Roll sulphur is frequently used alone as a cement for fastening iron bars in holes drilled in stone. The addition of brickdust, sand or resin, lessens its liability to crack. When the yellow colour of brimstone is an objection, a little graphite may be mixed with it.
White sugar, 1 oz.; starch, 3 oz.; gum arabic, 4 oz. These should all be separately reduced to a very fine powder, and then rubbed well together in a dry mortar; then little by little add cold water until the mass is of the thickness of melted glue; put in a wide-mouthed bottle, and cork closely. The dry powder itself, thoroughly ground and mixed, may be kept for any length of time in a wide-mouthed bottle, and when wanted a little may be mixed with water with a stiff brush. It answers ordinarily for all the purposes for which mucilage is used, and as a cement for labels it is specially good, as it does not become brittle and crack off.
(1) This material forms a very useful cement for many purposes. It is the only cement employed by opticians for uniting the lenses of achromatic objectives. For this purpose, it must be pure and colourless. It is easily bleached by exposure to sunlight. If too thick, it may be thinned with benzole. In cementing the two parts of an achromatic lens together, the surfaces should be thoroughly cleaned, and the glasses, having been previously warmed, should be laid on some surface which will not scratch them. By means of a rod of glass or metal, place a drop of balsam on the centre of one lens, and then gently lower the other, down upon it. Now apply a slight pressure, and the dark disc in the centre, indicative of optical contact, will rapidly increase in size, until at last the balsam reaches the margin and begins to ooze out at the edges, if the balsam be in excess, as it ought to be. By means of a piece of soft string, if the lenses are large, or a spring clip, if they be small, the lenses should be held firmly together and exposed to a gentle heat in an oven that is cooling, or before a fire, until the balsam at the edges has become hard and dry.
The string or clip may then be loosened, and all external traces of balsam removed, first by scraping, and afterwards with a little benzole or ether. The above directions, modified to suit circumstances, apply to the cementing of glasses for transparencies or opal pictures; also to the varnishing of magic-lantern slides, and the protection of any transparent surfaces from the air. (2) Canada balsam forms a very efficient and easily applied cement for the construction of small tanks used by microscopists for keeping minute plants and animals alive in water.
(1) Melt yellow beeswax with its weight of turpentine, and colour with finely powdered Venetian red. When cold, it has the hardness of soap, but is easily softened and moulded with the fingers, and for sticking things together temporarily it is invaluable. The consistence of the cement may be varied by changing the proportions of turpentine and wax, and, if a very firm cement is needed, a little resin may be added. (2) Slaked lime is beaten up with white of eggs; strips of linen are soaked in the mixture, and applied immediately, as it dries very rapidly. (3) 1/2 lb. pulverized chalk, 1 lb. rye flour, sufficient white of egg; the whole is formed into an almost liquid mass, which is brushed over strips of linen, and the latter are applied to the joints; an additional strip of linen is laid ever them, and pressed with a hot iron, which dries the compound.
To 3 parts of fresh-beaten blood are added 4 parts of slaked lime and a little alum; a thin, pasty mass is produced, which can be used immediately. Objects which are to be made specially waterproof are painted by the Chinese twice, or at the most three times. Dr. Scherzer saw in Pekin a wooden box which had travelled the tedious road via Siberia to St. Petersburg and back, which was found to be perfectly sound and waterproof. Even baskets made of straw became, by the use of this cement, perfectly serviceable in the transportation of oil. Pasteboard treated therewith receives the appearance and strength of wood. Most of the wooden public buildings of China are painted with schio-liao, which gives them an unpleasant reddish appearance, but adds to their durability. This cement was tried in the Austrian Department of Agriculture, and by the "Vienna Association of Industry," and in both cases the statements of Dr. Scherzer were found to be strictly accurate.
This cement consists of a strong solution of gelatine, to which has been added, for every 5 parts of gelatine, 1 of a solution of acid chromate of lime. The mixture becomes insoluble in water under the action of light, in consequence of the partial reduction of the chromic acid, and this property is utilized on several occasions in photography. Professor Schwartz has been experimenting with it as a cement for glass. With a fresh preparation of the solution he covered the surfaces to be united as evenly as possible, pressed them together, and then tied them together. He then exposed the glass to the sun, and at the end of a few hours the operation had perfectly succeeded. Boiling water has no effect on the oxidized cement, and the fracture could scarcely be recognized. Valuable objects in glass, which would be disfigured with common cement, can be satisfactorily repaired in this manner. It is probable that microscopic object-glasses could be better fastened with this than with black asphalte.
To render corks impervious to air, acids, alkalies and corrosive liquors generally, boil them for some time in melted paraffin. They must be kept under the surface of the melted material, and should be heated and allowed to cool several times, so as to get all the air out of the pores. Corks thus treated cut easily, and make very close joints. For cements for coating cork, see Bottle.