202, REPORTS ON THE STATE OF SCIENCE.—1914. 
the problem was settled finally. However, it was soon discovered 
that the coloured Giesel salts, although they look to the eye quite 
like the cathode-ray salts, in all other respects behave quite differently. 
For instance :— 
(1) The cathode-ray salts, as I mentioned before, are very sensitive 
to daylight: after an exposure to diffuse daylight of a few minutes—or 
in some salts even of several seconds only—the colouration diminishes, 
whilst the Giesel salts remain unaltered even when they are kept in full 
sunshine for days or even weeks. 
(2) The cathode-ray salts, if dissolved in distilled water, show 
absolute neutral reaction; the Giesel salts are strongly alkaline. 
(3) The cathode-ray salts give very marked photoelectric effects (as 
Elster and Geitel* observed); the Giesel salts are quite ineffective. 
(4) Under certain circumstances, which will be mentioned further 
on, the cathode-ray salts may emit a phosphorescent light, the Giesel 
salts none at all. Therefore the question arose again, whether there 
is not a marked internal difference between the cathode-ray salts and 
the Giesel salts, and what is the nature of the latter? 
I have succeeded in settling this question, having produced salts 
by cathode rays, the behaviour of which is in every respect absolutely 
identical with the Giesel salts. You may produce such substances if 
you allow the cathode rays to fall on the original salts not for a short 
moment only, but for a somewhat prolonged time, until the salts are 
strongly heated. Produced in this way the salts will keep colours; but 
the substances coloured in this way are not sensitive to light; they 
show no photoelectric effect; they give strong alkaline reaction, and 
they are not suited for phosphorescence—all like the Giesel salts. It 
is quite sure, and you may test it also directly by spectroscopic proof, 
that in this case, if for instance you have worked on sodium chloride, 
the chlorine is set free. Then of course an amount of free sodium is 
left, which dissolves itself in a deeper layer of unaltered sodium 
chloride, to which the cathode rays could not penetrate. I call these 
non-sensitive colours the after-colours of the second class, while the 
ordinary sensitive after-colours, produced in a short time on cool 
salts, are called after-colours of the first class. 
Now, u the after-colours of the second class are identical with 
the Giesel salts, then, of course, the very different substances of the 
first class cannot be also identical with the Giesel salts. Therefore the 
question arises anew what is the nature of the first-class after-colours? 
One observes with regard to solid solutions that the first-class colours 
depend not only upon the metal contained in the small admixture, but 
they vary greatly, for instance, in the case of the admixture consisting 
of potassium chloride or bromide or iodide. This indicates that the 
metals alone do not cause the after-colours. It becomes much more 
clear when we expose some ammonium salts to the cathode rays. (The 
ammonium salts are cooled by liquid air in the discharge-tube to prevent 
their evaporation.) Then you get strongly marked after-colours like- 
wise; for instance, ammonium chloride becomes yellow-greenish, the 
bromide becomes yellow-brown, the iodide becomes brown, and the 
4 J. Elster and H. Geitel. Wied. Ann. 59, 487. 
