198 HISTORY OF CHEMISTRY. 



difficulty by referring to processes and analogies, as 

 well as to substances, bring us back to the same 

 point in a circle of fallacies. If we say that an 

 acid and alkali are known by combining with each 

 other, we still must ask, What is the criterion that 

 they have combined f If we say that the distinc- 

 tive qualities of metals and earths are, that metals 

 become earths by oxidation, we must still inquire 

 how we recognize the process of oxidation? We 

 have seen how important a part combustion plays 

 in the history of chemical speculation ; and we may 

 usefully form such classes of bodies as combustibles 

 and supporters of combustion. But even combustion 

 is not capable of being infallibly known, for it 

 passes by insensible shades into oxidation. We can 

 find no basis for our reasonings, which does not 

 assume a classification of obvious facts and qualities. 

 But any classification of substances on such 

 grounds, appears, at first sight, to involve us in 

 vagueness, ambiguity, and contradiction. Do we 

 really take the sensible qualities of an acid as the 

 criterion of its being an acid? for instance, its sour- 

 ness? Prussic acid, arsenious acid, are not sour. 

 "I remember," says Dr. Paris 1 , "a chemist having 

 been exposed to much ridicule from speaking of a 

 sweet acid, why not?" When Davy had discovered 

 potassium, it was disputed whether it was a metal ; 

 for though its lustre and texture are metallic, it is 

 so light as to swim on water. And if potassium be 

 1 Life of Davy, i. 263. 



