PERCHLORIC ACID. 351 



This substance presents the same percentage composition as the 

 chlorite. It appears that its formation in the action of chlorous 

 acid on the alkalis must account for the discoloration of the 

 solution which takes place, as is known, after some time. 



3. THERMAL FORMATION OF PERCHLORIC ACID AND ITS SALTS. 



1. The result of the researches made on the oxy acids of 

 chlorine and other halogen elements led to the study of the 

 heat of formation of perchloric acid; the results, which are 

 with great difficulty obtained, demonstrate a certain number of 

 new chemical facts. They show at the same time how thermo- 

 chemistry explains the differences of stability and activity which 

 exist between pure perchloric acid and the same acid when 

 combined with a more considerable quantity of water. 



2. In fact, it is known, principally from the researches of 

 Eoscoe, 1 that there exist several hydrated perchloric acids, 

 namely, monohydrated acid, properly so called, or HC10 4 , a 

 crystallised hydrate, HCIOJIA and a hydrate, HC10 4 2H 2 0, 

 volatile at 200, and partly dissociable, even under the conditions 

 of its distillation. 



These experiments were reproduced; it was even succeeded 

 in obtaining the first acid in the crystallised form. It suffices 

 to take the liquid acid, which contains a slight percentage of 

 water in excess, and to place it in a cooling mixture. The acid 

 becomes crystallised, and the mother-liquid is decanted. It is 

 allowed to liquefy, and then recrystallised, which finally gives 

 an acid fusible at 15, a point of fusion which is still not high 

 enough. Its composition was proved by analysis. It is a body 

 which eagerly absorbs water and emits dense fumes on contact 

 with the air. 



3. The solution of monohydrated liquid HC10 4 , at 19, in one 

 hundred times its weight of water, liberates + 20*3 Cal. This 

 is rather a delicate experiment, owing to the rapidity with 

 which the acid attracts moisture while being weighed, and also 

 to the violence with which it reacts on the water at the time of 

 the calorimetric part of the experiment. The preceding figure 

 is enormous; it exceeds the heat of solution of all ordinary 

 monohydrated acids being, for instance, more than double that 

 of hydrated sulphuric acid, H 2 S0 4 . It is even nearly equal to 

 the heat of solution of anhydrous sulphuric acid (+ 187) and 

 anhydrous phosphoric acid (+ 20'8), which are the highest 

 hitherto known; but they refer to anhydrous bodies. The 

 figure + 20*3 also exceeds the heats of solution of hydracids, 

 although the latter are increased from 6 Cal. to 8 Gal, owing to 

 their gaseous state. 



This enormous heat of hydration of perchloric acid explains 

 3 " Annalen der Chemie und Pharmacie," Band cxxxi. s. 376. 1861. 



