368 PHOTOCHEMISTRY 



contrary, in tlic first case are only continuators, but they become roductors in the 

 presence of organic substances. 



MM. Bunseu and Roscoe have songlit to estimate tlie quantity of the cliemical 

 forces annually discharged on the globe by the sun; they caused the solar light 

 to penetrate by a very naiTow aperture to a vessel containing a mixture of chlo- 

 rine and hydrogen. These g'ases, which do not combine in darkness, comliined 

 over the whole tract of the luminous ray in proportion to the quantity of the 

 radiations absorbed. In a word, if the aperture by which the light amves be 

 doulded or tripled, the quantity of chlorhydric acid formed is also doubled or 

 tripled. In this way, MM. Bunsen and lloscoe found that the quantity of chemi- 

 cal rays aninially discharged by the sun is capal>le of combining a stratum 35 

 metres in depth of mixed hydrogen and chlorine gas. The terrestrial atmo- 

 ■sphero ai)Sorbs a part of these rays, so as to reduce to 17 metres the stratum of 

 chlorhydric acid W'hich would be formed under the normal inclination, and to 11 

 metres if the sun traversed the atmosphere at 45 degrees. 



It is also to photochemistry that the action exerted by light upon vegetables 

 is to be referred. Bonnet ascertained that leaves immersed in water and exposed 

 to the sun disengage a gas by their under surface. Priestley announced that 

 plants have the property of restoring its primitive purity to the air vitiated by 

 animals. Towards the close of his life, having repeated his experiment, he 

 arrived at a diHerent result, and failed to detect the secret of this ditlerence. It 

 was Ingenhousz who explained this phenomenon in 1779. He proved that under 

 the action of the solar rays the green parts of plants purify the air, while, on the 

 contrary, they vitiate it in darkness. By what rays are these effects produced'? 

 If the view which we have taken may be regarded as correct, r:e are justified in 

 saying that it is the red and the yellow rays which cause the production of oxy- 

 gen ; the others produce carbonic acid. The ex})eriment has been conducted l)y 

 M. Draper, under conditions which leave nothing to be desired. He took seven 

 tubes of glass containing water charged with carbonic acid, and introduced into 

 each a leaf of grass ; he then caused one of the seven colors of the spectrum to 

 fall on each tube. After an interval of time, oxygen was disengaged in the 

 tubes exposed to the yellow and red rays ; in the others there was none. The 

 red and yellow rays, therefore, are those alone which give to plants the property 

 of renewing the ox^'gen of the air. 



I shall conclude by stating in a few words the means employed in the practice 

 of photography. A plate of glass is taken, perfectly cleansed, and is covered 

 with a thin coat of collodion containing suitable proportions of bromides and 

 iodides. Before the plate is entirely dry it is plunged into the bath of silver, 

 the operation being conducted under protection from the light ; it is then exposed 

 in the camera obscura. To bring out the image it is enough to wash with pyro- 

 gallic acid. Lastly, to prevent ulterior alteration it is washed with the hyposul- 

 phite of soda. It is thus that a negative proof is obtained ; to obtain positive 

 proofs it suffices to apply this on paper rendered sensitive and to expose the whole 

 to the light. 



It is not probable that photography will be limited to the progress which it 

 Las made up to the present time. At this moment, great hopes are entertained 

 of obtaining, and that at no distant day, not only the outline of objects, but also 

 their proper color. Some time has already elapsed since M. Edmond Becquerel 

 conceived the idea of submitting to the action of the solar spectrum a plate on 

 which he had deposited a thin layer of iodide of silver. After the lapse of a 

 considerable time he thus obtained a perfectly distinct image of the spectrum, 

 with its stripes ; and, what is remarkable, even the obscure calorific rays were 

 also represented. But the point most to be remarked in this experiment is that 

 the spectrum was colored. Unfortunately it was impossible to fix these imagfj, 

 and only a very fugitive proof was obtained. M. Niepce de Saint Victor occu- 

 pied himself with these phenomena, and obtained in the camera obscura photo- 



