326 HISTORY OP CHEMISTRY. [LECTURE XV. 



experimentally by Helmholtz himself, has also been proved 

 experimentally by several other investigators, 171 especially by 

 Jahn. 172 



The relations between optical and chemical properties, 

 which are equally important in theory and in practice, can 

 only be touched upon here, as there are but few results of 

 general importance to be brought forward. The chemical 

 effects of light have been closely studied, especially in three 

 cases: i, That of the silver salts; 173 2, That of the process 

 of assimilation by the green parts of plants ; 174 and 3, That of 

 the mixture of chlorine and hydrogen. 175 



Draper endeavoured to prove that the chemical effect pro- 

 duced is proportional to the intensity of the light ; and the 

 proof was completed by Bunsen and Roscoe. It had already 

 been recognised by Scheele that all the rays do not participate 

 alike in the action of the light. This was confirmed by the 

 various subsequent investigators, and it was still more definitely 

 settled by Bunsen and Roscoe. As the principal action was 

 repeatedly found to take place in the violet rays, the idea of 

 specific chemical rays arose ; but this is now entirely dropped. 

 It may be looked upon as established that rays of every wave- 

 length can bring about chemical effects, although not with the 

 same intensity ; and the effects vary according to the chemical 

 nature of the sensitive substance concerned. It is further of 

 importance that the maximum effect in different chemical pro- 

 cesses has been found at different parts of the spectrum. It is 



171 Czapski, Wiedem. Ann. 21, 209; Gockel, ibid. 24, 618. 1? - Jahn, 

 ibid. 28, 21. 173 Scheele, Chemische Abhandlung von der Luft und dem 

 Feuer, Ostwald's Klassiker, 58,48^^.; Senebier, Memoires phys. chim.; 

 Draper, Phil. Mag. [3] 19, 195; etc. 174 Senebier, loc. cit.; Ingenhousz, 

 Versuche mit Pflanzen, Leipzig 1780; Daubeny, Phil. Trans. 1836, 149; 

 Draper, Phil. Mag. [3] 23, 161 ; Sachs, Bot. Zeitung 1864, 59 ; Miiller, 

 Bot. Untersuchungen 1872; Pfeffer, Pogg. Ann. 148, 86; Pringsheim, 

 Berlin. Akad. Ber. 1881, 504; Engelmann, Bot. Zeitung 1882, 663; 1883, 

 i, 17; 1884, 81, 97; Reinke, Bot. Zeitschr. 1884, I ; etc. 17S Bunsen 



and Roscoe, Pogg. Ann. 100, 43; 101, 254; 117, 529; Erganzungsband 



5> i77. 



