PHOTOGRAPHY 567 



By dissolving the nitrate of silver in common rectified spirits of wine instead of 

 water, we produce a tolerably sensitive nitrated paper, which darkens to a very 

 beautiful chocolate-brown. 



2. AMMONIA NITRATE of SILVER. Liquid ammonia is to be dropped carefully 

 into nitrate of silver ; a dark oxide of silver is thrown down ; if the ammonia-liquor 

 is added in excess, this precipitate is redissolved, and we obtain a perfectly colourless 

 solution. Paper washed with this solution is more sensitive than that prepared with 

 the ordinary nitrate. 



3. CHLOKIDE OF SILVEB. This salt is obtained most readily by pouring a solution 

 of common salt, chloride of sodium, into a solution of nitrate of silver. It then falls 

 as a pure white precipitate, which changes colour in diffused daylight. 



Chloridated papers are formed by producing a chloride of silver on their surface, by 

 washing the paper with the solution of chloride of sodium, or any other chloride, and 

 when the paper is dry, with the solution of nitrate of silver. 



The most sensitive paper. Chloride of sodium, 30 grains to 1 ounce of water ; 

 nitrate of silver, 12.0 grains to 1 ounce of distilled water. 



The paper is first soaked in the saline solution, and after being carefully wiped with 

 linen, or pressed between folds of blotting-paper and dried, it is to be washed twice 

 with the solution of silver, drying it by a warm fire between each washing. This 

 paper is very liable to become brown in the dark. Although images may be obtained 

 in the camera obscura on this paper by about half an hour's exposure, they are never 

 very distinct, and may be regarded as rather curious than useful. 



Less sensitive paper for copies of engravings or botanical specimens. Chloride of 

 sodium, 25 grains to 1 ounce of water; nitrate of silver, 99 grains to 1 ounce of 

 distilled water. 



Common sensitive paper for copying lace~worJc, feathers, $~c. Chloride of sodium, 

 20 grains to 1 ounce of water ; nitrate of silver, 60 grains to 1 ounce of distilled water. 

 This paper keeps tolerably well, and, if carefully prepared, may always be depended 

 upon for darkening equally. 



4. IODIDE OF SILVER. This salt was employed very early by Talbot, Herschel, and 

 others, and it enters as the principal agent into Mr. Talbot's calotype-paper (see CALO- 

 TYPE). Paper is washed with a solution of the iodide of potassium, and then with 

 nitrate of silver. By this means papers may be prepared which are exquisitely sensi- 

 tive to luminous influence, provided the right proportions are hit ; but, at the same 

 time, nothing can be more insensible to the same agency than the pure iodide of 

 silver. A singular difference in precipitates, to all appearance the same, led to the 

 belief that more than one definite compound of iodine and silver existed ; but it is 

 now proved that pure iodide of silver will not change colour in the sunshine, and 

 that the quantity of nitrate of silver in excess regulates the degree of sensibility. 

 Experiment has proved that the blackening of one variety of iodated paper, and the 

 preservation of another, depends on the simple admixture of a very minute excess of 

 the nitrate of silver. 



5. BROMIDE OF SILVER. Bromide is the most sensitive to light of all the salts of 

 silver. To prepare a highly sensitive paper of this kind, select some sheets of superior 

 glazed post, and wash it on one side only with bromide of potassium (40 grains to 

 1 ounce of distilled water) over which, when dry, pass a solution of 100 grains of 

 nitrate of silver in the same quantity of water. The paper must bo dried as quickly 

 as possible without exposing it to too much heat; then again washed with the silver- 

 solution, and dried in the dark. Such are the preparations of an ordinary kind, with 

 which the photographer will proceed to work. 



The most simple method of obtaining sun-pictures is that of placing the objects to 

 be copied on a piece of prepared paper, pressing them close with a piece of glass, and 

 exposing the arrangement to sunshine : all 

 the parts exposed darken, while those co- 

 vered are protected from change, the result- 

 ing picture being white upon a dark ground. 



For the multiplication of photographic 

 drawings, it is necessary to be provided 

 with a frame and glass, called a copying 

 frame. The glass must be of such thickness 

 as to resist considerable pressure, and it 

 should be selected as colourless as possible, 

 great care being taken to avoid such as have 

 a tint of yellow or red, these colours pre- 

 venting the permeation of the most efficient 

 rays. Fig. 1617 represents the frame, showing the back, with its adjustments for 

 secxiring the close contact of the paper with every part of the object to be copied. 



