 whole series of unknown oils, acids, alcohols, and such 

 like products, just as a designer might draw out a multi- 

 tude of patterns. The formation of many such substances 

 is a matter of course, but there is an interesting predic- 

 tion given by Hofmann, concerning the possible existence 

 of new compounds of sulphur and selenium, and even 

 oxides of ammonium, which it remains for the future to 

 verify 6 . 



Prediction by Inversion of Cause and Effect. 



There is one process of experiment which has so often 

 led to important discoveries as to deserve separate de- 

 scription and illustration I mean the inversion of Cause 

 and Effect. Thus if A and B in one experiment produce 

 C as a consequent, then antecedents of the nature of B 



e Hofmann's 'Introduction to Chemistry/ pp. 224, 225. 



