312 REPORTS ON THE STATE OF SCIENCE, ETC. 



does increase very lapidly with the density of the absorbing substance. With 

 certain photochemical leactions in solution Henri and Wurmser {/. de Physique, 

 3, 305 (11)13)) found that at a concentration of iV / 10 IbO molecules react lor 

 every phase quantum absorbed, wliilst at a concentration of A'/l this number 

 is increased to 1,360. JStill more striking is the fact recorded by Bodenstein 

 and Dux, that the number of molecules of hydrogen and chlorine which react 

 for every phase C[uantum absorbed by the chlorine varies as the square of the 

 density of the chlorine. 



The variation in the divergence from Einstein's law with the density of the 

 radiated energy has also been proved in the case of the reaction between hydrogen 

 and chlorine (Baly and Barker, Trans. Chem. ,S'or., 119, 6.53 (1921)). The 

 density of the energy radiated during the reaction depends on the velocity of the 

 reaction, and hence on the intensity of the incident light ; the amount of 

 hydrogen chloride, therefore, formed per minute was determined with different 

 intensities of incident light. The phase theory demands that the amount of 

 hydrogen chloride formed per minute should increase at a far greater rate than 

 is accounted for by the increase in the intensity of the incident light^ — that is 

 to say, the number of molecules of HC'l formed for each quantum absorbed 

 should rapidly increase as the intensity of the incident light is increased. Not 

 only was this found to be the case, but two additional phenomena were observed 

 which afford strong confirmatory evidence. 



In the first place, with a constant intensity of the incident light, the velocity 

 of the reaction is at first very small and then increases rapidly up to a constant 

 maximum. It is evident that during the first instant the reaction will obey 

 Einstein's law, but the reabsorption will then commence and the reaction velocity 

 will increase until the proportion of the radiated energy that is reabsorbed 

 reaches the maximum possible for the conditions existing. 



In the second place, when the incident light is cut off the reaction stops 

 instantly, but the chlorine reaches its normal condition very slowly. At least 

 thirty miiiutes are required for the normal state to be reached, for if the light 

 be again allowed to fall on the mixture of gases before this period of time has 

 elapsed the initial velocity of the reaction is abnormally great, and the constant 

 maximum rate is established sooner than is normally the case. The explanation 

 of this very interesting phenomenon is to be found in the presence of partially 

 activated chlorine molecules, i.e. molecular phases intermediate between the 

 reactive and normal non-reactive phases. These intermediate phases, although 

 they are not reactive towards hydrogen, contain more molecular quanta than 

 the normal non-reactive phase, and therefore require less energy to convert 

 them into the reactive phase. This partially activated condition of the chlorine 

 constitutes one of the strongest pieces of evidence in favour of the molecular 

 phase theory. 



One further conclusion also follows from the above — namely, that in an endo- 

 thermic reaction, in the second and third stages of which the energy radiated 

 is very small compared with the critical increment, E, necessary for the first 

 stage, Einstein's law should be obeyed. It is an interesting fact that in the 

 photo-chemical conversion of oxygen into ozone, which is the only highly endo- 

 thermic reaction yet studied quantitatively, Einstein's law is obeyed, since twf> 

 molecules of' ozone are produced for every phase quantum absorbed by the 

 oxygen. 



An exactly analogous explanation to that detailed above for the photo- 

 chemical comljination of hydrogen and clilorine is given by the phase theory 

 for purely thermal reactions, such as the thermal decomposition of phosphine. 

 in which great divergence from the Einstein law is observed. The number of 

 molecular quanta absorbed per second by the phosphine by the radiation from 

 the walls of the containing vessel is calculated from their temperature by the 

 well-known laws of radiation. The number of phosphine molecules decomposed 

 per second is measured and found to be some million times too large. Here, 

 again, since the reaction is exothermic, the energy evolved in the second and 

 third stages can be reabsorbed by the surrounding phosphine molecules. In 

 fact, the decomposing phosphine molecules form a radiating system, with the 

 result that the density of the effective radiation, i.e. the number of molecular 

 quanta available per second, is very much greater than that calculated from the 

 temperature of the walls of the containing vessel. 



