The laps used for polishing diamonds are made of a particular kind of soft iron, which is found to surpass any other metal in retaining the diamond powder. They are rotated at a high rate of speed, which is about 2000 to 2500 revolutions a minute, and the heat developed by the friction at this speed is too great for a cement to be used; a solder or fusible alloy, composed of one part tin to three parts lead, therefore takes its place. The solder is held in a hollow cup of brass which is from its shape called a ‘dop,’ an old Dutch word meaning shell. Its external diameter is ordinarily about 1½ in. (4 cm.), but larger dops are, of course, used for large stones. A stout copper stalk is attached to the bottom of the dop; it is visible in the view of the dop shown at _e_ on Plate VI, and two slabs of solder are seen lying in front of the dop. The dop containing the solder is placed in the midst of a non-luminous flame and heated until the solder softens, when it is removed by means of the small tongs, _c_, and placed upright on a stand such as that shown at _a_. The long tongs, _d_, are used for shaping the solder into a cone at the apex of which the diamond is placed. The solder is worked well over the stone so that only the part to undergo polishing is exposed. A diamond in position is shown at _f_. The top of the stand is saucer-shaped to catch the stone should it accidentally fall off the dop, and to prevent pieces of solder falling on the hand. While still hot, the dop with the diamond in position on the solder is plunged into cold water in order to cool it. The fact that the stone withstands this drastic treatment is eloquent testimony to its good thermal conductivity; other gem-stones would promptly split into fragments. It may be remarked that so high is the temperature at which diamond burns that it may be placed in the gas flame without any fear of untoward results. The dop is now ready for attachment to an arm such as that shown at _b_; the stalk of the dop is placed in a groove running across the split end of the arm, and is gripped tight by means of a screw worked by the nut which is visible in the picture. [Illustration: _PLATE VI_ APPLIANCES USED FOR POLISHING DIAMONDS.] [Illustration: _PLATE VII_ POLISHING DIAMONDS]