358 EEPOET— 1888. 



and (2) that the conductivities of electrolytes increase while those of metals diminish 

 as the temperature increases. 



' A little consideration will show that we could hardly expect to detect [polari- 

 sation] in the case of metals or alloys, for here instead of, as in electrolytes, the 

 property of splitting up being confined to a few molecules sparsely scattered 

 through a non-conducting solvent, the whole of the molecules can thus split up ; thus 

 the rate of disappearance of any abnormal condition would be almost infinitely 

 greater than in the case of electrolytes, so that if any polarisation were produced it 

 would probably die away before it could be detected. The only case in which we 

 could expect to detect the appearance of the constituents of the conductor at the 

 electrodes is that of the alloys, but even in this case Prof. Roberts-Austen was un- 

 able to detect any change in composition in the alloy round the electrodes. We 

 must remember, however, that an alloy differs very materially from an electrolyte, be- 

 cause while in the latter we have a few active molecules embedded in a non-con- 

 ductor, in the former it is as if the solvent as well as the salt conducted, so that the 

 discharge is not concentrated on a few molecules of defiuite composition, but can 

 travel by an almost infinite variety of paths. 



' Then, again, the statements about the effect of heat on the conductivity of 

 elements and electrolytes though true in general are subject to exceptions; thus, the 

 conductivities of selenium, phosphorus and carbon increase as the temperature in- 

 creases ; that of bismuth is said to increase at certain temperatures ; and I have 

 lately found that the conductivity of an amalgam containing about 30 per cent, of 

 zinc and 70 of mercury is greater at 80° than at 15°. We must remember, too, 

 that the rate of increase of conductivity with temperature for electrolytes diminishes 

 as the concentration increases. No sharp line of demarcation can therefore be 

 drawn between the two clas.ses of conductors on this account. 



' There does not seem any difference between metallic and electrolytic con- 

 duction which could not be attributed to the vastly greater number of molecules 

 taking part in metallic conduction, whilst assuming that in all cases the current 

 consists of a series of intermittent discharges caused by the rearrangement of the 

 constituents of molecular systems.' 



I call attention to Prof. J. J. Thomson's remarks both on account of their sug- 

 gestiveness and in order to point out that we are not in a position to make any 

 such statement as the above regarding selenium, phosphorus and carbon. All 

 observers hitherto have dealt with more or less iinjmre substances — everyone, in 

 fact, knows that the so-called carbon which conducts is not carbon at all in the 

 true chemical sense. The conductivity of the substances in question may well be 

 conditioned by the presence of the associated impurities, just as that of water 

 appears to be, and hence their behaviour as electrolytes in respect of temperature 

 changes; Shelford Bid well's experiments with selenium distinctly favour this view. 

 The differences between metals and non-metals are so great that the chemist has 

 difficulty in resisting the conclusion that there is some very radical distinction to 

 be drawn between metallic and non-metallic matter, although in practice it is im- 

 possible to say where metals end and non-metals begin. If it be found that the 

 more nearly pure a non-metal is the greater is the lesistance which it opposes, it 

 will some day be possible, perhaps, to cla.ssify the elements into metals and non- 

 metals on the'basis of their electrical behaviour. In the hope of obtaining evidence 

 on this point I have latterly been engaged in purifying selenium, with the object 

 ultimately of determining its specific resistance ; it appears highly desirable that 

 ' metalloids ' such as arsenic and antimony should also be carefully examined from 

 this point of view. 



The increase in conductivity of bismuth and of Prof. J. J. Thomson's zinc amalgam 

 at higher temperatures may well be due to a change in molecular composition, such 

 as occurs in solid silver iodide, for example ; both are cases in which the occurrence 

 of such a change may be considered as probable. That molecular structure is an 

 all-important element in influencing conductivity is all but estabhshed by the ex- 

 traordinary diminution in conductivity effected by minute quantities of foreign 

 elements in the case of copper, for example. In short, I am of opinion that as yet 

 there is no experimental evidence to justify any modification of the time-honoured 



