M. C. Lea — Image Transference. 35 



. Another very decisive experiment may be made in this way. 

 Paper prepared with tartrate, oxalate or almost any other salt of 

 silver, is to be exposed for a minute or thereabouts to a strong 

 light; (not under a negative). It is then taken into a dark room, 

 and marks are made upon it either with a glass rod or a camel's 

 hair pencil dipped in dilute hydrochloric or hydrobromic acid. 

 After letting the acid act for five or ten minutes, it is to be 

 washed off, the paper plunged into nitric acid, and after again 

 washing, it can be placed in a developing solution when the 

 marks made will appear black on a white ground. This mode 

 of operating gives a very convincing result. The nitric acid 

 treatment may be omitted, but when this is done, the number 

 of salts that can be used is more limited. The four salts first 

 above mentioned give good results, even without the nitric 

 treatment, but some silver salts undergo a spontaneous change 

 in the dark by keeping a short time, such that when they are 

 placed in a developing solution (without exposure to light) they 

 may blacken instantly all over. Silver tartrate is one of the 

 besi salts to operate with, though pyrophosphate, citrate, oxa- 

 late and some others do almost equally well. Sulphate, anti- 

 monio-tartrate, phosphate, nitrite and arsenite do not give good 

 results, except with the nitric acid treatment. 



"With a salt like tartrate or oxalate the experiment is very 

 striking. The paper imbued with it is exposed to light over its 

 whole surface, it is then taken to the dark room, and simply 

 marked with dilute HC1 or HBr and washed. Thrown into a 

 developer, all the marks of the halogen acid quickly blacken, 

 proving, first, that the effect of light is transferred from the one 

 salt to the other; second, that the effect as transferred to the 

 chloride or bromide is far more susceptible of development than 

 it was in the original salt. It seems a not unreasonable expla- 

 nation of this last-mentioned fact that the greater sensitiveness 

 of the haloid compounds may depend on their power to com- 

 bine with their own subsalts, so that the reduction may com- 

 mence with the subsalt, and quickly extend from it to the por- 

 tion of normal salt with which it is combined ; that on the other 

 hand, other silver salts may not share this power of uniting 

 with their subsalts, and are consequently more slowly and im- 

 perfectly attacked by the developing solution. This explana- 

 tion may or may not be correct, but seems not improbable. 



A curious fact incidentally presented itself in the course of 

 this investigation : that when paper prepared with silver salts, 

 other than the haloids, was exposed to light, and then marked 

 with HC1, the effect of a short exposure, so far as development 

 was concerned, was as great as that of a long one. A piece of 

 paper was prepared with a given salt by non-actinic light. It 



