158 APPENDIX. [No. IV. 



interesting subject, the practical student in this art finds that great 

 difficulties occur in every department of photogenic drawing. 



In the first place, he finds that the paper which he has prepared the 

 preceding evening is by no means equal in its qualities, as sometimes he 

 may have two or three sheets very excellent, so that, when they are 

 exposed to the light, they become in every part of a uniform dark colour ; 

 sometimes, on the contraiy, he finds that the paper, after it has been 

 similarly acted upon by the solar rays, becomes black over the greater part 

 of its surface, yet numerous white spots occur throughout which detract 

 much from the beauty and effect of drawings made with it ; and, lastly, it 

 occasionally happens that some sheets are not affected by the most 

 powerful light, except, perhaps, at a few points. 



Indeed, should the paper be good, and the drawings made, yet, with- 

 out the greatest care in the fixing of them, they may be found to have a 

 ground of an irregular tint, or they may be imperfectly stopped, and even 

 the colour may be altogether removed. 



To surmount with certainty these various difficulties, numerous 

 experiments have been performed in every department of the manufacture 

 of photographs, which we shall now describe ; first as regards the chemical 

 substances, then the paper, and, lastly, the most efficient stopping 

 solution. 



The various compounds of silver have been long known to be acted 

 upon powerfully by the solar rays : this property is possessed by far the 

 greater number of the preparations of that metal, yet not by all ; and 

 upon the former, the effect of light differs materially in its degree of 

 sensitiveness. 



The two soluble salts of silver with which we are most acquainted are 

 the nitrate and sulphate, both of which communicate to organic textures 

 and substances made from them a black stain when exposed to light ; but 

 these, neither on paper nor in combination with albumen, gelatin, gums, 

 or glutea, have sufficient delicacy to be applicable for the manufacture of 

 photogenic drawings. 



The ammonia-nitrate of silver will be found considerably more 

 delicate than either the nitrate or sulphate, and may be used where 

 rapidity of action is not required, particularly as it lessens the trouble, by 

 the application of only one solution to the paper. 



The chloride of silver is the substance to which we principally look 

 for the ready action of the solar rays, and the modes of its application to 

 the paper are numerous. It is by itself very insoluble in water, and, on 

 the contrary, easily dissolved by ammonia; but, unfortunately, the 

 ammonia- chloride of silver cannot with good effect be used for the pre- 

 paration of this paper, and thus we are compelled to form a chloride upon 

 it by a more circuitous process. This object may be effected by the 

 application of either chlorine, chloride of an oxide, chloride of metal, or 

 hydrochloric acid, first to the paper, and afterwards a solution of nitrate 

 of silver. 



When a nearly saturated solution of chlorine is used, it should be 

 applied lightly with a sponge to the paper, taking care that every part 

 is moistened by the liquid : the paper should then be allowed to dry, 

 and the solution of nitrate of silver applied also with a sponge, in a 

 similar way. 



