480 PRINCIPLES OF CHEMISTRY 



which fumes in the air and has a sp. gr. 1'78 at 15 (sometimes, after 

 being kept for some time, it decomposes with a violent explosion). It 

 explodes violently when brought into contact with charcoal, paper, 

 wood, and other organic substances. If a small quantity of water be 

 added to this hydrate, and if it be subjected to cold, then a crystallo- 

 hydrate, ClHO 4 ,H 2 Oj separates out. It is much more stable, but the 

 liquid hydrate HC1O 4 ,2H 2 O is still more so. The acid dissolves in 

 water in all proportions, and its solutions are distinguished for their 

 stability. 46 When ignited both the acid and its salts are decomposed, 

 with the evolution of oxygen. 47 



distils over below 100, and into the liquid hydrate HC1O 4 ,2H. 2 O. The acid HC1O 4 may 

 also be obtained by adding one-fourth part of strong sulphuric acid to potassium chlorate, 

 and carefully distilling and subjecting the crystals of the hydrate HC1O 4 ,H 2 O obtained 

 in the distillate to a fresh distillation. A liquid of the composition HC1O 4 then passes 

 over. Perchloric acid, HClO 4 ,when taken separately, does not distil, and is decomposed 

 in distillation until the more stable hydrate HClO 4 ,H.jO is formed ; this is decomposed in 

 distillation into HC1O 4 and HClO 4 ,2HoO, which latter hydrate distils without alteration. 

 This forms an excellent example of the influence of water on stability, and of the pro- 

 perty of chlorine to give compounds of the type C1X 7 , of which all the above hydrates, 

 C10 5 (OH), C1O 2 (OH) 3 , and C1O(OH) 5 , are members. Probably further research will lead 

 to the discovery of a hydrate C1(OH) 7 . 



46 According to Roscoe the sp. gr. of perchloric acid = 1*782 and of the hydrate 

 HClO 4 ,HoO in a liquid state (50) 1*811 ; hence a considerable contraction takes place in 

 the combination of HC1O 4 with HoO. 



47 The decomposition of salts analogous to potassium chlorate has been more fully 

 studied in recent years by Potilitzin and P. Frankland. Professor Potilitziii, by decom- 

 posing, for example, lithium chlorate LiClO 3 , found (from the quantity of lithium chloride 

 and oxygen) that at first the decomposition of the fused salt (368) is accomplished 

 according to the equation, 8LiClO 3 = 2LiCl + LiClO 4 + 5O, and that towards the end the 

 remaining salt is decomposed thus: 5LiClO 3 = 4LiCl + LiClO 4 -f 10O. The phenomena 

 observed by Potilitzin obliged him to admit that lithium perchlorate is capable of de- 

 composing simultaneously with lithium chlorate, with the formation of the latter salt and 

 oxygen ; and this was confirmed by direct experiment, which showed that lithium chlorate 

 is always formed in the decomposition of the perchlorate. Potilitzin turned particular 

 attention to the fact that the decomposition of potassium chlorate and of salts analogous 

 to it, although exothermal (Chapter III. Note 12), not only does not proceed by itself, but 

 requires time and a rise of temperature in order to attain completion, which again shows 

 that chemical equilibria are not expressed by the heat effects of reactions only. 



P. Frankland and J. Dingwall (1887) showed that at 448 (in the vapour of sulphur) 

 a mixture of potassium chlorate and pounded glass is decomposed almost in accordance 

 with the equation 2KC1O 3 = KC1O 4 + KC1 + O.,, whilst the salt by itself evolves about half 

 as much oxygen, in accordance with the equation, 8KC1O 3 = 5KC1O 4 + 3KC1 + 2O 2 . The 

 decomposition of potassium perchlorate in admixture with manganese peroxide proceeds 

 to completion, KC1O 4 = KC1 + 2O 2 . But in decomposing by itself the salt at first gives 

 potassium chlorate, approximately according to the equation 7KC1O 4 = 2KC10 3 + 5KC1 

 + 11O. 2 . Thus there is now no doubt that when potassium chlorate is heated, the per- 

 chlorate is formed, and that this salt, in decomposing also with the evolution of oxygen, 

 gives the former salt. 



I may further remark that the decomposition of potassium chlorate as a reaction 

 evolving heat from this very reason easily lends itself to the contact action of manganese 

 peroxide and other similar admixtures ; for such very feeble influences as those of contact 

 may evince themselves, as is observed either in those cases (for instance, detonating gas, 



