306 HISTORY OF CHEMISTRY. 



bodies as combustibles and supporters of combustion. But even com- 

 'justion is not capable of being infallibly known, for it passes by insen- 

 sible shades into oxidation. We can find no basis for our reasonino-s. 



O ' 



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 sourness ? 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 allowed to be a metal, is silicium 

 one, a body which wants the metallic lustre, and is a non-conductor of 

 electricity ? It is clear that, at least, the obvious application of a classi- 

 fication by physical characters, is attended with endless perplexity. 



But since we cannot even begin our researches without assuming a 

 classification, and since the forms of such a classification which first 

 occur, end in apparent confusion, it is clear that we must look to our 

 philosophy for a solution of this difficulty; and must avoid the embar- 

 rassments and contradictions of casual and unreflective classification, by 

 obtaining a consistent and philosophical arrangement. We must 

 employ external characters and analogies in a connected and systematic 

 manner ; we must have Class ificatory Sciences, and these must have a 

 bearing even on Chemistry. 



Accordingly, the most philosophical chemists now proceed upon this 

 principle. " The method which I have followed," says M. Thenard, in 

 his Traite de Chimie, published in 1824, "is, to unite in one group all 

 analogous bodies ; and the advantage of this method, which is that 

 employed by naturalists, is very great, especially in the study of the 

 metals and their compounds." 8 In this, as in all good systems of 

 chemistry, which have appeared since the establishment of the phlogistic 

 theory, combustion, and the analogous processes, are one great element 

 in the arrangement, while the difference of metallic and non-metallic, 

 is another element. Thus Thenard, in the first place, speaks of Oxygen 

 in the next place, of the Non-metallic Combustibles, as Hydrogen, 

 Carbon, Sulphur, Chlorine ; and in the next place, of Metals. But 

 the Metals are again divided into six Sections, with reference, princi- 



1 Life of .Davy, i. 263. " Pref., p. riii. 



