44 SECTIONAL ADDRESSES. 



few hours and is the absorption curve of the sulphonic acid. It was 

 found that the sulphonic acid prepared photochemically gave an absorption 

 spectrum almost identical with that represented by curve C. 



Exactly similar experiments were carried out with para- and ortho- 

 nitroanisole, the absorption curves of which in alcoholic and sulphuric 

 acid solution are very analogous to those shown in fig. 2. Both of these 

 ethers in strong sulphuric acid solution on remaining at 50° react to give 

 their sulphonic acids. These nitro compounds, however, differ from the 

 parent anisole in the fact that in alcoholic solution they exhibit no trace 

 of fluorescence. This suggests that the super-activated states produced 

 when they absorb light at their characteristic absorption bands do not 

 pass into the activated state required for the sulphonation reaction. 



Acidified solutions of each nitro compound were irradiated by the 

 light from the quartz mercury lamp for 96 hours and the solutions were 

 treated in exactly the same way as described above in the case of anisole. 

 The results were, however, entirely different. The nitro compounds were 

 recovered from the ether extract, no barium salt of a sulphonic acid was 

 found, and barium- ethylsulphate was obtained in considerable quantities. 



Although no more can be claimed for these observations than that 

 they are preliminary, yet the evidence they afford is in, striking agreement 

 with that obtained from the photoluminescence phenomena. In the 

 photochemical reaction the radiation of the fluorescence quantum Av.^ 

 during activation gives an independent proof of the formation of the 

 activated state, and also indicates that the critical increment of activation 

 of a molecule is numerically equal to hi^—liv.^. In the case of photo- 

 luminescence the radiation of the critical increment of activation as a 

 single quantum of phosphorescence per molecule indicates that this 

 critical increment of activation is in fact a single quantum per molecule. 

 It would thus seem that independent evidence has been obtained in 

 favour of the first part of the radiation hypothesis, although it has now 

 been shown that the supply of the activating quantum to the reactant 

 molecule cannot under any circiimstances be achieved by a simple process 

 of absorption. 



The theory of fluorescence now advanced may be considered as being 

 a reasonable one, but it is advisable, before the main argument is pursued 

 further, to examine it in more detail. In the first place the question may 

 be asked as to the course of events when phosphorescence is absent and 

 no chemical reaction takes place. All that the theory states is that any 

 molecule on exposure to radiant energy of its characteristic frequency Vg 

 in the visible or ultra-violet region absorbs a single quantum Avo and is 

 raised to a high energy level which has a very short life period. This 

 super-activated state tends to return to its initial state with the radiation 

 of energy numerically equal to lv\)^. Under conditions not yet defined there 

 can exist an intermediate level with a finite stability, and then the 

 molecule falls from the high level to this intermediate level, and in so 

 doing radiates the energy difference between these two levels as a single 

 quantum of fluorescence. The intermediate level may be sufiiciently 

 stabilised by the conditions to exhibit the phenomenon of phosphorescence 

 when the final fall to the initial level takes place, or, alternatively, the 

 intermediate level may have a very short life period and may be recognised 



