140 ILLINOIS STATE ACADEMY OP SCIENCE 



hydrogen, heptane, hexane, toluene and hexahydro-ben- 

 zene under the influence of light. The first took place 

 much less than was predicted by theory, several quanta 

 being required to cause one molecule to react; the next 

 three took place more than was predicted by theory and 

 hexahydrobenzene reacted according to theory. Since the 

 free energy changes in organic reactions are little known, 

 it is impossible to correlate these results with the rule 

 enounced above. 



The action of light on the hydrogen-chlorine reaction 

 has formed the subject of many studies. As a result, it 

 is impossible to state at present what the function of the 

 light really is. Stark 11 suggests that the action of light 

 is to loosen the valence electrons. It is impossible to 

 make any generalization of this sort, and it is probable 

 that the action in the hydrogen-chlorine reaction is not 

 connected with the valence electrons as such but with the 

 molecule as a whole. In some recent experiments 12 it has 

 been shown fairly conclusively that a mercury surface 

 which has been acted on by wave lengths below the pho- 

 toelectric threshhold reacts more readily with nitrogen 

 dioxide and with oxygen than an unactivated mer- 

 cury surface. Since fields which would tend to hinder the 

 elimination of electrons from the surface caused the 

 speed of the reaction to become normal, it seemed that 

 the emission of the electrons was the deciding factor. A 

 large number of molecules of HgO were formed for each 

 electron emitted, and since the free energy change is 

 negative, this reaction should be classified as a "second- 

 ary" light reaction. 



In conclusion, it seems that the field of photochemistry 

 is in a rather unsatisfactory state from a theoretical 

 standpoint. Much work is being done on synthesis of or- 

 ganic compounds, especially those compounds formed in 

 plants by the action of sunlight and of certain carbohy- 

 drates from formaldehyde, and these reactions may lead 

 to very important conclusions from the standpoint of the 

 biologist. 



"Stark, Atomdynamik. Leipzig:, 1911, Vol. II, p. 207. 



13 Moore and Noyes, to appear in the June number of the Journ. Amer. 

 Chem. Soc. 



