MODES OF DECOMPOSITION OF PERCHLORIC ACID. 353 



in the presence of acids, sodium amalgam in the presence 

 of pure acidulated or alkaline water, or by electrolysis. Per- 

 chloric acid and perchlorates in solution are even as stable as 

 sulphates. 



The hydrates HC1O 4 , 2H 2 O (liquid) and even HC10 4 , H 2 

 (crystallised), hydrates whose heat of solution only rises to 

 -f 5-3 Cal. for the former, and + 7*7 Cal. for the latter, 1 seem 

 to be scarcely more active than the diluted acid itself, from 

 determinations made with hydriodic gas, sulphurous acid, and 

 arsenious acid. 



Monohydrated perchloric acid acts quite differently ; which 

 can be explained by the fact that it also liberates -f- 20*3 Cal. 

 in its solution. When brought into contact with oxidisable 

 bodies it sometimes remains almost inactive just as in the case 

 of nitric acid and iron (the passive state) ; on the other hand, it 

 sometimes attacks them suddenly and with explosive violence. 

 It causes the ignition of hydriodic acid and sodium iodide ; it 

 attacks arsenious acid, etc,, very energetically. With hydro- 

 genised bodies the formation of water modifies the action by 

 converting a portion of the acid into a higher hydrate. 

 Arsenious acid does not present this disadvantage ; it produces 

 an oxychloride, intermediate between this body and arsenic 

 acid, and to which reference has already been made while 

 treating of the reciprocal displacements of oxygen and halogen 

 bodies. 2 Nevertheless, it has been impossible to utilise this 

 reaction for calorimetric measurements, even by dissolving these 

 products in soda, owing to the uncertain constitution of the 

 arsenic acid formed, which presents differences similar to those 

 of the several phosphoric acids. Hence it is that the saturation 

 of arsenic acid by soda liberates much less heat than that of 

 normal arsenic acid. It is this fact that interferes with all the 

 calculations. 



7. It is only necessary to cite the following figures, which 

 show the multiplicity of the simultaneous modes of decom- 

 position of perchloric acid. 3 1*175 grm. of this acid, in 

 presence of a great excess of arsenious acid, was distributed in 

 the following manner: 0*264 grm. yielded all its oxygen (0 4 ) 

 to the arsenious acid ; 0*139 grm. was destroyed in HC1 

 -f 4 , which fixed itself on the same acid ; 0*145 grm. in C1 2 

 + 7 (fixed on the arsenious acid) -f H 2 ; 0*645 grm. left 

 unchanged. 



According to a special determination made, only a few 

 milligrammes formed chlorous acid. 



The heat liberated in the combination of perchloric acid with 

 various bases, at 18, was measured. 



1 About + 11*7 in the liquid state. 



2 " Annales de Chimie et de Physique," 5" serie, torn. xv. p. 211. 



3 See pp. 6, 7. 



2 A 



