258 XIV RESIDUAL CHARGE The pro- conductivities measured at the corresponding times. gress of formation of the residual charge, as far as it was followed, is shown by the curves. The measurements are only relative; the conductivity at time 0 was too great to be measured.ยน 3. The conductivity and residual charge at the beginning are certainly due to impurities. For, in the first place, they could be reproduced, after they had once disappeared, by any action which introduced fresh impurities, e.g., by pouring the benzene into other vessels, by stirring it up, blowing in moist air, dipping in a wire of oxidisable metal, or mixing any powder with it. The effects thus produced could again be destroyed as before. In the second place, if a sample of benzene having a high conductivity was carefully distilled over calcium chloride, and only allowed to come in contact with vessels which had been rinsed out with purified benzene, its con- ductivity was very much reduced. But the highest grade of insulation could never be attained in this way. 4. The reduction of both of these effects is due, at any rate in part, to the action of the current. They certainly fell off even when the benzene was simply allowed to stand; but neither so rapidly nor so far as when the coatings of the jar were connected to the poles of a battery. In this respect different samples of benzene seem to behave differently. The resistance of the sample which I investigated only changed very slightly when no current was used, whereas the con- ductivity of that examined by Herr Heins fell to very low values simply by standing. It may have been that the former contained chiefly soluble impurities and the latter matter in suspension. Experiments made with the intention of finding out the nature of the active impurities were unsuccessful. I shall only remark that in such experiments the benzene can be tested between glass electrodes; for the resistance of the latter is negligible in comparison with that of the benzene. 5. In its behaviour electrically purified benzene comes extraordinarily near to that of an ideal liquid insulator. 1 In Fig. 35 it will be noticed that after every considerable interruption of the current the flow to the condenser becomes greater than it had been before. This is only very slightly, if at all, due to a decrease in the resistance; it rather expresses the fact that for a short time after each interruption the flow proper is reinforced by that due to the residual charge.