52 SECTIONAL ADDRESSES. 



total persistence of the emission will decrease. At any constant tempera- 

 ture between the lower and upper limits the intensity will have a definite 

 rate of decay. Just below the upper temperature limit where the stability 

 is vanishingly small the persistence will be vanishingly small and the 

 intensity will be the maximum. Up to this stage the phenomena will 

 be identical with those of a chemical reaction, the criterion of intensity 

 of phosphorescence being substituted for the criterion of reaction velocity. 

 When the upper temperature limit is passed the complex will no longer 

 have any stability and will no longer exist. No phosphorescence or 

 fluorescence will be possible, since these depend on the stable existence 

 of the complex with its power of retaining the energy which it absorbs 

 at its characteristic frequency in the ultra-violet. These phenomena are 

 identical with those observed by Lenard and Klatt. 



One further piece of evidence, which has hitherto not been mentioned, 

 may now be brought forward. The hypothesis of complete formation 

 demands that the defect in the rotational energy of the ' catalyst ' or 

 diluent component may be absorbed as infra-red radiation. In all that 

 has gone before this defect has been supplied by raising the temperature, 

 and the hypothesis cannot be considered as entirely justified unless it 

 be proved that resolution of the complexes can be achieved by exposure 

 to infra-red radiation. The fact that the most effective method of 

 deactivating an activated phosphore and of releasing the whole of its 

 phosphorescence is by exposing it to infra-red radiation adds a conclusive 

 argument in support of the hypothesis. 



It will be noted that there is a marked difference between the 

 phenomena in photoluminescence and chemical reaction at temperatures 

 above the upper limit, since in the former phosphorescence is no longer 

 possible, and in the latter the reaction velocity is a maximum. This 

 difference is due to the fact that a chemical reaction is the result of a 

 single process of activation, and when the activated molecules are set 

 free by the resolution of their complexes the reaction takes place 

 immediately. The phenomenon of photoluminescence is the result of a 

 two-stage process of activation, the second stage only taking place so 

 long as the complex is in being. When the complex is no longer stable 

 the second stage can no longer be achieved. 



It has already been pointed out that in the photosynthesis of carbo- 

 hydrates the activation to the high energy level necessary for the chemical 

 reaction is effected in two stages, namely, partial activation in the 

 absorption complex and completion by absorption of an energy quantum 

 at a frequency in the visible spectrum. There is therefore a close analogy 

 between this and the activation of a phosphorogen. Since there exists 

 in the latter an upper temperature limit above which the second stage of 

 activation does not take place, so it is to be expected that there n,ust 

 be an upper temperature limit above which no photosynthesis can take 

 place. This is actually the case, since, as was shown in fig. 3, there is a 

 rapid decrease in the yield of carbohydrates as the temperature is increased 

 above 31° and the reaction falls to zero at about 48°. 



This decrease in efficiency is a very remarkable fact, and, as is well 

 known, it is observed also in the living leaf. It has long been a source 

 of difficulty to plant physiologists and the generally accepted explanation 



