Mr. W. A. Dixon on the Constitution of Acids. 127 



mixtures of baryta and lime by carbonic acid, found that if 

 the ratio of the two oxides in solution be a and in the preci- 

 pitate /3, then a = kj3, and that within certain limits of a the 

 value of k was nearly coustant. By equation (5) it is seen 

 that if only a very small fraction of the material be preci- 



pitated, v=- f fi, or a= — /8; so that the values obtained by 



Debus for k represent approximately the ratio — f for this 

 particular case. 



Chizynski* employed solutions of MgCl 2 and CaCl 2 , to 

 which he added an insufficiency of phosphoric acid for com- 

 plete precipitation, and to the clear acid solution ammonic 

 hydrate, determining the amounts of CaO and MgO in the 

 precipitates with variable quantities of CaCl 2 and MgCl 2 . 

 These experiments, however, do not seem to exhibit fractional 

 precipitation in the true light ; for before the addition of the 

 ammonia the phosphoric acid has probably divided itself 

 between the two bases in a determinate manner, and on neutra- 

 lizing the acid solution the mixed phosphates are precipitated. 



XVI. On the Constitution of Acids. By W. A. Dixon, 

 F.l.C, F.C.S., Instructor in Chemistry, Sydney Technical 

 College, N. S. Wales}. 



IN lecturing on Chemistry the question arose in my mind, 

 Why do some acids form with the alkali metals alkaline 

 hydrogen salts, whilst the similar salts of other acids are acid ? 

 And I wish to bring forward the answer to the question which 

 seems to me probable. 



We know that in organic compounds there are two cases in 

 which hydrogen is readily replaceable by metals. The first is 

 when the hydrogen is in combination with oxygen as hydroxy], 

 as in alcohols, phenols, &c, and that this hydrogen is more 

 difficult to replace than in the second case, which is when the 

 hydrogen is in combination with two oxygen atoms as oxyhy- 

 droxyl. The latter is the mode in which the readily replace- 

 able hydrogen of all the organic acids exists. It seems to me 

 that hydrogen in both these states of combination exists in 

 inorganic acids, and that where both exist in the same acid 

 the hydrogen of the oxyhydroxyl is invariably replaced first. 

 It therefore happens that the principal acid function is in 

 connection with oxyhydroxyl ; and it is somewhat doubtful 

 whether an acid which contains hydroxyl only is not more 



* Ann. Ch. Pharm. Suppl. vol. iv. 1866. 

 t Communicated by Prof. Mills, F.E.S. 



