The Daguerreotype. 291 



PHOTOGRAPHY.— PROCESSES AND PRECAUTIONS* 



BY J. W. M f GAULEY. 



The principle of the Daguerreotype is essentially different 

 from that of every other photographic process. It consists in 

 the production of an exceedingly delicate compound of iodine 

 or bromine and silver, on a silver surface ; this compound is 

 changed by the action of light, but the change is imperceptible 

 until the picture is brought out by vapour of mercury. The 

 light appears to decompose the iodide or bromo-iodide of sil- 

 ver, and the liberated iodine and bromine act on the plate 

 underneath the sensitive film; the mercury, by forming an 

 amalgam with the reduced silver, produces the lights. If the 

 newly-formed iodide and bromide were not removed by the 

 fixing agent, they would attack the mercury. Paper covered in 

 various ways with an argentine surface, has been proposed as 

 a substitute for plated copper, but without complete success. 

 There is reason to suppose that the copper of the Daguerreotype 

 plate not only supports the silver but contributes to the effect. 



Removal of the undecomposed Silver Salts. The undecom- 

 posed iodide, etc., of silver may be removed by a solution of com- 

 mon salt, especially if the plate is touched with a zinc rod ; but 

 it is far better to use hyposulphite of soda. The use of this salt 

 for the ordinary purposes of photography is, however, attended 

 with several inconveniences ; among others, it sulphurs positive 

 proofs. Cyanide of potassium may be used instead of it, but 

 is a very dangerous substance. Sulphocyanide of ammonia is 

 very effective as a fixing agent, both with glass and paper, with 

 positives and negatives : though so energetic, it does no injury 

 to the middle shades ; but unless, before using it, the picture 

 is immersed in a strong gold toning bath, it will be likely to 

 have a reddish hue. Unlike cyanide of potassium it is a harm- 

 less compound. The great resemblance between the salts of 

 selenium and those of sulphur suggested the substitution of 

 selenio for sulpho-cyanide, and it was found to dissolve the 

 undecomposed silver salts ; but feeble acids, which have no ac- 

 tion on sulpho-cyanide, decomposes selenio-cyanide. 



Sensitization of Paper. To imbue paper with chloride of 

 silver, it is first washed with some substance which contains 

 chlorine ; this is changed into chloride of silver by the addition 

 of the nitrate. Common salt may be used for the purpose ; 



* This is the fourth and concluding paper — the first, on " The History of 

 Photography," is in No. 27 ; the second, on " Photographic Processes," in 

 No. 28; and the third, in No. 33. 



VOL. VI. — NO. IV. U 



