preserving the Sensitiveness of Collodion Plates. 351 



convenient opportunity provided it be within the week. Pre- 

 vious to development, the plate should be allowed to remain for 

 a few seconds in the original 30-grain silver-bath, then removed 

 and developed with either pyrogallic acid or a protosalt of iron, 

 and afterwards fixed, &c. in the usual manner. 



The advantages of this process can scarcely be overrated. 

 Besides the facility it affords of working in the open air without 

 any cumbrous apparatus, photography may now be applied in 

 cases where it would have been hitherto impracticable, owing to 

 the feebleness of the light, e. g. badly illuminated interiors, 

 natural caverns, &c. ; if necessary, the exposure could be pro- 

 tracted for a week, or possibly much longer, and the deficiency 

 of daylight compensated for by the employment of the electric or 

 other artificial light. It will also be found useful where the plate 

 must be kept ready excited, but the exact moment of exposure 

 may depend upon possible contingencies rather than on the will 

 of the operator, or in cases where it would be impracticable to 

 prepare the plate just before exposure ; for these reasons it might 

 prove a valuable adjunct on the eve of a naval or military engage- 

 ment, for accurately recording the positions of the forces. 



A small proportion of nitrate of zinc added to the ordinary 

 nitrate of silver bath in no way interferes with its action, and 

 might obviate the inconvenience sometimes felt during hot 

 weather in photographic rooms, of the film becoming partially 

 dry before exposure. If added in a still smaller proportion to 

 the silver solution used for exciting the ordinary Talbotype 

 paper* (without the employment of gallic acid), it is very pro- 

 bable that its sensitiveness may be preserved during a much 

 longer period than is generally possible. As far as our experi- 

 ments have gone, they tend to confirm this supposition ; but at 

 present we can hardly speak more confidently on this point, as 

 it is still under investigation. 



There are, no doubt, many other substances which might 

 equally well answer the purpose of nitrate of zinc ; besides those 

 already mentioned, the nitrates of cadmium, manganese, and 

 perhaps also those of copper, nickel, and cobalt might be found 

 serviceable. Glycerine at first seemed to promise very good 

 results, but the principal difficulty was the necessary impurity of 

 the commercial product, in consequence of its being obtained 

 from the exhausted leys of the soap boilers. 



* This addition of nitrate of zinc to the silver solution in the Talbotype 



Erocess was suggested, we have been informed, some time back by the 

 'rench, but not with reference to its keeping properties — only as an acce- 

 lerator. 



