_Gas Cautery._—The late Mr. Alexander Bruce perfected an instrument which employs the gas flame as a source of heat (see figs. 112, 113). A blowpipe flame plays on platinum discs of various sizes, and keeps them at a glowing heat. This hot solid point can be thrust into the tissue wherever it is desired. A larger form is also made for cauterising the pedicle in Ovariotomy, &c. In this a large flame is blown on a wedge-shaped surface of platinum, 1 inch long and ½ inch broad, and continued backwards for 2 inches by wire gauze to confine the flame against the platinum. The flame behind these platinum discs quickly heats them again if cooled by the blood. This cautery is very portable, and easily made ready for use. [Illustration: Fig. 113.—Gas cautery. A. Elastic gas reservoir. B. Gas jet. C. Tube for convoying air to the flame. 3. Platinum disc to be heated by the flame. ] Mr. Clover has also devised a cautery, very useful for small growths. A silver bead, the size of a pea, is set at each end of a horizontal metal rod 4 inches long, which rotates on a vertical pivot half a circle backwards and forwards, so that one or other bead is thrown into the flame of a spirit lamp placed at a proper distance; when the bead is heated, a touch of the finger causes the central pivot to rotate, which brings the hot bead away from the lamp, and carries the cold one into the flame, to be heated while the first is used. _Galvanic Cautery._—The instrument consists of a platinum wire, made to glow by passing through it a powerful galvanic current. The wire should be thick (about 1/12 of an inch), and all the other conducting surfaces sufficiently large to offer no impediment to the current where heat is not desired. The battery best adapted for this purpose is a Grove’s battery. The main advantage of a galvanic cautery is that the wire can be passed while cold exactly where it is required, and then heated when it is in place. It is exceedingly useful in fistulæ between the urethra and rectum, or in destroying vascular growths, nævi, &c., where it is desirable not to destroy all the skin covering the tumour. Again, by this means, an intense heat can be applied to a very limited area, and more quickly renewed than by any other plan, for the wire, even when plunged in the tissue, is never far below a red heat.