_Electrolysis_ acts by decomposing the blood and tissues into their constituent elements--oxygen and acids appearing at the positive, hydrogen and bases at the negative electrode. These substances and gases being given off in a nascent condition, at once enter into new combinations with anything in the vicinity with which they have a chemical affinity. In the nævus the practical result of this reaction is that at the positive pole nitric acid, and at the negative pole caustic potash, both in a state of minute subdivision, make their appearance. The effect on the tissues around the positive pole, therefore, is equivalent to that of an acid cauterisation, and on those round the negative pole, to an alkaline cauterisation. As the process is painful, a general anæsthetic is necessary. The current used should be from 20 to 80 milliampères, gradually increasing from zero, without shock; three to six large Bunsen cells give a sufficient current, and no galvanometer is required. Steel needles, insulated with vulcanite to within an eighth of an inch of their points, are the best. Both poles are introduced into the nævus, the positive being kept fixed at one spot, while the negative is moved about so as to produce a number of different tracks of cauterisation. On no account must either pole be allowed to come in contact with the skin, lest a slough be formed. The duration of the sitting is determined by the effect produced, as indicated by the hardening of the tumour, the average duration being from fifteen to twenty minutes. If pallor of the skin appears, it indicates that the needles are too near the surface, or that the blood supply to the integument is being cut off, and is an indication to stop. To cauterise the track and so prevent bleeding, the needles should be slowly withdrawn while the current is flowing. When the skin is reached the current is turned off. The punctures are covered with collodion. Six or eight weeks should be allowed to elapse before repeating the procedure. From two to eight or ten sittings may be necessary, according to the size and character of the nævus. _Excision_ is to be preferred for nævi of moderate size situated on covered parts of the body, where a scar is of no importance. Its chief advantages over electrolysis are that a single operation is sufficient, and that the cure is speedy and certain. The operation is attended with much less hæmorrhage than might be expected.