On the Effect of Heat on the Colour of Salts in Solution, 428 
retains its specific absorbent power when in combination. Two 
of the above-described cases, however, are slightly anomalous— 
the chromate of chromium, and the double iodide of platinum 
and potassium; and these are not the only, nor indeed the 
greatest exceptions, for the ferric ferrocyanide dissolved in oxalic 
acid transmits blue rays in great abundance, which are absorbed 
both by erdinary ferrocyanides and ferric salts. It must be 
borne in mind that the statement, “all the compounds of a par- 
ticular base or acid, when dissolved in water, have the same effect 
on the rays of light,” is not universally true, even when a coloured 
base is combined with a colourless acid; and hence we-might 
anticipate, what actual observation has shown to be the case, 
that variations would sometimes occur when both the acid and 
base are coloured. It may be therefore laid down as a general, 
though not an invariable law, that when an acid and a base com- 
bine, each of which has a different influence on the rays of light, 
a solution of the resulting salt will transmit only those rays which 
are not absorbed by either, or in other words, which are transmitted 
by both. 
XLIX. On the Effect of Heat on the Colour of Salts in Solution. 
By J. H. Guapstone, Ph.D., FRS. &c.* 
[With a Plate.] 
oo a general rule, the solution of a salt has the same power 
of absorbing or transmitting the rays of light at all tem- 
peratures. I am.not acquainted with any instance of a dissolved 
colourless salt which assumes a colour when the solution is either 
heated or cooled; nor does the converse seem ever to occur,— 
a salt coloured at the ordinary temperature, which loses that 
colour when heat is applied. Nevertheless it is not rare to find 
coloured salts, which, when dissolved in water, vary in shade or 
in tint according to the temperature. 
In some instances, heating the solution seems merely to inten- 
sify the colour. This is the case with the following red, orange, 
yellow, and green salts :— 
Meconate of iron—red. 
Terbromide of gold—red. 
Red nitrate of cerium. 
Bichromate of potash—orange. 
Ferrocyanide of potassium—yellow. 
Molybdous chloride—green. 
More frequently a change takes place in the character as well 
as in the intensity of the colour when the solution is heated. 
The following cases have been observed. 
: * Communicated by the Author. 
