356 OXYGENATED COMPOUNDS OF CHLORINE. 



could not become explosive; whereas, according to the 

 measurements made, the contrary is the case with chlorates. 



The sign of the phenomenon, however, does not seem neces- 

 sarily to change with elevation of temperature ; the molecular 

 heat of potassium perchlorate (2 6 -3), for instance, being smaller 

 than the sum of those of the chloride and oxygen (33'9) ; that is 

 to say that, towards 400, the divergence in absolute value 

 would be increased by about 3 Cal. 



11. The conversion of potassium chlorate into perchlorate is 

 therefore exothermal, as might be foreseen. 



At the ordinary temperature, 



4KC10 3 = 3KC10 4 + KC1 would liberate + 63. 

 This is, moreover, in conformity with the thermal relation 

 already observed between the hypochlorites and chlorates, the 

 latter being more stable than the former, but also being formed 

 with a smaller absorption of heat. 



12* The thermal relations also show that the decomposition 

 of ammonium perchlorate must be explosive, for 



NH 4 C10 4 (solid) = Cl + 2 + N + 2H 2 (liquid) liberates 



+ 58-3 Cal. 



NH 4 C10 4 (solid) = Cl + 2 + N + 2H 2 (water as vapour) 

 liberates + 38'8 Cal. 



With the salt in a melted state we should have, in addition, the 

 heat of fusion. This is verified by experiment. In fact, 

 ammonium perchlorate^ when heated, first melts ; then the 

 liquid becomes incandescent, assuming a spheroidal form; the 

 brilliant bead thus produced is decomposed with great rapidity 

 into free chlorine, oxygen and water, with the production of a 

 yellowish flame. The salt does not, however, detonate; at 

 least when only a small quantity is operated on. These 

 phenomena resemble those of the decomposition of ammonium 

 nitrate (nitrum flammans), but possess rather more intensity. 



13. Reference has been already made to the great heat of 

 solution (+ 20*3) of hydrated perchloric acid HC1O 4 , which is 

 more than double that of all the other monohydrated acids, and 

 is comparable to that of the most powerful anhydrous acids. 

 The greatness of the heat liberated is followed up to the 

 secondary hydrates. 1 That of the second hydrate, 



HC10 4 (liquid) + H 2 (liquid) = HC10 4 , H 2 ; 



liberates (the hydrate being solid) -f- 12*6 Cal. ; and, if con- 

 sidered as a liquid, about + 8*6. The formation of the third 

 hydrate 



HC10 4 , H 2 + H 2 = HC10 4 , 2H 2 (liquid), 



1 With regard to the heat of dilution of perchloric acid, which presents 

 some remarkable peculiarities, see researches made, " Annales de Chimie et de 

 Physique," 5 e se'rie, torn, xxvii. p. 222. 



