PHOTOCHEMISTRY. 365 



lines, and becomes a deep blue under the "white, we obtain, in reality, an inverse 

 iraajre. This is a negative proof. 



Thus we see that the solar radiations, in acting on different bodies, maj' give 

 rise to two very distinct orders of chemical phenomena : })henomena of redncti(jn 

 in all the metallic compounds ; oxidation, a tendency to favor combination, as 

 regards all organic bodies. 



If the opinions thus advanced are well fomuled, it is evident that bv mixing 

 with nu'tallic salts organic substances we shall double the efiects ]»y augmenting 

 the sensibility of the pliotochemic action. This may be demonstrated l)y numer- 

 ous experiments. The foUowing, which is one of the most decisive, we owe to 

 M. Niepce de Saint Victor, nephew of the inventor of photograph^^ lie took 

 a piece of porcelain presenting a fresh fracture and covered it with a solution 

 of nitrate of silver ; ho then exposed it to the sun while screened from all 

 organic matter. The nitrate of silver remained unaltered ; but when a leaf of 

 paper was imbued with this solution, or the nitrate of silver was mingled with 

 any organic matter, (gelatine, tartaric acid, for example,) ])efore covering the 

 porcelain, the action of the s(dar rays was immediately manifested, and the sil- 

 ver was reduced to the metallic state. It is evident that in this experiment tlie 

 active co-operation of the organic matter admits of no doubt. 



We may farther adduce an experiment of Professor Haghen, of Kosnigsburg, 

 which leads to the same conclusions with the preceding. M. llaghen took two 

 tubes and filled them, one with a solution of nitrate of uranium in distilled 

 water, the other ^\'ith an alcoholic solution of the same salt. The first of these 

 tubes was carefully closed, to exclude the access of any organic matter ; both 

 were afterwards exposed to the sun, and, agreeably to M. Haghen's previsions, 

 the second alone was altered and partially reduced. These results are con- 

 stantly tiunied to advantage in the art of photography. 



And, indeed, in the preparation of sensitive paper the only object in covering 

 the sheet with a thin layer of choloride of silver is to bring together in a state 

 of considerable division the reducible metallic salts and the organic matter des- 

 tined to effect the reduction. It is proper, however, to add that, in order to still 

 further augment the sensibility of the papers intended for photographic proofs, it 

 is the practice to dip them, in the last place, in a solution of a chloride, a bro- 

 mide, or an alkaline iodide. 



The following will serve as a final example of the energetic part which 

 organic substances play in these photochemic actions. About 1850, M. Poitcvin, 

 one of the most celebrated French photographers, took a salt of sescpiictxide of 

 iron, the ses(piicldoride, added to its solution an organic matter, and saturated 

 some leaves of paper with this mixture. Under the influence of light and by 

 virtue of the presence of organic matter this salt, wdrich the light alone could 

 not modify, was partially reduced, and passed to the state of a pliotochloride. In 

 taking a pliotogra})hic proof with such paper, it is sufhcient, in order to make the 

 image appear, to wash the sheet with a solution of c^'anoferridc of potassium or tan- 

 nin. These compounds reacting only on the perchloride of iron wherever it has 

 been preserved intact, that is to say, under the black portions, we obtain a positive 

 proof, colored blue or black, according to the compound employed for washing. 



I shall not further nmltiply these examples; but there is a fact of much more 

 importance, and to which I proceed to call your attention for a moment : I refer 

 to what are called, in photography, the revcalh^g phenomena. 



It is generally known that, when the sheet of prepared paper is withdrawn 

 from the camera obscura, the action of the light seems to have produced no par- 

 ticular effect on its surface ; and yet, when it is immersed in a certain hath, 

 the image immediately appears. This, in the language of the photograi)hers, 

 is the revelation of the image. We shall accept this word, amiutious as it is, 

 and endeavor to render an account of the phenomenon, the most important and 

 certainly the most curious of photochemistry. 



