A tenacious viscid substance, used chiefly for binding or cementing pieces of wood together: it is usually prepared from the cuttings and parings of hides, and from the hoofs and horns of animals. For this purpose the materials are first steeped in water for two or three days, then well washed, and afterwards boiled to the consistence of a thick jelly, which is passed, while hot, through ozier baskets, to separate the grosser particles of dirt, bones, etc. from it, and then allowed to stand some time to purify it farther; when the remaining impurities have settled at the bottom, it is then melted and boiled a second time. It is next poured into flat frames or moulds, from which it is taken out pretty hard and solid, and cut into square pieces or cakes, and afterwards dried in the wind in a coarse kind of net. This is the ordinary method of preparing the common glue for carpenters' work; but some few years back Mr. Yardley, of Camberwell, obtained a patent for manufacturing glue from bones, which, as chemists have long known, contain nearly one-half their weight of solid gelatin, besides a considerable portion of fat.
The glue thus obtained is said to be of very superior quality.
In the engraving on the next page a represents a section of the principal vessel of Mr. Yardley's apparatus, which is in the shape of a sphere or hollow globe of great magnitude, and made of cast or wrought iron; copper should not be used, as gelatin has a powerful action upon that metal. The first part of the process is to cleanse the bones by immersing them in a pit or cistern of water, where they are to remain about twelve hours; the water is then to be drawn off, and fresh water added to them; this operation may be repeated several times, to get rid of the adhering dirt. The water being withdrawn from the bones, a solution of lime, in the proportion of one bushel of the earth to five hundred gallons of water, is to be poured into the cistern for the more perfect cleansing of the bones, and the removal of superfluous matters. After three or four days' saturation the limy solution should be drawn off, and fresh water added, to get rid of the lime. Thus prepared, the bones are brought to the globular vessel a called the extractor, which is filled with them by removing the interior plate which covers the man-hole b; this aperture is of an elliptical form, and allows the plate (which is of a similar figure) to be slipped round and refixed in its place by turning the nut c, which draws it up tight against the interior surface of the globular extractor; and the junctures are made air-tight by luting.
The extractor turns upon a horizontal cylindrical shaft in the bearings e e; one-half of this shaft is made hollow, or consists of a strong tube f f, which tube also proceeds downwards from the centre of the vessel, to conduct the steam beneath the grating g, upon which the bones are laid. The steam, of about 15 lbs. pressure to the inch, is admitted from the boiler by turning the cock h, and, passing along the pipe by the safety valve i and the stuffing box k, it enters and proceeds first to the bottom of the extractor, then rises up through the grating, and amongst the bones, until the vessel is completely charged; previous to this, however, the air contained in the vessel is got rid of by opening the cock l for the steam to blow through, and afterwards closing it. Whilst the steam is acting upon the bones the extractor is occasionally turned gently round by hand at the winch m, the shaft of which carries a small pinion that takes into the teeth of the wheel n, and the latter being on the same shaft as the extractor, consequently gives it a rotatory motion. When at rest, as shown, a quantity of fluid gelatin is collected in the bottom of the extractor at o, from whence it is discharged by the cock p into a tub beneath, after opening the air-cock f to allow it to run off.
This done, steam is again admitted from the boiler into the extractor to act upon the bones for another hour, when the second portion of condensed liquor is to be drawn off. When the products thus obtained have become cold, the fat which has formed upon the surface is to be carefully removed by skimming, and the gelatinous portion only is to be returned into the extractor, by means of a funnel at the cock l. The steam is then readmitted to the extractor for another hour, after which it is finally drawn off into another vessel to undergo a simple evaporating process, until it arrives at a proper consistency to solidify when cold, previous to which some alum is added to clarify it. When the gelatin has become cold and solid, it is cut out into square cakes, and dried as usual in the open air.
Mr. Bevan found that when two cylinders of dry ash, 11/2 inch in diameter, were glued together, and, after twenty-four hours, torn asunder, it required a force of 1260 lbs. for that purpose, and consequently that the force of adhesion was equal to 715 per square inch of surface. From a subsequent experiment on solid glue, he found that the cohesion is equal to 4000lbs. on the square inch, and hence infers, that the application of this substance as a cement is susceptible of improvement. Glue is frequently prepared for more delicate purposes in the arts from parchment or vellum cuttings, or from isinglass. Parchment glue is made by boiling gently shreds of parchment in water, in the proportion of one pound of the former to six quarts of the latter, till it be reduced to one quart. The fluid is then strained from the dregs, and afterwards boiled to the consistence of glue. Isinglass glue is made in the same way; but this is improved by dissolving the isinglass in common spirit by a gentle heat: thus prepared, it forms a cement much superior to paste for joining paper, or for stretching it on wood.