352 THE BEGINNINGS OF PHOTOGRAPHY. 



exposed in the sumo way on china plates, but kept luoi.st witli alcohol. 

 In the case of the gold no change took place in an hour. The experi- 

 ment was interrupted l)y want of sun, and on subsequent exposure there 

 was a faint change of color and some reduced gold. With silver there 

 was more darkening, up to rc^ldish brown, but no l)lackening after 

 four days' exposure. In (i>) a sli}) of silk was dipped in alcoholic solu- 

 tion of silver nitrate and after veiy careful drying in darkness over 

 sulphuric acid in a phial was exposed to light over the acid for about 

 three months without the slightest change of color. 



From these experiments she concludes: (1) That water is essential 

 to the n^duction of metals by light; (2) that light does not reduce 

 metals by giving them phlogiston; nor (8) by fusing and expelling 

 their oxygen; (4) light is a combustible body, for it acts like hydro- 

 gen, piiosphorus, sul})hur, and charcoal in the reduction of metals, 

 and further, that it is obvious that light reduces metals b^- decompos- 

 ing water attracting the oxygen, while the h3'drogen unites, in its 

 nascent stiite, to the oxygen of the metal and reduces it, forming at the 

 same time a quantity" of water eipial to that decomposed. In chapter 

 10 she gives an intelligent explanation of the decomposition of water 

 which takes place when silver is precipitated in the metallic^ form hy 

 ii'on and other metals, the precipitant uniting with the oxygen, while 

 the hydrogen combines with the oxygen of the precipitated metal, 

 forming water, and reduces the metal. 



These principles are applical)le in the same wa}' to the reduction of 

 silver by acid iron solutions, as in the wet collodion process. She also 

 notes the reducing powei- of gallic, tartaric, and formic acids on 

 metallic salts. In considtn-ing her work it imist l)e remembered that 

 chemical science was in a very transitional state at the time she wrote, 

 but it is interesting ])ecause it led to further investigation of the action 

 of light on silver compounds b}^ Count Kumford, liitter, Berthollet, 

 and others. Her little treatise was translated into German by Lentin, 

 and fa\'orably reviewed by Ritter. A ver}- appreciative accoiuit of 

 the experiments with light is to be found in Placidus Heinrich's prize 

 essay, " Von der Natur und den Eigenschaften des Lichtes" (St. Peters- 

 burg, ISOcS. p. lo6). We find several of her experiments repeated in 

 a little ])()ok of chemical recreations (Rational Amusement, by W^. M. 

 Toulmin, Calcutta, 1822), among them methods of drawing silver or 

 gold figures of flowers, etc., upon silken ribbons. Many of her obser- 

 vations were confirmed and extended by Count Rumford in a paper 

 entitled "An iiKjuiry concerning the chemical properties that have 

 been attrilnited to light'' (Phil. Trans. R. S., 1798), in which he tried 

 to show that the reduction of the gold or silver on the tissues was 

 produced, not by any chemical combination of the matter of light with 

 such bodies, but merely by the heat which is generated or excited by 

 the light that is absorbed bv them. 



