250 REPORTS ON THE STATE OF SCIENCE.—1914. 
On Salts Coloured by Cathode Rays. 
By Professor E. GOLDSTEIN. 
[Ordered, on behalf of the General Committee, to be printed in extenso.] 
Preruars a part of the phenomena which I am about to discuss is 
already familiar to you all. I shall not bring forward many hypo- 
theses. So you will perhaps ask why I should speak at all. And, 
in fact, apart from reference to certain facts not published hitherto, 
my intention is mainly to invite the interest of men younger and abler 
than myself in a class of phenomena which seem to constitute a new 
condition of matter, but on which very few have yet worked. 
If cathode rays fall on certain salts—for example, common salt, or 
chloride of potassium, or potassium bromide—vivid colours are pro- 
duced immediately on these salts.1 Thus common salt becomes 
yellow-brown (like amber), potassium chloride turns into a beautiful 
violet, potassium bromide becomes a deep blue colour quite like copper 
sulphate. Here you see a specimen of common salt transformed in 
this way on the surface of the single crystals into a yellow-brown 
substance. I show also sodium fluoride, which takes a fine rosy 
colour. 
The colours so acquired in a very small fraction of a second may 
be preserved for a long time, even for many years, if the coloured 
substances are kept in the dark and at low temperatures. But in 
the daylight, and also under heat, the colours will gradually disappear 
till the original white condition is reached again. 
The colours of different salts are sensitive to heating in a very 
different degree. I could show you the yellow sodium chloride, pre- 
pared some months ago in Europe, but I cannot show you here the 
violet KCl and the blue KBr, because these colours, even in the dark, 
do not stand the heat of the Equator. The same salt, if dissolved, 
may keep very different colours, according to the medium in which 
it has been dissolved, even when the pure medium itself cannot be 
coloured at all by cathode rays. I am speaking of solid solutions, 
produced by fusing a small quantity—for instance, of common salt 
or of certain other alkali salts—together with a great mass of a 
salt which remains itself colourless in the cathode rays, as, for example, 
the pure potassium sulphate. Lithium chloride acquires a_ bright 
yellow colour in the cathode rays; but if dissolved in potassium sul- 
phate a lilac hue is produced, as you may see in this specimen. Like- 
wise the pure carbonate of potassium acquires a reddish tint, but 
after dissolving it in the potassium sulphate it becomes a vivid green 
in the cathode rays, as you see here. 
Very small admixtures are sufficient to produce intense colours. 
So soy Of carbonate will produce the green colour in the potassium 
1 E. Goldstein, Wiedem. Ann. 54,371; 60,491; Phys. Zeitschr. 8, 149 ; Sitzungsber, 
Berl. Akad. d. Wiss. 1901, 222. 
