Dr. Gladstone on the Colour of Salts in Solution. 419 
CHROMATES. 
Chromic acid in solution, and its combinations with colourless 
bases, such as potash, cut off instantly the more refrangible half 
of the spectrum, admitting only the blue rays near F for a short 
distance ; but they transmit the less refrangible half perfectly, 
See fig. 1. Plate II. The red bi-salts, as those of potash or baryta, 
absorb the green in the neighbourhoodof F likewise, and alittlered. 
Chromates were prepared by saturating the acid with the hy- 
drated oxide or carbonate of the base. 
Chromate of Copper is a green salt. The spectrum presented 
by it is represented in fig. 5, where evidently the absorption of 
the red ray is due to the base, that of the blue and violet to the 
acid. Compare figs. 1 and 9. 
Chromate of Nickelis of a yellowish-green, and presents nearly 
the same spectrum as the copper salt, as might be anticipated on 
comparing figs. 1 and 10. 
Chromate of Ferric Oxide is orange passing into red as the 
depth increases. It transmits only the red, orange, and yellow 
rays, which a comparison of figs. 1 and 4 will show to be trans- 
mitted in common by both constituents. 
Chromate of Uranium is yellow, like any other salt of that base ; 
but its prismatic appearance is totally different, the chromic acid 
having cut off all the remarkable luminous bands beyond F. 
Compare figs. 1 and 2. 
Chromate of Chromium is of a reddish-brown colour. The 
rays transmitted by it are represented in fig. 7, from which it 
will be seen that the whole of the blue or green maximum of an 
ordinary chromium salt (see fig. 6) is cut off by the acid; but 
an absorption of light between D and E does not occur to such 
an extent as might be expected from the presence of chromic 
oxide. From the chemical reactions of this body, however, I 
have some doubt about its right to be considered a salt. 
PERMANGANATES. 
Permanganate of potash gives the very characteristic spectrum 
represented in fig. 3. 
Permanganate of Uranium giyes a purple solution. Its pris- 
liquid by diagrams similar to my own. These experiments are an import- 
ant step in the history of photo-chemical research, and would have been 
noticed by me in my previous paper had I been aware of their existence. 
M. Miiller’s mode of representing the optical effects produced by the trans- 
mission of light through coloured bodies is an improvement on the original 
attempt of Sir John Herschel, and should give precisely the same figures 
as my method; yet he arrives only, after a series of observations in each 
ease, at the same result, which I, by using the hollow wedge, render appa- 
rent to the eye at once. His observations agree closely with mine; but 
they are very few in number, and have not led him to any of the generaii- 
zations deduced in my previous or my present paper. 
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