B.— CHEMISTRY. 45 



by virtue of its chemical reactivity. If this intermediate level does not 

 exist, then neither fluorescence nor phosphorescence will be exhibited, 

 and since optical resonance is unknown with compound molecules, the 

 energy numerically equal to /iVg is radiated in the infra-red. If the inter- 

 mediate level exists, then fluorescence will be exhibited as the molecules 

 fall to that level from the' high level first produced. If the molecule when 

 in the intermediate level undergoes no chemical reaction, the critical 

 increment of activation of the intermediate level will also be radiated 

 when the molecule finally reaches the initial level. It may be noted that 

 in all cases of gases and liquids, where phosphorescence never occurs, the 

 difference between the frequencies of the activating and fluorescence 

 radiations is small. The critical quantum of activation, which is the 

 difference between the absorbed and fluorescence quanta corresponds to 

 a frequency in the infra-red. 



As already stated, the theory involves the view that the activated 

 states responsible for phosphorescence are similar to those which enter 

 into chemical reaction, and it might be argued that they must be of 

 markedly different type, since the life-periods of the former may be very 

 long, whilst those of the latter are known to be very short. Such an 

 argument, however, is based on the assumption that it is not possible to 

 vary the life-periods of these intermediate states of activation by change 

 of conditions. There is no justification for this assumption ; and indeed 

 the evidence is against it, since remarkable variations in the life-period 

 can be produced by change of temperature alone. For example, Lenard 

 and Klatt showed that by raising the temperature the life-period of the 

 activated state in a phosphore could be reduced from days or hours down 

 to an exceedingly small fraction of a second. Then again von Kowalski 

 showed that many substances in alcoholic solution, which only exhibit 

 fluorescence at room temperatures, develop marked phosphorescence 

 when cooled in liquid air. 



Attention may be directed once again to the absorption spectra 

 observations which were recorded in fig. 2 on page 43, and in particular 

 it may be noted that the critical increment of the sulphonation reaction 

 ^Vj, is given by the product of the Planck constant into the difference 

 between the central frequencies of the absorption bands shown by the 

 anisole in solution in alcohol and in strong sulphuric acid. This follows 

 at once from the fact that the fluorescence quantum of the anisole in 

 alcoholic solution is equal to the quantum absorbed by the anisole in 

 sulphuric acid solution. The conclusion would seem to be obvious that 

 the absorption band of the anisole in sulphuric acid solution is that 

 characteristic of the activated state which in some way has been stabilised 

 by the sulphuric acid. The stabilisation is proved by the fact that no 

 measurable sulphonation takes place when the solution is allowed to 

 remain at ordinary temperatures. At 50°, however, the sulphonation 

 takes place with measurable velocity. 



Two points of interest may be mentioned which arise from this. In 

 the first place it would seem that the raising of a molecule from a lower 

 to a higher energy level is accompanied by a shift in the characteristic 

 absorption band in the visible or ultra-violet region towards the longer 

 wave-lengths. This also occurs when phosphores are activated, for the 



