110 Dr. W. J. Kussell. On the Action exerted ly certain 



removed from the tube and again the air sent through it for a 

 week, the sensitive plate showed no signs of any action having taken 

 place. 



To try whether any accumulation of the vapour, and hence an 

 increase of action could be brought about by an increase of zinc 

 surface, two small circular glass dishes were taken, about 1^ inches 

 in diameter and f inch in depth ; into one a single disc of bright 

 zinc foil was placed, and in the other twenty similar discs, then on 

 the top of both vessels a photographic plate was placed. The single 

 disc was raised on a piece of glass, so both end discs were at the same 

 distance from the plate. The discs were a little smaller than the 

 glass vessels, and, owing to their not being quite flat, there was a 

 space between each one. In two experiments there was no marked 

 difference between the density of the pictures produced, the single 

 disc produced as much effect as the twenty. A more direct way for 

 the passing off of the vapour was then made by catting a circular hole 

 f inch in diameter through the centre of all the zinc discs, and now 

 a very black central spot was formed by the twenty zincs, and of 

 course there was a white spot with the single disc, so that the 

 vapour accumulated to a considerable extent in this central space. 



It has already been mentioned that the statement in the former 

 paper that mercury was the most active of the metals is incorrect. 

 The error arose from not suspecting that a trace of any impurity 

 would affect the activity of the mercury, and, consequently, not 

 taking special precautions to insure its perfect purity. On repeat- 

 ing the former experiments with another sample of mercury it was 

 found that no action occurred, which seemed very remarkable ; 

 moisture was added, the temperature was increased, but still no 

 action took place ; the addition of a little zinc to the mercury was 

 then tried, and it was found that this made the mercury excessively 

 active. The presence of a very small quantity of zinc is able to 

 effect this change, certainly less than 1 /300th per cent. It is 

 very remarkable that so small an amount of the metal can cause so 

 strong an action on the photographic plate, for the exposure to the 

 vapour given off by such an amalgam need not, even at ordinary 

 temperature, be longer than two to three days. If other active 

 metals are dissolved in pure mercury they act in the same way, at 

 all events, this applies to magnesium and to lead. If silver, on the 

 contrary, be added it does not render mercury active, nor does 

 sodium. This action of mercury, which contains zinc or lead, the 

 most common impurities, is so readily recognised that it becomes a 

 valuable test for its purity, and a very interesting means of follow- 

 ing the effect produced by any purifying process. A specimen of 

 mercury containing not more than l/300th per cent, of zinc gave a 

 very dark picture ; this mercury was then treated first with sulph- 



