344 THE BEGINNINGS OF PHOTOGRAPHY. 



some black paper on the unchang-ed .side of the vessel to see if it would 

 protect the silver salt, and left it again exposed till next day, when, on 

 returning to it, he found that the luna cornea had turned violet ever}^- 

 where except in the parts protected by the paper. From this he con- 

 cluded that the change was due more to the action of the light than of 

 the air, and that the same is probabl}^ the case with the fading of the 

 colors of garments, for fullers when dyeing the more costh" cloths onl}^ 

 consider a dye good if the color remains unchanged after a long exposure 

 in full daylight, though thev probabl}' attribute the injury to the effect 

 of the air rather than of light. The 7"emainder of the paper is devoted 

 to experiments ])y Bonzo on the changes of color of silken ribbons in 

 light. Here, as in the case of Schulze, Beccari's experiment was more 

 photochemical than photographic. 



DR. WILLIAM lewis's INVESTIGATIONS. 



So far it had Ijeen recognized that the change of color of silver 

 compounds was due to the action of light, but nothing had been done 

 to show what chemical changes took place during this action or what 

 were the conditions to be fulfilled, and the first to make any investiga- 

 tion in this direction was Dr. William Lewis, M. D., F. R. S., the 

 author of many works on technical chemistry. In his Commercium 

 Philosophicum Technicum, or Philosophical Commerce of Arts (1763), 

 he has given a very full account of his inv^estigations into the cause of 

 the coloration of ivory, bone, wood, or stone treated with solution of 

 silver nitrate and exposed to sunshine. He repeated Schulze's experi- 

 ments with chalk moistened with solution of silver nitrate, both while 

 wet and after being dried, and notes that the color is produced only 

 on those parts on which the sun shines, and that distinct characters may 

 be exhibited on the mass by intercepting a part of the sun's light by 

 threads or cut paper. He found that the color thus produced on the 

 chalk}" mixture was not so deep as it was on bone or ivory and was 

 entirely superficial, so that b}" shaking up the mixture it again appeared 

 white. By exposing the mixture constantly to light for many weeks 

 and frequently shaking it, he was able to darken it throughout, though 

 weakly. The light of a candle or the ordinary warmth of a fire had 

 no effect, but at a considerable heat the matter became brown, though 

 it did not become black as it did in the sun. 



He also tried several earth}' bodies and found that those which 

 dissolve in acids, the ashes of vegetables, of bones and horns, darkened 

 in the same way as chalk and other mineral calcareous earths. Pow- 

 dered flint remained perfectly uncolored, even after six months' 

 exposure in the sun. White clay, plaster of Paris, and powdered talc 

 also remained uncolored; and even chalk itself, previously satiated with 

 vitriolic acid so as not to be acted upon 1)y the acid in which the silver 

 was dissolved, was imchanged. He concluded, therefore, that to pro- 



