During the winter session I had occasion to prepare several supersaturated solutions for lecture demonstration. Among them, was a solution made by fusing crystals of sodium thiosulphate. Next day this solution had deposited a considerable quantity of crystals, but had not solidified. The crystals were separated from, the liquid, washed, dried, and analysed. Their composition agreed approximately with the formula Na2S2O3, 2H2O. Other crystals having this composition were then obtained by driving off part of the water from fused crystals of the pentahydrate, and allowing the solution to cool.

They were analysed by conversion into sodium sulphate.

I.—1·2186 gm. substance gave 0·9027 gm. Na2SO4= 24·02 percent. Na.

II.—1·3977 gm. substance gave 1·0320 gm. Na2SO4 = 23·95 percent. Na.

In order to find out the temperature condition for the formation of this hydrate, a solubility curve was constructed from the data given by Mulder and by Kremers. There appeared to be a change in the curve between 50° C. and 60° C., but the data were insufficient to fix the temperature with accuracy.

I therefore made a series of solubility determinations, extending, from 0°C. to 100°C. The form of apparatus used was that described by van't Hoff. The stirrer was driven at about 3000 revolutions a minute by means of an electric motor; and the temperatures were constant to 0·1°C. Below 50°C. the solutions were in contact with Na2S2O3, 5H2O; above 50°C. in contact with Na2S2O3, 2H2O; and at 50°C. solutions were prepared in contact with each of them, and found to have the same concentration.