ON SALTS COLOURED BY CATHODE RAYS. 251 
which you will detect the nature of the different small ad- 
mixtures which adhere to the pretended pure preparations of the 
different factories. In this way a new analytical proof, much more 
sensitive than the ordinary chemical methods, is obtained, and im- 
purities may be detected even when a certain specimen of salt contains 
more than a single impurity, because the colours produced by different 
admixtures generally disappear with different speed in the daylight or 
under rise of temperature. For instance, the ordinary potassium 
sulphate turns to a dark gray with a slight greenish tint at first. 
After a short while the very sensitive gray will disappear, simply under 
the ordinary temperature of the laboratory room, and a vivid green 
comes out. The gray hue indicates a very small amount of sodium 
chloride, ;54555 or so, and the remaining green indicates the admix- 
ture of a carbonate. Here are some preparations of potassium sul- 
phate each containing a single small admixture (K,CO,, Li,CO,, 
LiCl, KCl, KBr). You will notice how different are the colours of 
the originally white substance, varying from green to bluish gray, ash- 
gray, grayish blue, and violet. 
By fractional crystallisation one may finally get a really pure pre- 
paration of potassium sulphate, which is no longer coloured by 
cathode rays (or only in a very slight degree, indicating minimal traces 
of sodium chloride). But there are other preparations which, so far 
as I know, cannot be acquired in pure condition by any means, not 
even by fractional crystallisation. I never came across a pure sodium 
sulphate—the purity exists only on the manufacturers’ labels. Even 
the best preparations of this salt contain an amount of sodium car- 
bonate which up to the present cannot be separated from it, not 
even by frequent fractional crystallisation. The colour produced by 
the small admixture, which always remains, is a very marked ash- 
gray. By an intentional further addition of sodium carbonate the 
colour becomes nearly black. 
The question arises: What may be the cause of these colourations 
in pure salts and also in solid solutions of them? Shortly after the 
colours of the alkali salts had been discovered, an explanation was 
given’, according to which the phenomenon mainly consists in a 
chemical reduction. For instance, in the case of potassium chloride 
the chlorine would be set free, while the remaining potassium is dis- 
solved in the unaltered main quantity of the salt, colouring it at 
the same time. And it seemed a convincing proof for this theory 
when Giesel® and also Kreutz, simply by heating rock salt in the 
vapours of sodium or of potassium, produced colours in this rock 
salt quite similar to those produced by cathode rays. It seemed that 
2 EK. Wiedemann and G. C. Schmidt, Wied. Ann. 54, 618. 
8 ¥, Giesel, Ber. D. Chem. Ges. 80, 156. 
