22 Prof. E. C. C. Baly on 



oxygen to benzoic acid, and that it is not converted to 

 benzaldehyde-sulphonic acid when sulphuric acid is added 

 to the solution. The reaction with oxygen, therefore, is 

 characteristic of one or both of the two molecular phases 

 present in alcoholic solution. If benzaldehyde is dissolved 

 in concentrated sulphuric acid, it exhibits two new molecular 

 frequencies — one in the visible and the other in the ultra- 

 violet region. Two further molecular phases therefore exist 

 in solution in sulphuric acid. In this case the benzaldehyde 

 is no longer oxidized by oxygen, but is readily converted into 

 sulphonic acid. 



Now, the question arises as to the amount of energy 

 necessary to convert one molecular phase into another and 

 the mechanism whereby this energy is supplied. The amount 

 of energy required per molecule is readily calculated, and is 

 equal to one or more quanta measured at the infra-red 

 fundamental of that molecule. If the frequency character- 

 istic of the first phase is x times the infra-red fundamental, 

 and the required . phase is characterized by a frequency 

 which is y times the infra-red fundamental, then the 

 energy required for each molecule is ,v —y quanta at 

 the infra-red fundamental. Obviously, the molecular 

 system can absorb this energy when exposed to radiation 

 of a frequency equal to its infra-red fundamental, or, as 

 explained in the preceding paper, it may absorb it at any 

 of the frequencies characteristic of its component atoms. 

 Lastly, the molecule may absorb one quantum at its 

 characteristic phase frequency, and under ordinary circum- 

 stances this energy will again be entirely radiated as quanta 

 at a lower phase frequency, the infra-red fundamental, or 

 the atomic frequencies. If there is present a substance 

 capable of reacting with a less condensed phase, then the 

 molecule is converted into that phase and reacts, the balance 

 of energy being evolved as infra-red radiation. The essential 

 point is that the necessary amount of energy to change the 

 molecular phase is x — y quanta at the infra-red fundamental, 

 and that when one quantum is absorbed at the phase 

 frequency the excess energy over and above that required 

 i"s radiated. The change of molecules from one phase to 

 another under the influence of light is readily enouah shown 

 experimentally, but it is necessary to stabilize the second 

 phase in some way, since otherwise it returns instantaneously 

 to the first phase. An interesting example is furnished by 

 tri nitrobenzene, an alcoholic solution of which contains a 

 molecular phase characterized by a frequency in the ultra- 

 violet. A piperidine solution contains a molecular phase o£ 



