PHOTOCHEMISTRY. 367 



ing itself in reproducing tlie j)lienomenon of pliotocliemistiy. However tliis 

 may be, we continue the study <^»f these phenomena. 



If we receive a pure spectrum on paper rendered sensitive, we shall find that 

 the calorific rays do not in general act : the more luminous take no effect ; the 

 more refrangilde rays, on the contrary, which also produce phosphorescence and 

 fluorescence, are the only ones capable of developing chemical action. For the 

 chloride of silver, the effect commences at the ray F, attains its maximum at H, and 

 becomes progressively weaker np to the limits of the ultra-violet rays. For the 

 chloride of gold the eflect is more capricious ; the action is slow in being pro- 

 duced, but, once commenced, it continues spontaneously, even in the dark. The 

 iodide of silver begins to alter in the red and presents two maxinmms. Theso 

 few examples suffice to show that each substance is impressible by particular 

 rays. 



We must not think, however, that the calorific and luminous rays are destitute 

 of all action. If a paper rendered sensitive be covered with glass of various 

 colors, blue, yellow, or violet, for example, we shall find, after an exposure of 

 some moments to the sun, that the red and yellow glasses have produced no 

 action, and that the blue glass alone has acted. If, however, we begin by caus- 

 ing the violet rays to act for a very short time, and the sensitive paper be then 

 placed nnder red glass, the action is continued. The red glass, which did not 

 possess the property of commencing the chemical action, has therefore the prop- 

 erty' of continuing it after it has commenced. It is for this reason that M. Ed. 

 Becquerel, who discovered these phenomena, has given to the rays of little refran- 

 gibility the name of confinuafor rcnjs. 



The resin of guaiacum presents a peculiar phenomenon : it becomes oxydized 

 only nnder the influence of the violet rays. M. Becquerel, having impregnated 

 a leaf of paper with resin of guaiacum dissolved in alcohol, exposed it to the 

 violet rays, which oxydized it in rendering it blue ; having then submitted it to 

 the action of light under a red glass, he found that it became deoxydized. In 

 the case of organic substances, then, there is an inverse action between the lumin- 

 ous rays and tlie chemical rays which it would be interesting to study, especially 

 with reference to the different phenomena of photochemistry. We have pre- 

 viously seen that, Avith the metallic salts, the chemical rays were ix\ox\eexcitators, 

 but that the luminous rays were contbmaiors. This very important distinction 

 between the different rays of the spectrum is again encountered in the daguerreo- 

 type. Every one knows how daguerrean proofs are made. The operation con- 

 sists in exposing to the vapors of iodine a well-polished plate of silver, for the 

 purpose of covering it with a thin layer of the iodide of silver ; it is afterwards 

 exposed to the light in the camera obscura. Wherever the light has struck it 

 the iodide of silver is reduced. It suffices next, in order to render the image 

 visible, to expose it to mercurial vapors; the mercury is fixed, wherever the light 

 has struck the plate, in the form of minute globules, giving it a deadened color; 

 it is now only necessary to wash the plate with hyposulphite of soda. M. Clau- 

 det has show'n, by the following experiment, that, even on dagueiTcan plates, 

 the luminous rays destroy the effect of the chemical rays. He exposed an iodized 

 plate in the camera obscura, and then cut it into four parts; he observed that the 

 first could condense the vapor of mercury and yield an image ; the second he 

 left in the dark, and placed the third under a red glass. At the expiration of 

 some time, the third had lost and the second had retained the property of giving 

 an image with mercury; the fourth, kept also under red glass, recovered its for- 

 mer sensibility, while it had lost all trace of an impression. It would therefore 

 serve anew to produce an image, a fact very important in practice, because it 

 admits of operating in the light. 



Thus we see that the violet rays possess the property of deoxydizing the metal- 

 lic salts and of acting as oxydants on organic substances. The red rays, on the 



