PHOTOCHEMISTRY 265 



found. The Grignard reagent (phenyl magnesium bromide) when 

 exposed to air gives a faint purpHsh glow which can readily be seen in 

 the dark. Substituted compounds of this type are known to give more 

 brilliant reactions and a considerable amount of research has been done 

 in this field (7). One of the most striking chemiluminescent reactions 

 has been described recently. When 3-aminophthalhydrazide is dis- 

 solved in ordinary hydrogen peroxide, the whole solution becomes brightly 

 luminescent so that it can be seen even in dayhght (10). A few other 

 chemiluminescent reactions are known, but they are comparatively rare. 

 It may be assumed that the phenomena of chemiluminescence would 

 become very much more common if sufficiently sensitive means for the 

 detection of light were available. In most chemical reactions the new 

 products are formed with the evolution of heat and accompanying it there 

 may occur frequently the production of some radiation. In many 

 cases light may be produced in a reaction, but this light in turn may be 

 absorbed by surrounding molecules so that little, if any, finally emerges 

 from the reacting system. The light emitted from molecules close to the 

 surface is more Hkely to emerge from the reacting system. General 

 rules for chemiluminescence have not yet been developed. The phe- 

 nomenon seems to be rather specific. It may be that violent reactions of 

 a particular type are necessary, together with an absorption coefficient 

 for the emitted radiation such that only sUght absorption occurs within 

 the system. 



Chemiluminescence in the infra-red has not been investigated, except 

 in flames and explosions, but it may be quite common. Chemilumines- 

 cence in the visible is rather infrequent. Chemiluminescence in the ultra- 

 violet is very rare. It is natural to expect that a small fraction of the 

 chemical energy may be converted into radiation and that under suitable 

 conditions this may be given out from a system in the form of chemi- 

 luminescence. The smaller the amount of energy in a quantum, the 

 more readily can such a quantum be produced. For these reasons one 

 would expect to find that chemiluminescence is more frequently found 

 with long wave-lengths than with short. As pointed out previously, 

 it is not the heat of reaction but the energy of activation that determines 

 the course of a chemical reaction. Accordingly, since the wave-length 

 of the chemiluminescent radiation is governed by the quantum theory, 

 the minimum wave-length of the emitted light can be calculated from the 

 energy of activation of the reverse reaction, by expressing this in ergs, 

 setting equal to Nhv and solving for v. 



Fluorescence. — When a system is excited by absorbing radiation, it is 

 possible that some of the excited molecules may return to a normal state 

 with the emission of radiation having a different wave-length from that 

 of the exciting radiation. This emission of light, different in wave- 

 length from that of the exciting light, is known as fluorescence. As a 



