r work, owing to a potential difference
appearing between the cups--at all events when the contacts are
inserted and however carefully this be done. A few drops of very pure
alcohol poured in above the mercury often cures this defect. The
surface of paraffin is by no means exempt from the defect of losing
its insulating power when exposed to damp air, but it is not so
sensitive as glass, nor does the insulating power fall so far.

Two useful appliances are figured.

Fig. 86. Fig. 87.

One, in which paraffin appears as a cement, is an insulating stand
made out of a bit of glass or ebonite tube cemented into an Erlenmeyer
flask, having its neck protected from dust when out of use by a rubber
washer, the other a "petticoat" insulator made by cementing a flint
glass bottle into a glass dish with paraffin. In course of time the
paraffin will be found to have separated from the glass. When this
occurs the apparatus may be melted together again by placing it on
the water bath for a few minutes.

Sec. 113. Vaseline, Vaseline Oil, and Kerosene Oil.

These, when dry, insulate almost, but not quite as well as solid
paraffin. H. Koeller (Wien Berichte, 98, ii. 201, 1889; Beibl. Wied.
Ann. 1890, p. 186), working with very small voltages, places the
final(?) specific resistance of commercial petroleum, ether, and
vaseline oil at about 2 X 1027 C.G.S. This is ten times higher than
the value assigned to commercial benzene (C6H6), and a hundred times
higher than the value for commercial toluene.

All these numbers mean little or nothing, but the petroleum and
vaseline oil were the best fluid insulators examined by Koeller. By
mixing vaseline with paraffin a soft wax may be made of any desired
degree of softness, and by dissolving vaseline in kerosene an
insulating liquid of any degree of viscidity may be obtained.

Hard paraffin may be softened somewhat by the addition of kerosene,
and an apparently homogeneous mass cast from the mixture. It will be
found, however, that in course of time the kerosene oozes out, unless
present in very small quantity. Koeller has found (loc. cit.) that
some samples of vaseline oil conducted "vollstaendig gut," but I have
not come across such samples. Vaseline oil, however, is sold at a
price much above its value for insulating purposes.

Kerosene oil is best obtained dry by drawing it directly from a new
tin and exposing it to air as little as possible. Of