THE BEGINNINGS OF PHOTOGRAPHY. 341 



He says very little about the eliloride, and bad apparently no idea of 

 the action of light or sunshine in producing the change of color in 

 organic matter by the action of silver nitrate. 



In the same way Robert Boj^le frequently mentions the stahiing of 

 the skin I33' solutions of silver or of gold and also the darkening of the 

 chloride in the air, but seems to have had no idea that it was due to 

 the action of light. Thus, in his General History of the Air (1692), 

 page 53, he says: 



If we precipitate a strong solution of good silver made in aqua fortis with a com- 

 jietent quantity of spirit of sea salt we shall have a pouder whieh at lirst will be very 

 white; but if the liquor l^e nut poured off, this being exposed for a good while to 

 the air, it would acquire on the surface a dark colour, which perhaps an attentive 

 eye will discern somewhat various, as this or that kind of saltness happens to be 

 predominant in the air. 



At page 52 he mentions that silver plate exposed to the air in Amster- 

 dam is very readily tarnished, and it is evident that he looked upon the 

 darkening of the chloride as something analogous and due to saltness 

 or impurit}" in the air. 



Nicolas Lemery also des^cribes the preparation of a medicinal '" lunar 

 tincture''' made by dissolving precipitated silver chloride in spirit of 

 urine and spirit of wine. He says, further, that the precipitate of sil- 

 vci- with salt darkens on drying, even in the shade, no doul)t on 

 account of a small quantity of copper present. 



sciiulze's observations. 



Although the writings of Kunckel and Stahl contain a good deal 

 about the chemistr}- of the compounds of silver and their reduction by 

 heat and chemical methods, we find no observations of the darkening 

 by light, and the first investigation of this action was made b}- Johann 

 Heinrich Schulze, who published an account of his experiments in the 

 Acta of the Cesarean Academy for 1727. (A full translation of this 

 paper appcuired in this journal for ISiKS, p. 53.) Though these exper- 

 iments were mentioned b}' Priestley and other authors, and, as I propose 

 to show, in all probabilit}^ led the way to Wedgwood's work, they 

 seem(Hl to ha\e entirel}' dropped out of sight until they were ])rought 

 forward l)y Doctor Eder in 1881, in his histor}^ of photography pub- 

 lished in the Photo. Correspondenz. Like manj'^ valuable discoveries, 

 th(\v originated with the investigation of an accident. St-hulze tells us 

 that while experimenting on the preparation of the Bolognian phos- 

 phorus, with a mixture of chalk saturated with some aqua fortis which 

 contained a small quantity of silver, at an open window, he was sur- 

 prised to rind tlie color of the surface changed to a dark puri)lish red, 

 while the part untouched by the sun's rays remained unchanged. 

 This curious fact struck him so forci})ly that he put aside liis original 

 experiments to investigate the cause of the darkening. His friends 



