One of the earlier operations of the glazier is to _prime_ the sash, that is, to give it a coat of thin paint, for the purpose of making the putty adhere more firmly to the wood. He next takes the dimensions, in inches and eighths of an inch, of the groove or rebate in which each square of glass is to be fixed, and then proceeds to cut squares of those sizes from the semicircular pieces in his crate. This requires much tact and judgment, since to procure square or rectangular panes necessarily entails a loss of some of the circular portions. The circular sheets are made of diameters varying from forty-eight to sixty-four inches, and these are cut at various distances from the central knot, so that the glazier is enabled to choose that piece which experience teaches him will entail least waste: sometimes it is better to cut the pane from a _table_ (the half which contains the knot), sometimes from a _slab_ (the remaining portion of the disc). In order to cut a table or slab, so as to procure a pane of the proper size, the straight edge of the table is placed near the glazier, and he cuts at right angles to it, by means of a diamond, and of an instrument called a _square_; and two other cuts, at the proper distances, are sufficient to give a pane of the required size. With respect to the power by which a diamond is enabled to cut glass, we may explain it by saying, that it is a general rule among mineralogists, lapidaries, and others concerned with stony or crystalline bodies, that the hardest among a certain number of bodies will _cut_, or at least _scratch_, any of the others:--in fact, tables of the _hardness_ of different substances are formed from the determination of what substances will mark or scratch others, that one being reckoned hardest which will scratch all others, without being equally affected by them in return. Now the diamond is the hardest body in nature, and cannot be cut by any substance but its own dust; but it can cut glass and other bodies, which are not so hard as itself. The Process of Glazing. The glass having been cut to the right size, it is next to be fitted into the sash; and among the many kinds of cement which might be suggested for this purpose, _oil putty_ is found to be the most advantageous, since it is conveniently soft when used, but hardens afterwards to the consistence of stone. Putty is made of whiting and linseed oil. The whiting is purchased in lumps, which are well dried, and then pounded and sifted. The linseed oil is poured into a tub, and the powdered whiting added to it, and stirred up with a stick. When some degree of stiffness is attained, the mass is taken out of the tub and placed upon a board, where more whiting is added, and the whole mixed up by hand. The mass is then beaten for a long time with a wooden mallet, until it attains a perfectly smooth and uniform consistency.