LECTURE X.] HISTORY OF CHEMISTRY. 195 



form two (and more) ethers, of which one (or more) is acid, 

 and one is neutral. The molecule of the latter, if it is assumed 

 to correspond to two volumes, contains one alcohol residue in 

 the case of monobasic acids, and two (or several) such residues 

 in the case of dibasic (or polybasic) acids. Further, the amid- 

 compounds, and also the anilid-compounds (discovered a short 

 time previously 61 ) furnish additional evidence. Thus whilst the 

 monobasic acids produce only one amide, one nitrile, and one 

 anilide, the acid ammonium salts of dibasic acids, by the loss 

 of water, give rise to the formation, besides, of an amid-acid 

 and of an imide ; and they alone can yield anilid-acids. 



Laurent had already drawn attention, a few years earlier, to 

 another difference between these substances. 62 According to 

 him, only the formulae of the dibasic and polybasic acids 

 permit the assumption of their containing water, whereas in 

 one molecule of a monobasic acid the constituents of only 

 half a molecule of water require to be present, for which 

 reason, the latter are not able to form anhydrides. 



Thus nitric acid is : HNO 3 = (HOj) + NO 2i ; while 

 sulphuric acid is : H 2 SO 4 = H 2 O + SO 3 . 



With Laurent, hypochlorous anhydride, which was already 

 known at that time, is C1HO in which one atom of hydrogen 

 is replaced by one atom of chlorine. 63 



Laurent's views as to the molecules of the elements were 

 also very important. It was a consequence of Avogadro's 

 hypothesis, with which Laurent identifies himself, 64 that the 

 molecules of certain elementary substances should be regarded 

 as composed of at least two atoms. Laurent tries to support 

 this view by means of chemical reasons. According to him 

 the so-called "dyads," such as hydrogen, chlorine, bromine, 



61 Compare p. 185. 62 Ann. Chim. [3] 18, 266. 63 These views 

 were looked upon as refuted by Gerhardt's discovery of acetic anhydride 

 etc., but strictly speaking, they were not. It was, in fact, an extension of 

 the word anhydride to apply it to the substances discovered by Gerhardt. 

 The relation between C 2 H 4 O 2 and C 4 H 6 O 3 on the one hand is not quite the 

 same as that between C 4 H 6 O 4 and C 4 H 4 O 3 on the other. w He does 

 not seem to be aware, however, that it was first announced by Avogadro. 



