493 



experimental investigation of this question, with other allied matters, 

 the suhject of a future communication. 



III. "On Perchloric Acid and its Hydrates." By HENRY 

 ENFIELD ROSCOE, B.A., Ph.D., Professor of Chemistry in 

 Owens College, Manchester. Communicated by Professor 

 A. W. WILLIAMSON. Received December 12, 1861. 



Stadion* in the year 1816 showed that perchlorate of potassium 

 contains 45' 92 per cent, of oxygen, and that its composition is 

 therefore represented by the formula C1O 4 K (requiring 46'17 per 

 cent, of oxygen). Mitscherlichf and Serullas in 1830, and Marig- 

 nac in 1841, confirmed this result, their experiments respectively 

 showing that the salt in question contains 46'06, 46'20, and 46' 17 

 per cent, of oxygen. 



Upon these determinations is based nearly all the knowledge we 

 possess of the quantitative relations of perchloric acid. The anhy- 

 dride, C1 2 O 7 , has not been isolated ; no analyses of the hydrated acid 

 itself have been published, and the composition of only one or two 

 of its salts has been ascertained. Our acquaintance even with the 

 general characters of this substance is also most limited ; and we can 

 only account for the neglect with which chemists have treated the 

 highest and yet the most stable of the oxides of chlorine by the fact 

 that the preparation of the acid in large quantity has hitherto been 

 attended with great difficulties. 



In the following communication I have to detail the results of ex- 

 periments which I believe somewhat enlarge our views respecting the 

 nature and properties of this interesting acid. 



The first point to which attention was naturally directed, was the 

 best mode of preparing the pure aqueous perchloric acid in quantity. 

 After trial of a great number of methods, the following modification 

 of that recommended by Serullas was adopted as yielding the best 

 results. A large quantity of a saturated solution of hydrofluosilicic 

 acid was prepared by heating dry sand and fluor-spar with more than 

 the equivalent quantity of twice-rectified oil of vitriol in large stone- 

 ware bottles, and leading the gaseous fluoride of silicon with the usual 



* Gilbert's Annalen, lii. pp. 197 and 339. f Pogg. Ann. xxv. p. 287. 



J Ann. de Ch. et de Phys. xlv. p. 270. Ann. Ch. Pharm. xliv. p. 11. 



VOL. XI. 2 N 



