Chemistry and Physios. 69 



nates there is mostly direct isomorphism ; between sulphates and 

 chroniates, isodimorphism is common ; while between sulphates 

 and molybdates or tungstates it is the rule. — Zeitschr. physikal. 

 Chem., viii, 6, July, 1891 ; J. Chem. Soc, lx, 1151, October, 

 1891. g. r. b. 



2. On Crystallized Persidphates. — In 1878, Berthelot obtained 

 persulphuric oxide S 2 7 by subjecting a well cooled mixture of 

 sulphurous oxide and oxygen to the silent electric discharge, and 

 persulphuric acid by electrolyzing strong sulphuric acid.* Mar- 

 shall, in the course of an investigation on the oxidation of 

 double cobalt salts by electrolysis, observed the production of 

 colorless crystals in the electrolytic cell, which at first were sup- 

 posed to be potassium sulphate ; but which gave only a slight 

 precipitate with barium chloride, until the. solution was heated 

 and were therefore persulphate. To prepare the persulphate in 

 larger quantity, a saturated solution of hydrogen potassium sul- 

 phate was electrolyzed, the kathode being contained in sulphuric 

 acid placed in a porous cell. On passing a current of 3 to 3-J 

 amperes for two days, a white crystalline deposit began to form, 

 which was separated from the solution, drained and dried on 

 porous plates ; the process being repeated with the mother-liquor 

 until sufficient material was collected. By solution in hot distilled 

 water and cooling, the salt is deposited in minute prisms, the 

 decomposition being inconsiderable. By slow cooling large tab- 

 ular crystals are formed, apparently asymmetric. On analysis, 

 the formula KS0 4 was obtained, the size of the molecule being- 

 determined from the conductivity of the solution. Potassium 

 persulphate is sparingly soluble in cold water, 100 parts of water 

 at 0° dissolving 1-75 parts of KSO^ Its solution gradually de- 

 composes at ordinary temperatures with evolution of oxygen, 

 though slowly; a pure solution even after three months con- 

 taining the greater part of the persulphate undecomposed. By 

 similarly electrolyzing hydrogen ammonium sulphate, ammonium 

 persulphate was obtained, in the form of long thin prisms, appar- 

 ently rhombic, which on spontaneous evaporation, may become 

 large, transparent lozenge-shaped monosymmetric tables. It is 

 very soluble in water, 100 parts at 0° dissolving 58*2 parts of the 

 salt. In consequence of this solubility, this salt was used in the 

 preparation of the other persulphates. By treating a saturated 

 solution of ammonium persulphate with barium hydrate, bariurn 

 persulphate is produced, which after re-crystallizing at a low 

 temperature, is obtained in beautiful prisms ; or when the mother- 

 liquor is concentrated in a vacuum, in large interlocking prisms 

 having a monosymmetric appearance. This barium salt has the 

 formula Ba(S0 4 ) 2 . (H 2 0) 2 . It is very soluble in cold water, 100 

 parts dissolving 39- 1 parts of the anhydrous or 52-2 parts of 

 the crystallized salt. Lead persulphate was obtained as a solid 

 crystalline mass, very deliquescent and having the probable for- 



* This Journal, III, xv. 209, March, 1878. 



