Metals and otlier Substances on a Photographic Plate. 42$ 



soaked in the following solutions, alum, potassium chromate, zine 

 sulphate, and quinine sulphate, renders the paper quite opaque to the 

 action of the zinc. 



No doubt this action of alum accounts for certain papers not 

 allowing the action to pass through them. Some singular develop- 

 ments of this subject have arisen from experiments made while 

 examining the metals. A piece of polished zinc was coated with 

 copal varnish with the object of ascertaining whether the action 

 would take place through such a medium, and in case it did, as it 

 was thought at the time, of demonstrating that the action could not- 

 arise from metal vapour. The experiment was quite successful ; the 

 photographic plate, notwithstanding the varnish, was strongly acted 

 on. The experiment was repeated several times, and always with the 

 same result; but the pictures seemed rather too good, darker than, 

 those given by the zinc alone, and on trying the copal on plain glass, 

 instead of on zinc it proved that effects apparently similar to those- 

 obtained with zinc were produced. What is known as picture copal 

 answers very well for these experiments. That prepared by Winsor 

 & Newton has been used. This is painted or poured on a clean, 

 warm glass plate, and allowed to harden completely. The plate can 

 then be used in the same way as the zinc plates. If a photogT&phic 

 plate be laid on the hardened varnish for two to seven days, a picture 

 of the varnish, showing the streaks it happens to have dried 

 in, is produced. If screens be interposed so as to prevent con- 

 tact between the copal and the plate, the action still occurs, and, in> 

 fact, readily passes down a tube 1 inch long. Therefore, as with the- 

 zinc, any figure cut out in an inactive screen is readily produced on the 

 photographic plate. Substances which are transparent or opaque to 

 the action of the metals seem to act in the same way towards copal. 

 It is rather more active than zinc. Glass is perfectly impervious to 

 its action, but celluloid, gutta-percha tissue, and gelatine it per- 

 meates more readily than zinc does. The activity of the copal varies, 

 considerably under different conditions. If the powdered gum be 

 sprinkled on a glass plate and then fused, it is not so active as when 

 picture varnish is used. If the solid gum be dissolved in pure alcohol 

 and ether, and applied to a glass plate as before described, it is far 

 more active than after fusion. Heating it in a water bath for a con- 

 siderable length of time certainly deprives it of a considerable- 

 amount of its activity ; but this can be revived by wetting it with 

 ether and allowing it again to dry at ordinary temperatures. As 

 with zinc, increase of temperature increases its activity to a great 

 extent. Experiments similar to those with zinc were made with 

 copal. A coated glass was exposed to a heat of about 70°, and a 

 similar one was kept at 0°. This one after five hours gave only a 

 faint picture, whereas the heated one gave a dark picture, and a con- 



