348 - THE BEGINNINGS OF PHOTOGKAPHY. 



liina cornea, when expo.sed to liyht, moistened with nitric acid, docs 

 not change color. In section •'>('. lie demonstrates the presence of 

 phlogiston in light, and shows that light itself is not phlogiston by 

 placing- in the colored rays of the solar spectrum paper on which some 

 luna cornea had been spread, when it w-as found that the darkening 

 took place much more rapidh' in the violet rays than in the others; 

 i. e., the calx of silver more quickl}^ separates the phlogiston from, the 

 violet than from the other rays. He thus shows that light can not be 

 considered as a simple substance or an element. These observations 

 of Scheele's, scanty as they are, mark a very distinct advance in photo- 

 chemical knowledge, and demonstrate fully the decompositions that 

 take place by the exposure of silver compounds to light, so far as the. 

 imperfect chemical theories of the time allowed. It may ))e noted that 

 Scheele did not discover the solubility of silver chloride in ammonia, 

 as it apparently was known to some of the alchemists, and, as we have 

 seen, is distinctly mentioned by Glauber and I^emery; also, in 1761, 

 b}^ Marggraf, who describes the preparation of anunonia (spiritum 

 urinosum), and says that he can onh^ sa}' of it that it dissohc^s luna 

 cornea in the cold. (Chymischer Schriften, pp. «)2 and 28-1.) 



SENEBIER AND PHOTOMETRY. 



Scheele's observations were repeated and developed further ))y Jean 

 Senebier, but more particularly with reference to the influence of light 

 on vegetation. His book, Memoires Plu'sico-chymiques sur Tinfluence 

 de la lumiere solaire pour modifier les .etres des- trois regnes de la 

 Nature, et surtout ceux du regne vegetal (1782), contains a vast num- 

 ber of interesting- observations on the disengagement of air or gas 

 from leaves under water in sunshine, on the production and devel- 

 opment of confervte in water, on etiolation and the effects of colored 

 lights and of the different colored rays of the spectrum on the growth 

 of plants. He recognized the greater activity of the violet ray. He 

 preceded Herschel in the examination of the temperature of the differ- 

 ent rays of the spectrum but failed to note the special heating power 

 of the ultra-red rays or the extension of action beyond the violet. In 

 the latter case he placed the chloride in saucers and threw the spectrum 

 on them. Had he used strips of paper he would probably have 

 observed the ultra-violet rays. In many of his experiments he used 

 cut-out masks or shades of metal or other material to cut off the light 

 of the sun from fruit or plants under observation. He, like others of 

 his time, believed that light w^as a material substance, and also in the 

 existence of phlogiston, so that it is sometimes difficult to translate his 

 meaning- into conformity with modern ideas. 



He investigated the changes of color in various woods by the action 

 of light, using slips of sheet lead as shades, also glazed boxes fitted 

 with various colored y-lasses or different thicknesses of the same glass. 



