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VI.—The Significance of Maximum Specific Electrical Conductivity in Chemistry. 
By Professor John Gibson, Ph.D. 
(Read July 13, 1908. MS. received June 13,1911. Issued separately October 20, 1911.) 
PHOTOCHEMICAL ACTION. 
The first step made in this investigation was the recognition of increased specific 
electrical conductivity as a general characteristic of photochemical action. It was 
argued that if there be any common characteristic in photochemical changes it must be 
found in the simplest as well as in the more familiar and more complex reactions 
which are characteristic of the metabolism of plants. 
No chemically simpler case suggested itself than the increase in electric conductivity 
of crystalline selenium under the influence of light.* This change might indeed be 
held almost to lie outside the range of chemistry proper, and to belong to the class of 
change often spoken of as merely physical change. But no sharp line can be drawn 
between physical and chemical changes, and the clue proved most useful. 
The action of light on crystallised selenium seems to give evidence of a directive 
action of light rather than of an inherent tendency of the crystalline selenium itself 
towards increased conductivity, for the gain in conductivity persists only as long as the 
exposure to light. Placed in the dark, after exposure to light, crystallised selenium 
reverts to its initially lower conductivity. The same remark applies to all cases of 
photochemical changes which are not permanent. It is otherwise with changes which 
are permanent. A few instances of permanent photochemical change in homogeneous 
systems may be cited :— 
(1) The conversion of yellow phosphorus, under the action of light, into red 
phosphorus. 
(2) The gradual conversion of red amorphous selenium into the black crystalline 
form. 
(3) The conversion of red crystalline mercuric sulphide into the black amorphous 
form. 
These diverse instances of photochemical change are correlated by the fact that the 
change in each case is accompanied by a gain in specific conductivity. 
An apparent exception to this rule is discussed in a very interesting paper by 
Meyer?) on the action of light on the chromo-gelatine film. The main reaction, viz. 
the photochemical reduction of potassium bichromate in presence of soluble organic 
* On Photochemical Action, Proc. Roy. Soc. Hdin., 1897, vol. xxi. p. 308. 
+ Comp. Zeit. physik. Chemie, lxvi. p. 58. 
TRANS. ROY. SOC. EDIN., VOL. XLVIII. PART I. (NO. 6). 19 
