500 Major-General j. Waterliouse. On the Sensitiveness 



weighing 82 milligrammes. In two other experiments, in which silver 

 was fused in a more oxygenated atmosphere, as much as 158 c.c. of 

 oxygen, weighing 226 milligrammes, and 174 c.c, weighing 249 milli- 

 grammes, were obtained from 1 kilo, of pure silver in each case. He 

 found also that silver which contains oxygen does not lose it all in a 

 cold vacuum. 



Mallefs Observations. — Professor J. W. Mallet, in some investigations 

 on the Atomic Weight of Aluminium, notes these results obtained by 

 Dumas, and states that in his own observations of the same kind, but 

 using a lime support for the silver heated in a hard glass tube to a 

 moderate redness, he obtaiaed only 34*63 c.c. of oxygen from 1 kilo, 

 of silver.* 



These different observations show that silver, apparently pure, may 

 contain a very considerable quantity of oxygen, and I believe it is 

 a well-known fact, especially in the Mints, that oxygen is nearly 

 always present in silver, with or without hydrogen, carbonic acid, and 

 other gaseous impurities in much smaller quantities. 



Whether occluded oxygen is the cause of the photographic effects 

 produced on surfaces of apparently pure silver, might readily be 

 proved by extracting all the oxygen from a silver plate by the above 

 methods, and then exposing the resulting purified silver to light, a 

 similar plate from which the oxygen had not been extracted being 

 exposed at the same time, I have not the appliances at my disposal 

 for making this experiment. 



Effects of Heating Silver Plates to Redness. — With regard, however, to 

 the effect of simply heating the silver plates to redness, the following 

 experiments may be of interest, and seem to show that the heating, 

 whether accompanied by subsequent "blanching" or "pickling" in 

 dilute sulphuric acid or not, causes a distinct loss of sensitiveness, and 

 in some cases seems to destroy it entirely unless the exposure is great!}/- 

 prolonged. 



A piece of thin, pure silver plate was first of ii\ heated to a red 

 heat over a spirit lamp, then plunged into dilute sulphuric acid, and, 

 after being washed with distilled water and dried, was exposed for 

 about two days and a half, partly in sunshine, at the end of September 

 last, under a screen of mica carrying a cut-out design in tinfoil, which 

 had also been passed through the flame of the lamp. No visible 

 image was produced, nor did one appear by breathing on the plate. 

 Development with acid ferrous sulphate and silver nitrate brought out 

 traces of the opaque tinfoil mask and of parts cut out of the mica. 

 The free ends of the plate, which were exposed to the air all the time, 

 did not show any distinct attraction for the developer. 



Another plate of pure silver, well cleaned with " meteoric " polish 

 (No. 2) and exposed as usual under a mica screen with black paper 

 * ' Phil. Trans.,' 1880, p. 1020. 



