352 OXYGENATED COMPOUNDS OF CHLOKINE. 



the great difference which exists between the reactions of this 

 acid when diluted with water a condition under which it is 

 almost as stable as dilute sulphuric acid and those of the 

 monohydrated acid, which ignites hydriodic acid gas, and acts 

 with explosive violence on oxidisable bodies. This subject will 

 be referred to again later on. 



4 Monohydrated perchloric acid decomposes spontaneously, 

 as Eoscoe observed. It is at first colourless, but it assumes a 

 yellow, then a red and brownish-red colour, and eventually 

 liberates gases which render the containing vessels liable to 

 burst ; this is the more to be feared since the necks of emery 

 flasks soon become stopped up through the formation of crystals 

 of the second hydrated perchloric acid. 



The acid which has suffered a partial decomposition is not 

 suitable for measuring the heat of hydration on account of its 

 becoming less and less considerable, owing to the formation of 

 water which accompanies this decomposition. Notwithstanding 

 this formation of water, the acidimetric value of the acid 

 referred to the equivalent weight of perchloric acid does not 

 fall, and it may even apparently increase a little, since the 

 lower oxygenated acids of chlorine have an equivalent less than 

 that of perchloric acid. This is a cause of error which should 

 be noted. 



5. A similar decomposition is produced under the influence 

 of heat, and prevents the re-distillation of perchloric acid. It 

 takes place in the very conditions of its preparation from 

 potassium perchlorate and sulphuric acid, as is shown by the 

 constant liberation of chlorine which accompanies the dis- 

 tillation. It appears that the monohydrated acid cannot be 

 separated unless carried away in a current of gas ; it is only 

 obtained in small quantities. This depends on the fact that the 

 decomposition of perchloric acid liberates heat. Even in its 

 preparation from potassium perchlorate and concentrated sul- 

 phuric acid, the reaction, when once started by the action of an 

 external source of heat, continues of itself, after the removal of 

 this source, and sometimes with a violence sufficient to give 

 rise to an explosion. This fact proves that the reaction is 

 exothermal. At the same time, chlorine and oxygen are 

 liberated, which carry away the perchloric vapour and render 

 its condensation difficult. 



6. Some details may now be given respecting the oxidising 

 characters exhibited by perchloric acid. 



In dilute solution, this acid is not reduced by any known 

 body. No action is caused by sulphurous, hydrosulphuric, 

 hydrosulphurous, 1 or hydriodic acid, by free hydrogen, zinc 



1 It was especially proved by accurate weighing, that this acid, which was 

 recently declared capable of reducing perchlorates, does not act in reality 

 except on the small quantities of chlorates often contained in perchlorates. 



