﻿Electrical Changes induced by Ultraviolet Light. 411 



4. Our next series of experiments dealt with compounds of 

 several elements. The sulphides and iodides, so far as they 

 were investigated, have a perfectly definite rate of discharge 

 for negative electricity ; this rate is much slower than in the 

 case of the metals, and presents a phenomenon similar to that 

 observed with the various specimens of iron, which may 1)8 

 regarded as compounds in a state of more or less purity. 

 The sulphides measured were prepared in two different ways. 

 For Series I. the plates of the metals were placed either in a 

 solution of sulphuretted hydrogen or ammonium sulphide until 

 they were completely coated with a film of the sulphide. 

 Then they were removed, washed, and thoroughly dried with 

 filter-paper. For Series II. the plates were made by preci- 

 pitating the sulphide from a solution of its salt, and after it 

 had been washed and dried it was made into a paste with a 

 starch solution and coated on plates of the requisite size and 

 dried at 80° C. In either case the plate was clamped on to 

 the electroscope and measured as shown in fig. 2. The two 

 series of measurements ran parallel, as will be seen from 

 Table VII., although the values of the same sulphide differ 



Table VII. 



Sulphide of 



Series 1. 

 Eate of discharge. 



Series 2. 

 Rate of discharge. 



Antimony, Sb 2 S 3 ... 

 Copper, CuS 



3361 

 338 

 413 

 7500 

 313S 

 7650 



5060 



603 



1732 



12750 



7610 



Lead, PbS 



Tin, SnS 



Iron, FeS 



Cadmium, CdS 



in the two series. This is not of much importance, because 

 the starch in the composition of the plates must have some 

 influence on the rate of discharge. The figures given in 

 Table VII. are corrected for the natural leak, and calculated 

 to the same arbitrary standard adopted in Table V. It will 

 be noticed that with the exceptions of antimony and tin 

 sulphides, the rates of discharge run in a series in the opposite 

 direction to the electropotentials of the metals. It would 

 have been interesting to have made experiments with the 

 metals with a more rapid discharge, but their sulphides are 

 not obtainable in a condition suitable for measurement. A 

 possible reason for the non-agreement of antimony lies in the 



