426 On the Effect of Heat on the Colour of Salts in Solution. 
No chemical change, however, appears to be concerned in the 
alteration of colour produced by heat in the other cases men- 
tioned above. The elevation of temperature seems merely to 
heighten the absorbent power of the dissolved salt, so that the 
light absorbed by a certain quantity of the heated solution is the 
.same as would have been absorbed by a larger quantity of the 
same solution, if cold. This will fully account for the changes, 
not merely in such salts as meconate of iron, where an increased 
intensity of colour is all that is observed, but for those changes 
which involve the character as well as the depth of the colour. 
These latter substances are in fact more or less dichromatic, 
that is, they present different colours according to the quantity 
of salt which the light has traversed before it reaches the eye ; 
and the reason of this will be apparent on glancing at the spec- 
trum of bichloride of platinum represented in fig. 20. It will 
be there seen that a thin stratum of the salt transmits all the rays 
from the extreme red to the fixed line F, and a little blue and 
violet beyond; but that as the thickness increases, the rays 
about 4 and afterwards about E are absorbed. The general 
impression conveyed by the rays that traverse a thin stratum is 
yellow; but when the green rays are absorbed, it changes natu- 
rally to orange, becoming more and more red as the stratum in- 
creases. Now the effect of heat upon a thin stratum, or a weak 
solution, is solely to produce the same amount of absorption as 
would be produced at the ordinary temperature by a thicker 
stratum, or a stronger solution. 
In reference, however, to the change that ensues when a solu- 
tion of polysulphide of potassium is heated, I doubt whether 
any increased thickness of the same liquid would give so intense 
ared. Can it be of the same nature as the modification of co- 
lour that takes place in melted sulphur at a far higher tempera- 
ture? This is rendered more probable by the fact, that the 
yellow colour of the potassium salt in solution is due to the sul- 
phur ; yet on the other hand, sulphur dissolved in naphtha shows 
no indication of redness when the liquid is boiled. 
It is scarcely necessary to remind either physicists or chemists 
of the observation made by Sir David Brewster, that the absorp- 
tion bands of peroxide of nitrogen are increased by heating the 
gas ; or of the observations of Schcenbein and others, that several 
solid substances, such as oxide of zine, or gallate of iron, absorb, 
when heated, rays that they transmit or reflect when cold. The 
above observations on dissolved salts give, therefore, results 
which are perfectly in harmony with the little that was known 
before about the effect of heat on the chromatic phenomena 
presented by other pure chemical substances. 
