No. IV.] APPENDIX. 



159 



This form of chloride is not quite so delicate as some others, and 

 requires a long time to become quite black. It has its advantages from 

 enabling the most highly-glazed papers to be prepared with great facility 

 and certainty, and it becomes of a beautiful brown, which is but slightly 

 altered by the stopping agents. 



The chlorides of oxides, such as the chlorides of soda and of lime, 

 may be advantageously applied in some cases where the chlorine is useful. 

 .... The chloride of soda, however, must not be used for absorbent 

 papers, such as those used in printing ; but with the glazed papers it 

 becomes very delicate and sensitive to light, whether it be applied before 

 or after the solution of silver. The strength which was found most useful 

 was that usually employed for medical purposes. 



The solution of chloride of lime was made by adding twelve grains of 

 chloride of lime to an ounce of water, and allowing any insoluble part to 

 subside. This is found applicable both to printing and to glazed papers, 

 but is more certain when used prior to the nitrate of silver. 



The chlorides of metals, as common salt, require more care in their 

 proportions than the foregoing substances ; and an experiment which was 

 tried, shows the absolute necessity of using an excess of nitrate of silver. 



A weak solution of nitrate of silver (twenty grains to the ounce) was 

 treated with excess of chloride of sodium, when an insoluble chloride was 

 precipitated : this was exposed to the direct rays of the sun, without the 

 slightest change; the supernatant liquor was then poured off, and the 

 precipitate well washed two or three times with distilled water, to remove 

 any superfluous salt which might perchance be present ; the chloride of 

 silver was again exposed to the light for many hours, when only a slight 

 brown tint was produced. On the contrary, when the nitrate of silver was 

 treated with such small quantities of salt that part of the solution of 

 silver remained in excess, the light speedily blackened the chloride exposed 

 to its action Similar experiments were tried with chlorine, chlo- 

 ride of lime, and chloride of soda, when excess did not prevent the black- 

 ening; but when muriatic acid was used, the same phenomenon was 



observed Without 'endeavouring to explain the difference of the 



action of light under these different circumstances, a,n important practical 

 inference is to be drawn from them ; for if any circumstance prevents the 

 nitrate of silver being in excess, no action will be produced. 



The proportions given by Mr. Golding Bird are evidently so designed, 

 that an equivalent proportion of each substance should be used; for 

 although he employs only a 20- grain solution of nitrate of silver to the 

 ounce, with a 12-grain solution of salt, yet, by using the silver twice, 

 it becomes equal to the single application of a 40-grain solution. To 

 insure success, the ratio of the chloride of sodium to the nitrate of silver 

 should be about one to five. As the relative proportions of these two sub- 

 stances are of importance, great care must be taken in the application of 

 the salt in the first place to the paper. A 10-grain solution of salt should 

 be sponged over one surface of the paper, and all superfluous moisture 

 carefully removed by the sponge wrung dry ; the paper ought then to be 

 allowed to dry, but taking care that the salt does not settle in any part, 

 and thereby cause an excess ; when the paper is dry, the solution of nitrate 

 of silver is to be applied in a similar way. An advantageous mixture 

 can be made of the chlorides of oxides and chlorides of metals: thus, 



