THE CHEMICAL ACTION OF LIGHT 279 



was not isolated). Benzophenone and diphenylmethane 1 give 

 aa/3/S-tetraphenylethanol, 



Ph.CO + CH,,Ph 2 -> CPh 2 (OH).CHPh, 

 Paraffins and hydrocarbons are transformed by benzophenone 

 in sunlight into unsaturated compounds which then combine 

 with the ketone ; the compounds formed are often complex. 2 



Photochemical action in relation to the refrangibility of the 

 active light. Here it is possible only very briefly to touch upon 

 this question. In an early series of experiments 3 Ciamician and 

 Silber, in 1902, showed that the photochemical results they had 

 obtained up to that date were due to the blue-violet rays and 

 were not produced by light of greater wave-length nor by the 

 heat-rays of the solar spectrum. In this respect, the changes 

 recorded were similar to ordinary photographic changes. On 

 the other hand, in the case of chlorophyll, photochemical change 

 is produced chiefly by the rays which are most strongly absorbed ; 

 the maximum activity is in the red between the lines B and C, 

 another maximum occurring in the blue near F, with a minimum 

 in the green corresponding with the transmitted rays. 4 In the 

 case of the red-tinted pigmented cells (Floridaceae) and yellow- 

 ish-brown (Diatomaceae), assimilation was proved by Engelmann 

 to be most active in that coloured light which was most com- 

 pletely absorbed by the pigment of the cell (" chromophyll "). 



From the recent measurements of Brown and Escombe 5 of 

 the actual energy absorbed by the green leaf during the period 

 of assimilation it appears that under the most favourable condi- 

 tions nearly 100 per cent, of the total light-energy absorbed is 

 utilised in bringing about chemical change. The leaf seems, in 

 fact, to be an almost perfect photochemical machine ; moreover 

 the photochemical change produced in the leaf differs from all 

 others, not only as regards the enormous amount of energy 

 actually absorbed but in the fact that this energy is mainly 

 taken up from a portion of the spectrum which is usually in- 

 active photochemically : in other words, chlorophyll has pro- 

 perties which distinguish it from most other colouring matters. 



1 Paterno and Chieffi, Gazzetta, 1909, 39, ii. 415. 



1 Compare Paterno and others, Atti R. Accad. Lincei, 1909, 18, i. 104 ; 

 Gazzetta, 1909, 39, i. 341, 449 ; ii. 415 ; 1910, 40, ii. 321. 



3 Atti R. Accad. Lincei, 1902, 11, ii. 145 ; Ber. 1902, 35, 3593. 



4 Engelmann, Bied. Centr. 1883, 174. 



5 Brown and Escombe, Proc. Roy. Soc. 1905, 76 B, 29 (Bakerian Lecture); 

 see also Weigert, Chem. Wirk. Lichts, p. 288. 



