THE BEGINNINGS OF PHOTOGEAPHY. 345 



dnce a l)lMck stain from solution of silver, it was necessary not onl}' 

 that the sul)stance moistened with it should be exposed to the action 

 of solar light, ])ut that it should contain some matter which the nitrous 

 acid mig-ht dissolve preferably to the silver which it already held dis- 

 solved. He observes that thoug-h this is plaiidy the case with bones, 

 horns, hair, marble, and other bodies which ai'c stained by the silver 

 solution, there are also some stones, such as agate, in which a sub- 

 stance soluble by the acid has not yet discovered itself. (In another 

 place he refers to Du Fay's experiments with agates already noted.) 



He goes on to say that this production of a dark color by the action 

 of the sun is not peculiar to solution of silver, or to a combination of 

 this solution with soluble earths, and notes that precipitated nitrate of 

 bisnmth and mercurius dulcis, a combination of quicksilver with the 

 marine acid (calomel), suffer a like change, but do not become l)lack 

 like silver. He does not mention the action of light on silver chloride. 



In the early part of the book he notes also that the solution of gold 

 in aqua regia stains the skin and other animal and vegetable substances 

 purple, the coloring being hastened b}" exposure to the sun and free 

 air and favored by the presence of moisture. He mentions several 

 other preparations of the muriate of gold combined with sea salt, 

 niter, or sal anuuoniac, as well as solutions in ether and volatile oils. 

 He also discusses the staining power of a solution of platimnn in acpia 

 regia in the light and finds it nmch less than that of gold. It gives a 

 brown stain to organic materials dipped in it, but in most cases water' 

 washes off the stain. 



CONNECTION BETWEEN SCHULZE AND WPHXIWOOU. 



These very interesting researches are, like the previons ones of 

 Schulze and Beccari, mor(> photochemical than pht)tographic, though 

 the}" form a noteworthy contri])ution to the bistor}- of silver printing 

 which has hitherto been quite overlooked. They are the more impor- 

 tant because it appears extremely probal)le that they form the con- 

 necting link between Schulze and Wedgwood, for we tind in Dr. 

 Thomas Thomson's History of Chemistry (Vol. I, p. SGO), that at Doc- 

 tor Lewis's death, in 1781, all the manuscript volumes containing his 

 experiments and collections from other authoi's which had ])een com- 

 pih'd by hU assistant, ]\Ir. Chicholm, who liad 1)een with him for man}' 

 years, were purchased ])y ^Ir. Wedgwood, who jilsotook Mr. Chicholm 

 into his own service and put him in charge of his la])oratory. Accord- 

 ing to Miss Meteyard (Life of Josiah Wedgwood, "2, }). 465), the name 

 of this assistant was Alexander Chisholm; he had been thirty 3'ears 

 with Doctor Lewis, of Kingston-on-Thames, and entered AVedgwood's 

 service at Etruria as secretary and chenucal assistant in 1782, and 

 was for a long tinu^ liis right-hand man. He di(Kl in 1S()7. From 

 Mr. Litchtield's "'Tom AN'edgwood" (p. 5), it appears that C/hisholm (or 



