THE BEGINNINGS OF PHOTOGEAPHY. 847 



We next find Schulze's experiment included by Dr. A\'illiam Hooper 

 in hi« Rational Recreations (1774, 4, 141}) under the heading- "Writing 

 on glass b}' the rays of the sun." It was repeated by Ilalle in 1784 

 in his Magie, oder die Zauberkrafte der Natur, who mentions writing 

 with sympathetic inks made of solutions of gold or silver, and it 

 appears also in later collections of chemical experiments. Here we 

 have the first distinctl}^ graphic application of Schulze's experiment, 

 and nothing else of the kind is given by Hooper. For the purpose of 

 this experiment the cut-out stencils (or, as Schiendl calls them, " nega- 

 tives") were more suitable than the positive prints or projections used 

 by Wedgwood and Davy. No thought of lixing or of multipl3dng 

 copies of his light images seems to have occurred to Schulze or to an}^ 

 of those who described his experiment, and, indeed, from the nature 

 of it, it was not likely to do so. In this sense he certainh^ fell short of 

 the photographic ideal which Davy and Wedgwood undoubtedly had 

 before them. 



scheele's observations. 



In 1777 Carl Wilhelm Scheele published his well-known observations 

 and experiments on air and lire (Aeris atque Ignis examen chemicum, 

 Upsala and Lips, 1777), of which translations were published in Ger- 

 man, French, and English. He also was a believer in the prevailing 

 theory of light being a body, and that the light of the sun was the 

 same as of a burning candle. He sets himself to prove the presence 

 of an inflammable principle in light (sec. 60), and starts with the 

 blackening of a solution of silver nitrate exposed to the sun on a piece 

 of chalk, noting that reflected white light produces the same efl'ect, 

 but heat does not; then he asks whether this black color should be 

 real silver, and in the following sections describes a series of experi- 

 ments he made to prove it. The most important, photographically, 

 are in section 63, in which he describes how he first of all prepared 

 silver chloride by precipitation with sal aumioniac from solution of 

 the nitrate, washed and dried the precipitate, and exposed it on paper 

 to the sun for two weeks, when the surface of the white powder grew 

 black. The powder was then stirred and the operation repeated 

 several times. He then poured some caustic spirit of sal ammoniac 

 upon the darkened chloride, and found that the annuonia dissolved a 

 quantit}' of the chloride, though some black powder remained. This 

 was washed and dissolved in pure nitrous acid and was again precipi- 

 tated as luna cornea. Consequently the blackness which the luna 

 cornea acquires from the sun's light, and likewise the solution of silver 

 poured on chalk, is siher by reduction. In further experiments he 

 showed that during the exposure of luna cornea to light under water 

 the latter takes up muriatic acid, which can be proved by its again 

 precipitating luna cornea m a solution of silver nitrate; also that 



