158 ANNUAL OF SCIENTIFIC DISCOVERY. 



the dark, a positive image immediately appears, having the usual maroon 

 color. To fix it, it is only necessary to wash in pure water. If the nitrate of 

 silver is replaced by chloride of gold, the image appears of a deep blue. 

 These pictures resist the action of the cyanide of potassium, even on ebulli- 

 tion; they are therefore far more stable than photographs taken in the ordi- 

 nary way. Tartaric acid has the same property, though in a less degree. 

 Heat increases the sensibility of the reaction. For on covering with a plate 

 of iron heated to 50 C. both the pasteboard which bears the impression 

 from the sun and the leaf of sensitive paper prepared with chloride of silver, 

 the image will appear at the end of a few minutes, while at (P C. it requires 

 several hours to obtain a faint impression. 



g neral result of the researches of M. Xiepce is this, that the bodies 

 which preserve the greatest activity when exposed to the sun, are, with the 

 exception of the salts of uranium, those which are the least disposed to fluor- 

 escence. This chemical activity which certain bodies may contract under 

 the influence of the sun's rays, or insolation, is greater or less according to the 

 nature of the substance; it has its limits. When a substance has reached its 

 maximum of activity, continued exposure does not add anything to it. 

 Paper prepared with the nitrate of uranium changes color in the light, 

 and becomes insoluble; in the dark it is decolorized, and it becomes soluble 

 after some hours, to be colored again in the light. It reduces the salts of 

 gold and silver, so much so as to become colored and insoluble. 



A body rendered active by the sun, will transmit this activity, by contact 

 in the dark, to another body tartaric acid, for example. 



M. Xiepce proposes to investigate whether the permanent activity com- 

 municated to a body by the solar rays is capable of determining the combi- 

 nation of chlorine and hydrogen, and whether it can be acquired in a 

 luminous vacuum. An engraving wet, and subjected to the sun, reproduces 

 itself on sensitive paper. But, if it is covered with some millimetres of 

 water, the effect fails, even with a solution of salt of uranium or tartaric 

 acid. 



After having shown that certain bodies acquire, by exposure to the sun, 

 the property of reducing in the dark salts of gold and silver, M. Xiepce ob- 

 serves further, that the reduction does not take place without the interven- 

 tion of an organic substance. Paper is very good for this purpose, while no 

 action is obtained if we take, for example, the edge of a porcelain plate 

 which has just been broken; on impregnating this edge with a solution of 

 nitrate of uranium, no effect is obtained in the sun; but there is an action if 

 we put on the edge a solution of nitrate of silver, containing a little starch 

 or gum, and then sulphate of iron or gallic acid. A coloration is seen in the 

 part subjected to the sun. It is the same if sih'er be used in place of ura- 

 nium. The reagents which M. Nicpce employs by preference for demon- 

 strating this action of the light, are the salts of gold and silver, tinctures of 

 litmus and turmeric, iodide of potassium, for paper prepared with starch. In 

 many substances that have been exposed to the sun, the activity communi- 

 cated is apparent in the insolubility; it is, on a similar principle, acquired 

 tinder the sun's action, by gelatine containing bichromate of potash, that 

 Mr. Talbot has founded his photoglyphy. Heat and humidity promptly 

 cause the loss of this property. M. Xiepce cites many examples in which 

 the same results are obtained on inverting the course of operations ; thus, a, 

 leaf of paper impregnated with gallic acid and exposed to the sun, treated 

 by iodide of potassium, gives a feeble image, which becomes very decided if 



