498 



the decomposition became more rapid, and a sharp explosion occurred ; 

 the receiver was broken to fragments, whilst the residual liquid in the 

 retort became colourless, and, on cooling, solidified to a crystalline 

 mass which contained 87*76 per cent, of C1O 4 H. I propose to 

 investigate the composition and mode of formation of this singular 

 body at a future time ; it may possibly prove to be the black explosive 

 substance described byMillon* as produced by the action of organic 

 matter on the so-called chloroperchloric acid. Perchloric acid likewise 

 undergoes spontaneous decomposition at the ordinary atmospheric 

 temperature, even when not exposed to light. The liquid, which 

 when freshly prepared has a slight yellow colour, changes to a deep 

 brown colour when kept in the dark for a few days ; and after the 

 lapse of about a fortnight the bulbs containing the acid are found to 

 have exploded. Hence if is impossible to preserve pure perchloric 

 acid for any length of time ; it must be prepared when required from 

 the crystallized acid. 



2. Perchloric Monokydrate, C1O 4 H+H 2 O. 



The first hydrate of perchloric acid is the solid crystalline substance 

 discovered by Serullas, and supposed by some chemists to be the 

 anhydride f. 



It is best prepared in the pure state by carefully adding water to 

 perchloric acid ; great heat is thereby evolved, and the crystals which 

 form on cooling generally possess a slight yellow colour, which rapidly 

 disappears when they are exposed to sunlight. 



By partially melting the crystals several times, and separating the 

 liquefied portions, the residue was found to be in a fit state for ana- 

 lysis, as is seen by the close approximation of the experimental num- 

 bers to the calculated composition in the following determinations. 



Analysis No. 1 . Weight of colourless twice recrystallized acid 

 employed 0'67 10 grm. Treated according to the method already 

 described, this quantity of crystallized acid yielded 0'7820 grm. of 

 perchlorate of potassium, corresponding to 0*6698 grm. C1O 4 H + 

 H 2 G, or to 84-55 per cent, of C1O 4 H. Of this potassium-salt 0-6360 

 grm. lost by heating 0-2935 grm., and required 0-4925 grm. of 

 silver for complete precipitation. 



* Ann. de Chira. et de Phys. ser. 3. t. vii. p. 333. 



f Pelouze et Fremy, Traite de Chimie, "Acide Perchlorique," t. i. p. 418. 



