ON SALTS COLOURED BY CATHODE RAYS. 253 
fluoride a deep blue. In the daylight these colours are gradually 
destroyed, quite like other after-colours of the first class. The colours 
themselves—yellow-greenish for the chloride, yellow-brown for the 
bromide, and so on—induce us to presume that the after-colours in this 
case are produced by the haloids, and not by the hypothetical 
ammonium radical. This presumption becomes a strong conviction 
when we observe that also a great number of organic preparations 
which contain no metal at all (and not any metal-like radical) acquire 
marked after-colours of the first class in the cathode rays also. (The 
part of the discharge-tube which contains the organic substances is 
cooled by liquid air.) 
Then you may observe that solid acetic acid (C,H,O,) remains 
quite colourless in the cathode rays; but if you substitute a hydrogen 
atom by chlorine, the substance thus produced (the monochloro-acetic 
acid) acquires a marked yellow-green after-colour. If you introduce 
an atom of bromine instead of chlorine, you get C,H,BrO, and the 
after-colour is of a marked yellow. Bromoform (CHBr;,) turns into 
the colour of loam, and chloral (C,HCI],0) becomes a deep yellow. 
In this way we see that not only salts, but likewise substituted acids, 
substituted hydrocarbons, and substituted aldehydes acquire after- 
colours if they contain any haloid. 
Now, it seems highly improbable that in the case of alkali salts the 
electro-positive component is absorbed only (producing the after-colour), 
and that, on the other hand, in the ammonium salts and in the organic 
substances the electro-negative component is efficient only. The most 
probable inference is that in each case both components remain and 
that both are efficient, but that under the same conditions the haloids 
produce a slighter colour than the metals, so that in the case of the 
salts the haloid colour is overwhelmed by the metal colour. 
Therefore we are compelled to suppose that we have not to deal 
with a decomposition in the ordinary form, by which the different com- 
ponents are finally separated from each other and at least one of them 
is set entirely free, but that the components detained by absorption 
remain at a quite short distance from each other, so that they may 
easily meet again. I realise that—for instance, in the case of sodium 
chloride—at every point of the coloured layer there is an atom (or 
perhaps a molecule) of chlorine and an atom (or a molecule) of sodium ; 
but they cannot combine, because they are fixed by absorption and dis- 
tended from each other by the absorptive power, which in this case 
surpasses the chemical affinity. But the absorptive power may be 
weakened by heating and the chemical affinity or the amplitude of the 
molecular vibrations may be strengthened by the energy of daylight. 
If we grant these assumptions, it is immediately evident why the 
reaction of all dissolved colour substances of the first class is a 
neutral one, for the two components may combine again and re- 
establish the original substance. The other special qualities of the 
first-class colours, and especially their differences from the Giesel 
salts, which contain the electropositive component only, may be de- 
duced likewise from this retention of both components and their oppor- 
tunity of meeting each other again when the absorptive power is 
