18. Plates thus cast are always dull and uneven. To render them good reflectors, it is necessary to grind and polish them. The plate to be polished is first cemented with plaster of Paris to a table of wood or stone. A quantity of wet sand, emery, or pulverized flints, is spread upon it, and another glass plate, similarly cemented to a wooden or stone surface, is placed upon it. The two plates are then rubbed together, until their surfaces have become plane and smooth. The last polish is given by colcothar and putty. Both sides are polished in the same manner. 19. _Moulding._--Ornamental forms and letters are produced on the external surface of vessels, by means of metallic moulds; and the process by which this kind of work is performed is called moulding. In the execution, the workman gathers upon the end of his iron tube, a proper amount of the material, which he extends, and brings to a cylindrical form, by rolling it upon his table. He then expands it a little by a slight blast, and afterwards lets it down into the mould, which is immediately filled by blowing still stronger through the tube. 20. The vessel is then taken from the mould, and disengaged from the tube. The same tube, or a punting iron having been attached to the bottom, the other end is softened in the fire, and brought to the proposed form with appropriate tools, while the iron is rolled up and down upon the long arms of the glass-blower's chair. The ornamental moulds are made of cast iron, brass, or copper, and are composed of two parts, which open and shut upon hinges. The moulds for plain vials, castor oil bottles, small demijohns, &c., are made of the kind of clay used for the crucibles. These consist merely of a mass of the clay, with a cylindrical hole in it of proper diameter and depth. 21. _Pressing._--This process is applied in the production of vessels or articles which are very thick, and which are not contracted at the top. The operation is performed in iron moulds, which consist of two parts, and which have upon their internal surfaces the figures to be impressed upon the glass. The material, while in an elastic condition, is put into the lower part of the mould; and the other part, called the _follower_, is immediately brought upon it with considerable force. 22. Every species of glass, before it can be used with safety, must be _annealed_, to diminish its brittleness. The annealing consists merely in letting down the temperature by degrees. Small boys, therefore, convey the articles, whatever they may be, as fast as they are made, to a moderately heated oven, which, when filled, is suffered to cool by degrees.