198 HOWARD E. PULLING 



Regarding the first part of this statement, it may be noted that 

 no general distinction can be made between waves that influ- 

 ence chemical reactions and those that do not. In fact, as 

 photo-chemical investigation is extended the conviction grows 

 that there is no portion of the spectrum (not excepting even the 

 very long infra-red waves) that can be definitely said to be 

 without effect upon some chemical reaction. 



The second part of the statement is often misstated to the 

 effect that those waves that are absorbed are chemically active. 

 There is, however, no simple relation between absorption and 

 chemical action. There are many cases knowm of strong ab- 

 sorption accompanied by no detectible chemical action and other 

 cases in which there is marked chemical action and only slight 

 absorption. This diversity occurs because the radiation effect 

 is not always solely because of the addition of energy to the 

 system, although of course the contribution of energy is all 

 that the light makes, and among other effects there are those 

 that for want of a better term are called catalytic. 48 It must 

 be further noted that even in those cases in which the ab- 

 sorption and the speed of reaction are definitely and simply 

 related the reaction does not proceed strongly in a limited 

 portion of the spectrum, immediately bounded by regions of 

 no activity. Instead, there is a more or less gradual increase 

 in the effectiveness of the waves up to a maximum and a similar 

 decrease beyond. Thus, while there is the possibility of ob- 

 taining measurements that may be interpreted in terms of 

 energy transmitted in any desired portion of the entire spectrum 

 by the proper choice of a chemical reaction with which to record 

 it, it does not follow that the interpretation will be either exact 

 or easy. 



One class of photo-chemical reactions are, within certain 

 ill-defined limits, of considerable usefulness for actinometric 

 use. These reactions depend for their rate upon the intensity 

 of the radiant energy absorbed and are accordingly rather 

 precise means for obtaining relative measures of its intensity. 



18 Lewis, W. C. McC, Phys. Chem., vol. 2, cited in note 2. 

 Sheppard, S. E., Photo-chemistry. London, 1914. 



