64 Proceedings of Royal Society of Edinburgh. [sess. 
oxidising power was estimated by titration with ferrous sulphate 
and potassium permanganate. The extra oxygen thus found was 
5*92 per cent, (equal to 35*5 of S0 4 ). Theory requires 5’93 (35*6 
of SO*). 
I have since prepared a considerable quantity of the salt by 
electrolysing a solution of potassium hydrogen sulphate in a 
divided cell. After some hours the persulphate crystallises out 
from the liquid surrounding the anode. 
Potassium persulphate dissolves fairly readily in water at the 
ordinary temperature, easily in hot water. If the solution be 
boiled, especially if it is acid, decomposition with evolution of 
oxygen occurs. By solution in warm water, and cooling, the salt 
can be recrystallised in prisms resembling those of potassium per- 
manganate, with which the persulphate is evidently isomorphous. 
Dr James Walker has kindly determined the electric conductivity 
of the solution, and his results show that the formula is KS0 4 , the 
solution behaving in a manner comparable to one of potassium 
perchlorate (which is also isomorphous with the permanganate). 
The solution of the pure salt is neutral to litmus, and appears to 
be stable at ordinary temperatures. It gives no precipitate with 
solution of barium salt. With silver nitrate it gives no immediate 
precipitate, but what appears to be silver peroxide separates out on 
standing. When mixed with potassium iodide solution, iodine is 
liberated only gradually, but more quickly on heating. The solu- 
tion is not decomposed by peroxide of hydrogen. It is attacked by 
ferrous sulphate in the cold, ferric and potassium sulphates being 
produced. If some of the solid substance be added to a small 
quantity of strong ferrous sulphate solution, as the salt dissolves 
the green colour changes to brown, and the liquid becomes warm. 
When the solid is gently warmed with strong nitric or sulphuric 
acid, oxygen highly charged with ozone is evolved. Hydrochloric 
acid gives chlorine. 
The properties of the salt have been as yet but superficially 
examined, and no attempt has been made to prepare other persul- 
phates. I am, however, engaged in a fuller investigation of the 
subject, and also in examining the behaviour of salts of other acids 
when electrolysed in a divided cell. 
