1886.] 



Effects of the Specfown on Silver Salts. 



251 



pump below F to the apparatus attached to M ; but when the opening 

 p is turned away from the groove the pump is cut off. 



Suppose now that above p there is a vacuum, and that p is turned 

 round so as to cut off the pump. Let the stopper at C be cautiously 

 raised. Mercury flows from the cup C, and in the first place fills 

 up the space below, and fresh mercury must be supplied to the cup 

 and the supply kept up. The whole of the lower part of the space 

 being filled, the mercury rises in the tube CB, lifts the glass float, and 

 closes the opening aa with great pressure. To hold up the stopper at 

 C during the flowing in of the mercury requires considerable force 

 with an opening at C of an ordinary size ; but as soon as the whole 

 space from aa down to the bottom of D has been filled, the part of 

 this force which is due to air pressure vanishes, and the stopper may 

 be separated from C safely. The mercury in the tube AB does not 

 drop out, as the orifice at e is very small ; and thus there is nothing 

 to prevent the apparatus under exhaustion being handled in any way 

 that may be desired. 



When the apparatus is to be reconnected with the pump, it is only 

 necessary to replace the stopper in the cup C, and turn the hole p 

 round to meet its groove. The mercury in the tube AB then drops 

 into the pump. The float falls into its lowest position, and everything 

 is once more as it was before the removal of the apparatus from the 

 pump. 



III. " Comparative Effects of different parts of the Spectmm on 

 Silver Salts." By Captain W. de W. Abney, R.E., F.R.S. 

 Received March 2, 1886. 



In 1881 I communicated to the Royal Society (" Proc. Roy. Soc," 

 vol. 33) the results of a research I had made on the comparative 

 effects of different parts of the spectrum on the haloid salts of silver, 

 and I pointed out that a mixture of iodide and chloride, and iodide 

 and bromide of silver gave rise to a very curious photographic 

 spectrum, a minimum of action taking place at Gr, the point where the 

 iodide is mostly affected, two maxima consequently occurring. ] 

 also gave some theoretical reasons why this should be. About a year 

 afterwards Herr Schumann, of Leipzic, called in question this result, 

 as applied to bromo-iodide of silver, when the two salts were formed 

 simultaneously, i.e., when mixtures in water of soluble bromides and 

 iodides were precipitated together by silver nitrate. He subse- 

 quently found that a mixture of the two salts after separate precipi- 

 tation did give rise to a double maximum. Now my own experiments 

 showed that in either case such double maxima existed, but perhaps 



