All the cements containing lime and gelatinous substances become so very hard that they cannot be separated from glass vessels without the aid of a sharp knife and some force; and, therefore, they can scarcely be applied to very thin vessels. They will not confine very corrosive acid vapours for a great length of time; but are excellent lutes for preserving a complicated apparatus steadily united and air-tight; and they will bear nearly a red heat. They are also the most useful kinds of cement for any accidental crack or failure of a lute already applied, although a stream of vapour may be bursting through at the time.1 c. Earthy Lutes are intended for operations which require a high temperature. The following are the best of this class :

1. Mix burnt gypsum, in powder, with water to the consistence of a thick cream, and apply it immediately. This forms a lute which sets as soon as it is applied, and is firm; but a slight blow will easily crack it.

2. Dissolve one ounce of borax in half a pint of boiling water, and add as much slacked lime as will make a paste. By using a smaller portion of lime, this lute forms an excellent glazing for earthenware retorts, over which it should be spread with a brush; but when dry, a coating of slacked lime and linseed oil, beaten till the mixture is plastic, should be laid over the whole of the lute.

3. A very valuable fire lute may be made of about one part of glass of borax, five parts of brickdust, and five parts of clay, finely powdered together, and mixed with a little water when used.

4. The same composition which has been already described as a proper coating for the inside of furnaces, is also an excellent earthy lute.

5. Six parts of clay, and one of iron filings formed into a paste by means of linseed oil, form an excellent cement for stopping cracks in iron retorts or boilers.

1 Aikin's Dictionary of Chymistry.

If the beak of a retort be too small to fit accurately to the neck of a receiver, the vacancy should be filled up by introducing short pieces of soft wood or of cork, a, a, a; and if the disproportion be very considerable, a cork must be fitted to the neck of the receiver, and a circular hole made in it sufficient to admit the beak of the retort. The curved tube of a Woulfe's apparatus, when not fitted accurately by grinding, may be also fixed by means of corks. After the parts are thus firmly joined, the luting must be neatly and closely applied over the junctures; and the whole covered with slips of wet bladder, or with linen spread with one or other of the above-described cements. The application of the lutes, although apparently very simple, yet requires some management, lest the luting of one juncture should disturb another already luted, which is apt to happen when applying the fillets and ligatures. When an operator, therefore, is not pressed for time, he should always allow the luting of one joint to dry before he applies luting to another; indeed, it is preferable not to apply the fillets and ligatures, until after the luting has been applied to all the joints, and is nearly hard.