282 HartLEy—The Action of Heut on the Absorption Spectra und 
The work of Lapraik, ‘On the Absorption-Spectra of some Compounds of 
Chromium,”* dealt chiefly with chromium salts of organic acids, the only inorganic 
compounds examined being chromium chlorides. His measurements agree with 
those given above ; but as he examined greater thickness of liquid 2'5 ¢.m. or 1 inch, 
he noted more particularly the bands in the red. The thickness examined by me, 
in each case, was not more than two or three millimetres. 
The chromium salts of oxalic acid examined by me, in 1874, were the 
following :—chromium oxalate, the red and blue potassium chromoxalates and 
a potassio-calcium chromoxalate. 
Blue Potassium Chromoxalate-—The solution was almost opaque when hot; the 
second readings were made with great difficulty. It is evidently the fifth band in 
Lapraik’s spectrum of a similar solution (2) which was measured above. 
Red Potassium Chromoxalate.—-A very fine spectrum, the blue rays well seen, the 
red and green very bright, red the brighter. The dark line observed by Brewster 
was not seen with this thin wedge. 
It was difficult to reconcile these measurements with those of Lapraik (solution 
(5)), as he made use of a very much thicker layer of solution; accordingly a satu- 
rated solution of the salt at 16° was examined by sun-light, using a dispersion of 
four quartz prisms, and photographing the spectrum, because it is known that there 
is a powertul absorption in the ultra-violet. The liquid was contained in a test- 
tube 15 mm. in diameter, and the sun-rays were concentrated upon it, by a quartz 
condenser of 3-inch diameter and 6-inch focus. 
A series of four photographs was taken at different temperatures : namely, 100°, 
75°, 50° and 20°. The absorption is greatest at the highest temperature, where 
three bands in the red are easily seen, two of them degraded towards the less 
refrangible side. These bands are not as sharp as they would appear in a cell 
with parallel sides, the liquid being maintained throughout the exposure of the 
photographie plate at exactly the same temperature, because the bands widen by 
rise of temperature, and of course become narrow on cooling. (See Plate X XI.) 
The action of heat on the hydrated compounds of chromium ts not simply a dissociation 
of water-molecules or of acid from base, but a true decomposition resulting in the pro- 
duction of a different class of salts with different generie proper ties. 
Regarding the salts of chromium and their change of colour by heat, let us 
examine, first, the observations of Schrotter on the chromic sulphates.t There 
is an anhydrous red insoluble salt, Cr,O;3SO;; a green uncrystallizable but 
soluble variety, Cr,O0;°38O0; + 5 or 6H,O, and a blue crystallized modification, 
Cr,05°3803;15H,O0. The last can be converted into the second by desiccation at 
100°C., by the action of a temperature of 65°C. or 70°C. on an aqueous solution ; 
* Chemical News, vol. 67, pp. 207, 219, 231, 245, and 255, 1893. 
+ Gmelin’s ‘‘ Handbook of Chemistry,” vol. iv., pp. 126-28. 
