Action of Hyposulphites. 297 



positives ; it imparts brilliancy to tlie paper, augments its 

 sensitiveness, and causes moisture to have no effect upon it; 

 it affords good proofs from even feeble negatives, and the slight 

 tinge of cream colour causes it to impart vigour. 



Another cause of deterioration consists in the presence of 

 hyposulphites ; these are to be removed only by the most 

 careful washing, with abundance of water, which is frequently 

 changed. The washings may be from time to time tested, by 

 adding to a small quantity of them some hydrochloric acid and 

 metallic zinc ; if an hyposulphite is present, the nascent hydro- 

 gen will form sulphuretted hydrogen : and this will be detected 

 by paper that has been steeped in a solution of acetate of lead, 

 and, while moist, exposed to the vapour given off by sesqui- 

 carbonate of ammonia. The test paper, before being used, 

 should be held in the vapour of boiling water. If no discolora- 

 tion is produced on the lead, no danger to the picture need be 

 apprehended from the presence of a hyposulphite. 



Albuminate of silver, which is acted on by light, is in some 

 instances the cause of deterioration ; it is not removed with 

 hyposulphite, whatever care may be taken. The nature of the 

 combination formed by albumen and silver depends on the 

 strength of the silver bath ; the stronger the latter, the more 

 insoluble the albuminate ; a weak bath would, however, dissolve 

 off albumen, and render the picture less effective. The albu- 

 minate may be removed by sulpho- cyanide of ammonia; but 

 when this is used as a fixing agent, stronger copying and toning 

 are required. 



We have treated this interesting subject at considerable 

 length, but we are far from having exhausted it. Desiring to 

 keep within as narrow bounds as possible, we have avoided 

 minor details, confining ourselves to those which might be 

 considered interesting, from their importance, their novelty, or 

 the suggestions to which they gave rise. We have endeavoured 

 to present to those who are not photographers, a general view 

 of the subject ; and to those who are, an abstract of its funda- 

 mental principles, with a view to the more accurate performance 

 of the processes which depend upon them : as well as a brief 

 account of the discoveries which have been made within a. 

 very recent period. 



