PHOTOGRAPHY. 27 



by the name of chlorophyll. Its solution is red by trans- 

 mitted, and green by reflected light. Here we have a piece 

 of paper which has been brushed over with this alcoholic 

 solution : the colour is a sort of primrose green. By exposure 

 to light it has become bleached. This bleaching is principally 

 due to the yellow light and not to the blue light, which 

 acts on the salts of silver. Again, if you take the leaves of 

 stocks, common wallflowers, violets, or roses, and treat them 

 with alcohol, you can extract the colouring matter, and if, 

 having brushed it over, you expose it beneath a negative, you 

 will get prints of various colours. If you treat the rose extract 

 with a small quantity of acid and brush it over a sheet of 

 paper and expose it to the light, you will find the natural 

 pink colour intensified and the subsequent change will be 

 increased. Again, take the common violet, treat its extract 

 with ammonia, and it gives you a green solution, but the 

 green colouring matter is bleached by the action of light, 

 and experiment proves that the parts of the spectrum to 

 which the colouring matter is sensitive are not the same 

 as those to which the silver salts are sensitive. There is 

 a wide range of experimental work yet to be undertaken 

 with respect to this colouring matter of flowers. 



I should here like to call your attention to the fact that 

 some gaseous bodies as well as solids are affected by light. If, 

 for example, we take hydrogen and chlorine in proper propor- 

 tions in a glass bulb, and keep them in the dark, no com- 

 bination takes place ; but if we take such a bulb into sunlight, 

 they combine almost instantaneously, the light causing the 

 atoms to swing in such a way that they mutually attract 

 each other, and form hydrochloric acid. As Dr. Tyndall has 

 shown, it is not heat-waves that cause the atoms to com- 

 bine, but light- waves ; he enclosed the gases in a collodion 

 balloon, and then caused them to combine by a concentrated 

 light, and the film was found unburnt. In diffused light 

 the combination takes place slowly and without explosion. 

 For similar reasons, if chlorine be passed into water in the 

 daylight, hydrogen is abstracted from the water and hydro- 

 chloric acid is formed, the oxygen forming another compound 

 with the chlorine. 



The bodies to which I next shall call your attention as 

 sensitive are metallic compounds. Sir John Herschel was 

 the first to investigate the action of light on iron com- 



