Joiners fasten one piece of their work to another by _glue_, made by boiling down refuse animal matter containing the animal principle called _gelatine_ in abundance, such as hoofs, horns, tendons, skin, gristle, &c.: it is a property of gelatine to dissolve in hot water, and to harden again when cold, and the water evaporates. Accordingly the glue, which is only concentrated impure gelatine, is dissolved by heat in a small quantity of water, and being applied to the clean faces of the wood to be united, by a coarse brush, these faces are closely pressed and retained together till the water evaporates, when such is the tenacity of the glue, that the wood may be broken in another place as easily as at the glued joint. To enable glue, however, to act in this manner well, the wood should be clean, the parts to be glued well warmed before the glue is applied, and the joint should be close, or the parts accurately brought together. Besides the before-mentioned tools and materials, and some others, such as hammers, axes, &c., which need not be described, carpenters and joiners use instruments for measuring and setting out their work, and for drawing on the surface of the material the forms into which it is to be reduced, or the shape and situations of portions of the material to be removed for the purposes of framing. The instruments are compasses, squares, rules, levels, plumb-lines, and so on, common to all artificers who form their materials into geometrical shapes: and, like the mason, the carpenter and joiner must be conversant with the more elementary problems of practical geometry. A Window-sash, as an example of Joiner’s Work. In illustration of the nature of joiner’s work, we may point out the mode of proceeding in making a window-sash, which is one of the most delicate operations of the common joiner. The outer part of the sash is made broader and stronger than the intermediate cross-bars which receive the panes of glass, in order to give strength and rigidity to the sash. This outer part is framed together at the four angles by mortises and tenons, the latter coming quite through the stuff, and having a small sharp wedge driven into the middle of the tenon when inserted into the mortise: by means of this wedge, the tenon is expanded at its end into a wedge-shaped form, by which it fits more tightly into the mortise, and is retained in its place, the wedge-shape not allowing the tenon to be withdrawn again. But it may be here remarked, that, besides this precaution, all small mortises and tenons are put together with glue, to ensure the stability of the joint.