DECOMPOSITION OF CYANIDES AND CHLORIDES. 321 



exists in the solid salts, if we always reckon from the diluted 

 hydracids. These latter being monobasic, the thermal inequality 

 that has been just mentioned indicates that dilute hydrocyanic 

 acid must entirely displace hydrochloric acid from its combina- 

 tion with mercuric oxide. 



Here is an experiment that fully bears out this supposition : 



/ [Hg(CN) 2 (1 eq. = 16 litres) + 2HC1 (1 eq. = 4 litres)] + 0\M - M t = + 5-9 

 \|[HgCl 2 (1 eq. = 16 litres) + 2HCN (1 eq. = 4 litres)] + 5'9 /calculated + 6'0 



The reaction is all the more remarkable, as, according to 

 thermal observations made, the dilute hydrochloric acid 

 completely displaces the hydrocyanic acid in dissolved potassium 

 cyanide. It was, moreover, easy to foresee that this would be 

 the case in the last instance, for 



i[2HCN (in solution) + K 2 (diluted)] gives off + 2'96) M M . ., 

 J[2HC1 (in solution) + K 2 (diluted)] + 13'59f M i~ 



The decomposition of dissolved mercuric chloride by dilute 

 hydrocyanic acid is all the more remarkable from the fact 

 that solid mercuric cyanide is decomposed by concentrated 

 hydrochloric acid ; it is in this way that pure hydrocyanic 

 acid is prepared. But this decomposition the opposite of 

 that which takes place in weak solutions is easily explained 

 by thermal theories. In fact, it is due to the action of the 

 anhydrous hydrochloric acid contained in the liquors, when 

 we are operating without heat ; or formed under the influence 

 of heat, when we proceed by distillation. Now, this anhydrous 

 hydracid possesses, in relation to the hydrate of the same acid, 

 the energy which the latter has lost in forming a definite 

 hydrate ; the magnitude of which energy is sufficient to reverse 

 the reaction. 1 



Moreover, hydrochloric acid gas displaces, immediately and 

 without heat, the hydrocyanic acid gas of crystallised mercuric 

 cyanide. This process for preparing the latter gas has been 

 mentioned. According to calculation, the reaction disengages 

 + 5*2 Cal. Attention 2 has already been called to these two 

 reactions and their mechanism, which is frequently met with 

 under other circumstances, such as when we are comparing the 

 reactions of concentrated .acids or alkalis with those of the 

 same acids or alkalis diluted. It is the existence of a certain 

 proportion of acid (or alkali), either not combined with water 

 or combined in the state of a less advanced hydrate in the 

 concentrated liquids, and also the formation of such an acid, 

 dehydrated under the influence of heat, that causes the inverse 

 reaction ; and this is in proportion to the excess of energy that 

 the anhydrous acid possesses, in comparison with the hydrate of 



1 " Annales de Chimie et de Physique," 5 e se"rie, torn. iv. p. 465, and 4 s 

 se*rie, torn. xxx. p. 494. 



2 " Essai de Me'canique Chimique," torn. ii. p. 547. 



Y 



