and chiefly on the use of the word Acid. 265 



media vel composita. The separation of acids from neutral and 

 alkaline salts, and their recognition as a distinct class, resulted 

 chiefly from Lavoisier's discovery of the presence of oxygen in 

 several of the best-known acids, and his consequent conclusion 

 that they were in reality a particular class of oxides*. This view 

 was of course applicable with strictness only to the anhydrous 

 acids, but it did not for a good while occasion any difficulty in 

 the application of the word acid : the essential nature of the dif- 

 ference between the bodies since known as anhydrides, and the 

 bodies formed by their union with water, although insisted upon 

 by Davy, was not generally recognized until experiment had 

 shown how different was the behaviour of these two sets of sub- 

 stances respectively with ammonia and almost all organic com- 

 pounds. But these investigations had been pursued only a very 

 little way before it became evident, to all who took the trouble to 

 examine the question, that it was utterly illogical and unphilo- 

 sophical to continue to designate by the same name bodies which 

 differed so essentially from each other as did many of those which 

 had been hitherto classed indiscriminately as acids. Laurent and 

 Gerhardt, whose teaching contributed more than that of any 

 other chemists to set this point in a clear light, got rid of the in- 

 consistency by strictly confining the application of the name acid 

 to salts of hydrogen, such as SO 4 H 2 , and calling the bodies 

 (actually or conceivably) produced from these by loss of water, 

 such as SO 3 , anhydrides. In retaining the term acid for SO 4 H 2 

 and its analogues, rather than for SO 3 and its analogues, they 

 undoubtedly followed the prevailing usage of chemical language ; 

 for though the latter class of bodies were described as acids in all 

 systematic works on chemistry, yet in perhaps ninety-nine cases 

 out of a hundred, when an acid was spoken of as taking part in 

 or resulting from a reaction, it was a hydrogen-salt, and not an 

 anhydrous acid, that was meant. Hence the course which they 

 adopted obviously involved a much smaller departure from esta- 

 blished usage than the choice of the other alternative would have 

 done ; and if this was true twenty years ago, it is true to so much 

 greater an extent now that the reversal of Laurent and Gerhardt' s 

 system would necessitate the rewriting of almost the whole of 

 organic chemistry. 



Hence it appears that, so far from the original meaning of the 

 word acid having been clear, it would be more correct to say that 

 this word was never used in a strictly scientific and logical sense 

 at all, until Gerhardt defined acids to be salts whose base was wholly 

 composed of hydrogenf. 



* See particularly his Traite elementaire de Chimie, vol. i. p. 163. 

 f Precis de Chimie Organique (Paris, 1844), vol. i. p. 70; Introduction 

 a V etude de la Chimie par le Systeme Unitaire (Paris, 1848), p. 103. 

 Phil. Mag. $. 4. Vol. 29. No. 196. April 1865. T 



