368 PHOTOCHEMISTRY 



contrar r, in the first case are only contiuuators, but they become reductors in the 

 presence of organic substances, 



MM. Bunsen and Roscoe have sought to estimate the quantity of the chemical 

 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 gases, which do not combine in darkness, combined 

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

 radiations absorbed. In a word, if the aj)erture by which the light arrives be 

 doubled or tripled, the quantity of chlorliydric acid formed is also doubled or 

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

 cal rays annuallj^ discharged by the sun is capable of combining a stratum 35 

 ijictres in depth of mixed hj^drogen and chlorine gas. The ten-estrial atmo- 

 sphere absorbs a part of these rays, so as to reduce to 17 metres the stratum of 

 chlorliydric acid which 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 l)y their under surface. Priestley announced that 

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

 animals. I'owards the close of his life, having repeated his experiment, he 

 arrived at a different result, and failed to detect the secret of this difference. It 

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

 the action of the solar rays the green paits 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 coiTcct, we 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 experiment has been conducted by 

 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 ra3's, therefore, are those alone which give to plants the property 

 of renewing the oxygen 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 ojieration 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 probal)le that photograph}- Avill be limited to the progress which it 

 has 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 jn'ojjer 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 images, 

 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 



