282 Hartley — The Action of Heat on the Ahsorption Spectra and 



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 cliromium chlorides. His measurements agree with 

 those given above ; but as he examined greater thickness of liquid 2-5 cm. 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 cln-omoxalate. 



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. — Avery 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 powerful 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 ijarallel sides, the liquid being maintained throughout the exposure of the 

 photographic plate at exactly the same temperature, because the bands widen by 

 rise of temperature, and of course become narrow on cooling. (See Plate XXII.) 



IVte action of heat on the hydrated compounds of chromium is not simply a dissociation 

 of ivater-molecules or of acid from base, hut a true decomposition resulting in the pro- 

 duction of a different class of salts with different generic properties. 



Regarding the salts of chromium and their change of colour by heat, let us 

 examine, first, the observations of Schrotter on the chromic sulphates. f There 

 is an anhydrous red insoluble salt, Cr203'3S03 ; a green uncrystallizable but 

 soluble variety, CrjOs'SSOs -|- 5 or 6H2O, and a blue crystallized modification, 

 Cr203'3S03"15H20. The last can be converted into the second by desiccation at 

 100°C., by the action of a temperature of 65° C. or70°C. on an aqueous solution; 



* Chemical News, vol. 67, pp. 207, 219, 2.31, 245, and 255, 1893. 

 f (imelin's " Handbook of Chemistry," vol. iv., pp. 126-28. 



