350 BEPOET — 1888. 



temperatvire. It was much easier to obtain it by first making some pure Tl,S and 

 then gently heating this with the theoretical amount of finely-divided sulphur. At 

 about the melting-point of sulphur (120° C.) combination ensues, and at a slightly 

 higher temperature appears complete. Thus prepared, TljSj is a dark grev com- 

 pound, behaving like an extremely viscous body. Under comparatively" slight 

 pressure, and at ordinary summer temperatures, it can be easily bent and moulded. 



In testing whether this compound could be electrolysed, we enclosed a little of 

 it in a glass tube and then pressed a metallic electrode into contact with it at 

 each end. Sometimes these electrodes were made of silver, but in most of the 

 experiments they were of platinum. 



Three Grove's cells were employed. The resistance of the trisulphide was very 

 high, and the deflection, even when using a high-resistance Thomson reflecting 

 galvanometer, was generally small. At 12° C. the current was very weak, but it 

 showed a tendency to increase if the battery was left on for an hour or two. The 

 polarisation was not detectable with such a small current. At temperatures rang- 

 ing from 20° to 40° or 50° C. the current increases, and polarisation becomes 

 evident. The values obtained for the polarisation E.M.F. vary so much with the 

 temperature and the duration of the electrolising current that only an approxi- 

 mation can be stated. Generally speaking, the polarisation E.M.F. increased with 

 the electrolising current, and reached in some cases a value somewhat over half a 

 volt. In one case a potentiometer measurement indicated approximately 0-8 volt. 



It was observed, after all the experiments, that the pellet of trisulphide was no 

 longer plastic, but had become hard and brittle. As this might be indicative of 

 a change in composition, a little of the sulphide, simply enclosed in a Hass 

 tube, was sometimes placed in the water-oven along with the specimen imdero-oing 

 electrolysis. At the close the two specimens presented about the same decree of 

 hardness and brittleness ; and we therefore concluded that the trisulphide is subject 

 to a molecular change through heat, independent of any effect due to electrolvsis. 



The effect of the molecular change was e^-ident during the experiments. After 

 an hour or so of heating the current would begin to diminish and fall towards 

 zero. The diminution was probably caused by an increase in the resistance of the 

 trisulphide, seeing that the polarisation E.M.F. was little altered so long as it could 

 be measured. It was this phenomenon that generally brought the experiments to 

 a close. 



"We thought this increased resistance might be due to a decomposition of the 

 compound Tl^Sg at the temperature employed (30°-50°). accompanied by a separa- 

 tion of sulphur. On trying, however, to obtain evidence of such separation, bv 

 treating the specimens with carbon bisulphide, we quite failed. Only a trace of 

 sulphur was separated, and the quantity was practically the same from both the 

 original and the changed specimens. 



The alteration in properties, which we have supposed to be due to a molecular 

 change, has a considerable bearing on the interpretation of the results : for in its 

 plastic form the trisulphide can hardly be looked upon as a sohd comparable with 

 the cuprous, argentic, and thallious sulphides. In the hard and brittle form it can 

 hardly be classed as a conductor. Thus our previous conclusion is not invalidated 

 by it/ 



On the Polarisalion of small Electrodes in Dilute Sulphuric Ar.id. 

 By Dr. FRixz Eichaez, of Berlin. 



I examined the greatest value the electro-motive force of galvanic polari- 

 sation possesses with platinum wires of very small surface in the electrolysis of 

 dilute sulphuric acid. 



I. Older experiments by Buflf (Pogg. 'Ann.' 130, p. 34], 1867) and Fromme 

 ("\\"ied. 'Ann.' 33, p. 80, 1888) have given under those conditions 3-3 and 

 4'3 Daniells for the maximum. In these experiments the polarisation is calculated 

 from measurements of the intensity of the galvanic current during the electrolysis, 

 tacitly assuming that the resistance of the decomposition cell is independent of 

 the intensity of the galvanic current. The correctness of this supposition has not 



