424 Mr. AY. Ackroyd on Selective Absorption. 



increase of absorption without any structural alterations in the 

 absorbing- medium ; for the cupric sulphate remains the same 

 during the two observations, and the temperature is unaltered. 

 The only difference is one of extent of medium. For this 

 kind of absorption we propose, at the suggestion of Dr. 

 Guthrie, the term transverse absorption. 



Professor Gladstone *, when speaking of the action of heat 

 on coloured solutions, thus tacitly refers to the two kinds of 

 absorption: — "The elevation of temperature seems merely to 

 heighten the absorbent power of the dissolved salt, so that 

 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." Notwithstanding this, the two 

 kinds of absorption seem not to have been formally separated ; 

 and the mixed inferences from one and the other have natu- 

 rally produced a confusion in the minds of students which has 

 made selective absorption a subject little understood. We 

 shall confine our observations to structural absorption. 



Structural Absorption. — Alteration of structural absorption 

 upon elevation of temperature is, as a rule, obvious to the eye 

 as a change of colour. This colour-change, for which I have 

 elsewhere proposed the name of metachromatism, has been 

 studied by Schonbein, Gladstone, Houston and Thompson, 

 and, lastly, by myself. Many theories have been offered to 

 account for the phenomenon ; these were discussed in my 

 paper on Metachromatism f, read before the Chemical Society 

 on the 3rd of February last. 



If a little oxide of zinc be heated strongly on a piece of 

 white porcelain, it changes from white to orange and yellow. 

 Chromate of lead treated similarly changes from orange to 

 deep orange, brick-red, and black-red. These bodies quickly 

 regain their original colour with a return of the normal tem- 

 perature ; hence one might suppose that the chromate of lead, 

 if cooled far below the normal temperature, would acquire in 

 succession the yellow and white of the zinc oxide, thus (in the 

 order of cooling) — black-red, brick-red, deep orange, orange, 

 yellow, and white. 



Such a sequence of colour-change is exhibited by nitric per- 

 oxide ; a ruddy vapour at the normal temperature, it becomes 

 orange, yellow, and below zero a colourless liquid. From a 

 study of such metachromatic facts and their sequence we have 

 arrived at the following scale of change : — 



* Phil. Mag. [IV.] vol. xiv. p. 423. 



t Chemical News, vol. xxxiv. pp. 76, 77. 



