(From soluble). Lute. Caementum. Many chemical vessels require to be covered with coating, to preserve them from being broken or melted in the fire, or to close exactly their junctures. These coatings are, in general, called lutes. Glass vessels may be covered with a mixture of equal parts of coarse sand and stiff clay, mixed with water and a little hair, so as to form a liquid paste, which must be spread with a brush upon the glass; and when dry, covered with another coat, until the covering is sufficiently thick. When a glass is to be exposed to the action of the fire, a coating of fat earth and fresh horse dung is recommended: the earth is suffered to macerate for some hours in water; and, when properly softened, it must be kneaded with the horse dung into soft paste, to be spread with the hand upon every part of the retort. The horse dung is useful, as containing a serous fluid, which hardens by heat, strongly connects all the parts together, and with filaments of hay, contributes to cement them very firmly. The dung must be fresh: for, when altered by fermentation, it does not possess the same virtues. Retorts, luted in this manner, resist the action of the fire very powerfully, and the adhesion of the lute is such, that, even should the glass break during the operation, the distillation may be still carried on.

The lutes with which the joining of vessels are closed are of different kinds, according to the nature of the substances to be distilled. When vapours of watery liquors, and such as are not corrosive, are to be prevented from escaping, it is sufficient to surround the joining of the receiver to the nose of the alembic, or of the retort, with slips of paper, or of linen covered with a mixture of wheat flour and water, of the consistency of soft paste; slips of bladder, wetted, will be often sufficient if carefully applied. When more active vapours are to be secured, the lute may consist of a soft paste made of quick lime extinguished in air, and mixed with the white of egg. When corrosive acid vapours are to be confined, what is called the fat lute is necessary. Fine clay, well dried and powdered, must be sifted through a silken searce, then moistened wiih water, and beat into a stiff paste with boiled lintseed oil, applied to the junctures, and secured by means of slips of linen: these slips must be covered with the lute made of quenched lime and white of egg. Chaptel's Elements of Chemistry.