294 Precautions with Silver Bath. 



be made a positive by reflection, without ceasing to be a nega- 

 tive by transmission. This is effected by covering it, after 

 having been washed once or more times, with chloride of gold, 

 which blackens the details, and renders them visible by reflected 

 light j as occurs in toning, the gold is precipitated by the 

 metallic silver ; and if, instead of the latter, copper or zinc 

 were present, the precipitation would be still more rapid. A 

 solution of bichloride of mercury might be used instead of the 

 gold, but its effect would not be so certain. 



Accidents and Failures to which Photographic Processes 

 are liable. 



Silver Bath. Notwithstanding very great care in pre- 

 paring materials, and performing the various manipulations, 

 accidents occasionally occur, and defects are perceived, for 

 which it is sometimes difficult to account. Most of these, how- 

 ever, are due to the state of the nitrate of silver bath. If, 

 with albumenized paper, it is too weak, in proportion to the 

 amount of albumen, or too acid, the picture will have a disa- 

 greeable reddish tone. In the former case, the addition of a 

 small quantity of nitrate is the remedy ; in the latter, powdered 

 chalk and filtration. A weak silver bath may be the cause of 

 many imperfections in the picture ; but the addition of nitrate 

 of soda will cause it to give more brilliant positive proofs than a 

 strong bath containing only the silver salt. After a bath has been 

 used for some time with albumenized paper it is darkened and 

 rendered impure by the albumen it has dissolved. Evaporating 

 to dryness will cause the organic matter to be destroyed by nitric 

 acid ; adding a little nitric acid will change the reduced silver 

 into nitrate ; evaporating again to dryness, and slightly fusing, 

 will afford perfectly pure nitrate of silver ready for solution. 

 No silver is lost during the process. 



The sensitizing bath used with collodion requires special 

 precautions. The collodion and the bath must be both acid, 

 or both neutral, otherwise there will be a want of sensibility. 

 The most effectual means of giving the greatest possible degree 

 of sensibility to a collodionized plate intended for a negative, 

 would be the use of a silver bath saturated with iodide of silver, 

 and absolutely neutral. But since, with such a bath, the pic- 

 ture will often appear clouded, and especially in the places not 

 acted on by light, it is usual to give the bath a slight degree of 

 acidity ; this, however, will be unnecessary if a small quantity 

 of iodine is added to it. In about fifteen or twenty days after, 

 the bath will indeed become slightly acid; but it may again be 

 rendered neutral with carbonate of silver. If, before adding the 

 iodine, the bath was not saturated with iodide of silver, some 

 of the latter will be formed ; in which case, nitric acid being 

 set free, great acidity will result. If the bath is feeble, com- 



