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further knowledge as to how it may be possible that both should 
be true at the same time.” 
3. The Photo-chemical Equivalent 
Another illustration of Einstein’s hypothesis is provided by 
his photo-chemical law : when light of given frequency is incident 
on a system sensitive to such light, for each single light quantum 
absorbed one molecule of the absorbing substance is decomposed. 
In other words, the number of molecules affected is equal to the 
number of light quanta absorbed. As Jeans points out: “ The 
law not only prohibits the killing of two birds with one stone, 
but also the killing of one bird with two stones. Since the 
energy of a quantum hv is proportional to its frequency v we 
understand how a small amount of violet light can accomplish 
what no amount of red light would suffice to do, a fact familiar 
to every photographer—we can admit quite a lot of light if only 
it is red, but the smallest amount of violet light spoils our plates.” 
It must, however, be said that most photochemical processes 
are complicated by secondary reactions which make it extremely 
difficult to test the direct applicability of the law. It is now 
generally admitted that the absorbing molecule absorbs energy 
in quanta; but the number of molecules decomposed may differ 
considerably from the number of absorbing molecules. In a few 
reactions, and within certain narrow spectral limits, these 
numbers have been found the same. It is, then, extremely 
probable, if not certain, that the primary process consists in 
the absorption of an energy quantum ; but the subsequent 
changes are frequently so complicated that it is not at. present 
possible to express them in terms of the quantum theory. 
Suggestions have been made by Trautz, Lewis, and Perrin 
that radiation is an important factor in all chemical action, 
Perrin in particular having developed the view that “ ordinary ” 
chemical reactions may be regarded as due to radiation, that is 
as photo-reactions. But the observed velocities of reaction are 
much in excess of those required (apparently) by this hypothesis. 
Although the radiation hypothesis is in some respects attractive, 
its supporters have not yet been able to present it in such a 
form as to make it rank as a theory capable of giving quanti 
tative expression of the results of experiment. Indeed it would 
seem that far more attention than it has yet received should be 
paid to the effect of electron collisions in promoting chemical 
change.* 
* “ Photochemical Reaction in Liquids and Gases,” Faraday Society 
Discussion, Trans., vol. 21, p. 438, 1926.