308 Proceedings of Royal Society of Edinburgh. [sess. 
absorbers, but it has to be observed that a part at least of the 
halogen thus absorbed goes to produce substances having a greater 
electrolytic conductivity than the sensitizers employed (e.g., HC1). 
On the other hand, the addition of substances which are good 
conductors diminishes the sensitiveness of the whole system to the 
action of light. 
For instance, the action of light on chlorine water is greatly 
retarded by the addition of hydrochloric acid, potassium chloride, 
sodium chloride, &c. ; the retardation being greatest in the case of 
hydrochloric acid, which is also the best electrolytic conductor (20). 
It would be premature to suggest a theoretical explanation 
of the rule to which I have called attention. The fundamental 
conceptions cannot be better expressed than by the following 
quotation from Clerk Maxwell : — 
“We know that the ether transmits transverse vibrations to 
very great distances without sensible loss of energy by dissipation. 
A molecular medium, moving under such conditions that a group of 
molecules once near together remain near each other during the 
whole motion, may be capable of transmitting vibrations without 
much dissipation of energy ; but if the motion is such that the 
groups of molecules are not merely slightly altered in configuration, 
but entirely broken up, so that their component molecules pass into 
new types of grouping, then, in the passage from, one type of 
grouping to another, the energy of regular vibrations will be 
frittered away into that of the irregular agitation which we call 
heat.” 
To this singularly lucid statement I would merely add the 
suggestion that, whenever molecules or groups of molecules are 
thus broken up, the new grouping immediately consequent upon 
the frittering away of the energy of regular vibrations will be 
itself always such as to oppose less resistance to the passage of 
an electric current than before. If the vibrations of the ether 
which we call light be indeed electro-magnetic, this result seems 
to follow as a natural consequence. 
It is necessary, however, to distinguish between primary and 
secondary changes. It does not follow that increased conduc- 
tivity necessarily implies chemical stability. The new grouping 
brought about by the action of light, while offering, so long as it 
