_c._--It has the property of clinging to the metallic conductor, by which is meant, that should it (conductor) be cut through, and any strain be brought on the cable, there is a tendency on the part of the gutta percha to cling to the conducting wire, thereby not increasing the fault. The defects of such an insulator are:-- _a._--Its liability to become hard and brittle when exposed to dry heat, and consequently it requires to be stored under water. _b._--It becomes comparatively a bad dielectric at 100° F. _c._--It becomes plastic at high temperatures, which causes the conducting wire to alter its position. In some particulars ordinary india rubber is a better insulator than gutta percha, but this substance is equally inferior to Hooper's material, &c. The advantages possessed by this substance are:-- _a._--It is not easily affected by a dry heat. _b._--It is a very excellent dielectric. The defects of this mode of insulation are:-- _a._--It must be put on the conducting wires in a series of jointed pieces. _b._--It does not cling to the conducting wire, so that if the electric cable be cut, and any strain be brought on it (cable), the previous fault is increased. _c._--It absorbs 25 per cent. of water. _Hooper's Material._--This insulating material consists of an inside coating of pure india rubber, then another similar coating in conjunction with oxide of zinc, which is termed the separator, and an outside coating of india rubber combined with sulphur. The use of the separator is to prevent any damage to the conducting wires by the action of the sulphur. The three coatings are then baked for some hours at a very high temperature, which fuses the whole into a solid mass, and vulcanises the outer coating. The properties of the pure india rubber which is in contact with the metallic conductor are thus preserved, while any decay of the outer covering is prevented by the vulcanising process. The advantages claimed by Mr. Hooper for this mode of insulating electric submarine cables, are:-- _a._--High insulation. _b._--Flexibility. _c._--Capability of withstanding the bad effects of dry heat. The qualifications essential to a perfect insulated electrical cable for use with submarine mines are as follows:-- 1.--Capacity to bear a certain amount of strain without breaking. 2.--Perfect insulation, or at least as nearly so as it is possible to obtain, and composed of a substance capable of being readily stored, and kept for a considerable length of time without being injured. 3.--Pliability so that it may be wound on, or paid out from, a moderately sized drum without injury. 4.--Provided with an external covering capable of protecting the dielectric from injury when used in situations where there is a rocky or shingly bottom, &c.