ON SUPERSATURATED SOLUTIONS. T33 



I took a solution of the sulphate which had deposited abundance 

 of the modified 7-atom salt, but this latter had dissolved again 

 owing to the warmth of the weather. I cleaned a glass plate 

 carefully and rubbed a little oil over a large surface, leaving it 

 moderately thick at one side and then graduating in thinness till 

 the other side was rubbed quite dry. Drops of the solution on the 

 thick and thin films and on the unoiled parts all remained liquid j 

 I rubbed one hard with my finger till quite dry ; fresh drops on 

 this surface remained liquid in every case. Repeated this on 

 various parts of the plate. One of the drops crystallised sponta- 

 neously and two others while being rubbed, but the great majority 

 remained liquid and evaporated, giving the 7-atom salt. These 

 experiments were not made in my laboratory, because the air 

 there would almost certainly have introduced something which would 

 have caused crystallisation. I have done a great deal of work there 

 with that salt, and it is quite possible that the dust contains some 

 crystals. But to continue, — I then put some earth which had 

 been exposed to the air in my laboratories for 9 hours into a good 

 many of the drops, some of which were depositing the 7-atom salt at 

 the time ; always inactive ; when these were nearly quite dry they 

 were inactive to fresh drops of the solution. I then put some of the 

 earth into the mother liquor in the test tube ; left all these exposed 

 to the air all night in my bedroom ; next morning the drops were 

 quite dry 3 at 11.30 I put fresh drops on the oily part, on the 

 unoiled part, on the smears, on the earth and modified crystals 5 

 one drop out of four crystallised on the oil ; none of the rest 

 except two put purposely on the two smears, which had crystallised 

 the night before ; these crystallised instantly. The fresh drops on 

 the plate gave the 7-atom salt, and also the amhydrous salt in 

 octohedra by evaporation exposed to air. The day being cool the 

 mother-liquor deposited the 7-atom salt on the earth. During the 

 next two days similar results were obtained. The test tube with 

 the solution was then carried into my laboratory at night ; it 

 remained liquid till next day, making three days that it had been 



