366 Royal Society. 



Captain Beaufort, dated the 22nd of January last, at which time he 

 ■was ignorant that it had been considered by Mr. Talbot, or by any 

 one in this country. As an enigma to be solved, a variety of pro- 

 cesses at once presented themselves, of which the most promising 

 are the following ; 1st, the so-called de-oxidizing power of the che- 

 mical rays in their action on recently precipitated cliloride of silver; 

 2ndly, the instant and copious precipitation of a mixture of a solu- 

 tion of muriate of platina and lime-water by solar light, forming an 

 insoluble compound, which might afterwards be blackened by a va- 

 riety of agents ; 3rdly, the reduction of gold in contact with de- 

 oxidizing agents ; and, 4thlj'-, the decomposition of an argentine 

 compound soluble in water, exposed to light in an atmosphere of 

 peroxide of chlorine, either pure or dilated. 



Confining his attention, in the present notice, to the employment 

 of chloride of silver, the author inquires into the methods by which 

 the blackened traces can be preserved, which may be effected, he 

 observes, by the apphcation of any liquid capable of dissolving and 

 washing off the unchanged chloride, but of leaving the reduced, or 

 oxide of silver, untouched. These conditions are best fulfilled by 

 the liquid hyposulphites. Pure water will fix the photograph, by 

 washing out the nitrate of silver, but the tint of the picture result- 

 ing is brick-red ; but the black colour may be restored by washing 

 it over with a weak solution of hyposulphite of ammonia. 



The author found that paper impregnated with the chloride of 

 silver was only slightly susceptible to the influence of light : but an 

 accidental observation led him to the discovery of other salts of silver, 

 in which the acid being more volatile, adheres to the base by a weak 

 affinity, and which impart much greater sensibilitj"- to the paper 

 on which they are apphed ; such as the carbonate, the nitrate, and 

 the acetate. The nitrate requires to be perfectly neutral ; for the 

 least excess of acid lowers in a remarkable degree its susceptibilitj''. 



In the application of photographic processes to the copying of 

 engravings or drawings, many precautions, and minute attention to 

 a number of apparently trivial, but really important circumstances, 

 are required to ensure success. In the first tranfers, both light and 

 shadow, as well as right and left, are the reverse of the original : 

 and to operate a second transfer, or by a double inversion to repro- 

 duce the original effect, is a matter of infinitely greater difficulty ; 

 and in which the author has only recently ascertained the cause of 

 former failures, and the remedy to be applied. 



It was during the prosecution of these experiments that the au- 

 thor was led to notice some remarkable facts relating to the action 

 of the chemical rays. He ascertained that, contrary to the prevail- 

 ing opinion, the chemical action of light is by no means proportional 

 to the quantity of violet ra}'s transmitted, or even to the general 

 tendency of the tint to the violet end of the spectrum : and his ex- 

 periments lead to the conclusion that, in the same manner as media 

 have been ascertained to have relations sui generis to the calorific 

 rays, not regulated by their relations to the rays of illumination and 

 of colour, they have also specific relations to the chemical spectrum. 



