364 PHOTOCHEMISTRY. 



Tins admirable discovery, wliicli is the principle of all tlie phenomena of 

 pliotocliemistry, remained long- unexplained, under the chemical point of view. 

 The explanation was given by MM. Girard and Duvanne. These two chemists, 

 having exposed chloride of silver to the solar rays, found that this blackened 

 chloride was susceptible, in part, of being attacked, with disengagement of 

 hyponitric acid, by nitric acid, a manifest proof of a partial reduction of the 

 chloride into chlorine and into metallic silver under the influence of the light. 



But this compound is not the only one in which this phenomenon is observed. 

 Light acts as a reductive agent on the nitrate of silvei", the chloride of gold, the 

 chloride of platina, and in general on almost all the chlorides, bromides, and 

 iodides of metals the least oxidizalile. The same is the case with a great 

 number of oxygenated metallic compounds. Every one knows that concentrated 

 nitric acid, entirely colorless when exposed to the light, is quickly colored yellow, 

 inconsiderable quantities of this acid becoming decomposed into hyponitric acid 

 and oxygen. The same may be said of chromic acid, which undergoes a par- 

 tial reduction, loses a certain quantity of oxygen, and is transformed into sesqui- 

 oxyde of chromium. 



In all these cases the light acts on the metallic compounds as an agent essen- 

 tially reductive. We are now about to see this same agent, in another order of 

 experiments, produce ver}' varied oxidations, and favor combinations in general. 

 Let us take, for instance, a mixture of chlorine and hydrogen, and expose this 

 mixture, enclosed in a flask, either to the solar radiations or the brilliant flame 

 of magnesium. A sharp detonation will immediately announce the instantaneous 

 combination of these two gases in forming chlorhydric acid. 



There is here, then, no longer an action of reduction, but an action of com- 

 bination, which the light has exerted. The phenomenon is the same for all 

 organic substances. We know, for example, that if hydrogenated compounds 

 be exposed to the action of the sun in presence of chlorine or bromine, combi- 

 nations are always formed in which the chlorine or bromine is substituted, equiv- 

 alent by equivalent, for the h3'drogen of the organic compound. This, in effect, 

 is the starting point of the theory of substitutions devised by M. Dumas. The 

 process of bleaching cloths by exposing them to the sun is a phenomenon familiar 

 to every one, and is also owing to a slow oxidation of the tissue. 



Among these different examples of oxidation joroduced under the influence of 

 light, there is one which it is especiallv incumbent on us to cite, because it has 

 been rendered famous by an ingenious experiment, made in 1813, by Jean 

 Nicephore Niepce, an experiment 'which was in some sort the starting point of 

 photography. 



Nicephore Niepce, having taken bitumen of India, (asphaltum,) dissolved it 

 in oil of lavender, and was thus enabled to spread it on a plate of glass, which 

 he exposed to the sun after having covered it with an engraving. Now, bitiuneu 

 has the curious property of becoming insoluble in volatile oils when it has been 

 for some time exposed to the light. This light penetrated to the bitumen through 

 all the white parts of the engraving, but was arrested by the black lines ; and 

 by afterwards washing his plates with oil of lavender, M. Nicdphore Niepce 

 obtained a reproduction of his engravings. The white parts therein Avere rep- 

 resented by the bitumen, now become insoluble and of a milky-white appearance, 

 the parts from which the bitumen had been removed representing the black por- 

 tions. 



This experiment, as I have said, was the point of departure for photography. 

 Still another example of these phenomena of oxidation remains to be noticed. 

 It is aftbrded by the resin of guaiacum, which has the singular property of 

 changing to a deeji blue in the b"ght at the same time that it is oxidized. If, 

 therefure, it is dissolved in alcohol, and a sheet of paper be impregnated with it, 

 we may o]:)tain copies, as with tlie bitumen, by covering the paper with an 

 engraving and exposing it to the sunj but as it remains white luuler the black 



