(1) In 1869, Hirzel obtained, by triturating litharge with glycerine, a mass which he found useful as a cement for vessels containing benzol, ethereal oils, etc, as it possessed the property of soon hardening. During the same year, Pollack recommended the same mass as a cement for stone and iron ware, and pointed out that it was attacked only by strong acids; also, that its durability is the greater, the more water the litharge had absorbed, since the latter, when entirely dry, yielded a cement of feeble adhesiveness only. Ross, in 1870, found this cement proof against concentrated [?] and diluted acids, alkaline lyes, ether, alcohol, benzol, and carbon bisulphide.

The somewhat contradictory statements regarding the power of resistance against acids of this cement, as well as the desire to find out whether glycerine entered into a chemical combination with litharge, led Theodor Morawski to investigate the subject. He obtained a definite compound, crystallizing in fine needles, which was found to be a glyce-ride of lead.

From a large number of experiments instituted to ascertain the most favourable conditions for the production of a perfect cement, Morawski obtained the following results: The hardest cement is produced by triturating 50 grin. (1 3/4 oz.) of litharge with 5 cc. (1/3 cub. in.) of glycerine. If more glycerine is used, the mass hardens much more slowly and imperfectly.

The small proportion of glycerine, however, makes it impracticable to prepare large quantities of the cement at a time. For this purpose, it will be necessary to take more glycerine, in order to facilitate the trituration. But as it was also proved that the addition of a small quantity of water produced an equally durable cement, provided the proper proportions are observed, he found, after many trials, that the most favourable results are obtained by adding 2 volumes of water to 5 of glycerine (sp. gr. 1.240); 6 cc. of this liquid are. incorporated with 50 grm. of litharge. This mass requires a shorter time than any other proportions to produce a hard cement, 10 minutes only being required to harden moderately, while, after 2 hours, it becomes even harder than any mixture containing litharge with glycerine alone. But, after a few days, the latter compound (prepared without water) overtakes the former in hardness, and remains so. If it is desired to produce a cement which rapidly hardens, and still has considerable firmness, it is advisable to use water with the glycerine. - Dingler's Polyt. Journal,

The applications of this cement are innumerable. Chemists and others know well the difficulty of keeping very volatile liquids. Bottles of ether, for example, are shipped for India, and when they arrive are found to be more than half empty. The chemist sometimes puts a bottle of benzole or bisulphide of carbon on his shelves, and when he next requires it, he finds the bottle empty and dry. The usual remedy for this is a luting of melted sulphur, which is difficult to apply and hard to remove. Glycerine cement, however, is easily prepared and applied, and is said to prevent the escape of the most volatile liquids. It is merely painted around the cork or stopper. It quickly dries, and becomes extremely hard, but can be easily scraped off with a knife, when it is necessary to open the bottle.

(2) Pollack's. Litharge and red lead, equal parts: mix thoroughly, and make into a paste with concentrated glycerine to the consistence of soft putty. This cement takes some time to dry, but it turns almost as hard as stone, and resists moisture and heat very well. Pollack used it to fasten the different portions of a fly-wheel with great success; while when placed between stones and once hardened, it is easier to break the stone than the joint.

Gum Arabic - Gum arabic is the product of various species of Acacia. It is the material from which true mucilage is made, and it forms one of the most valuable cements. Faraday says there is no cement which exceeds it in strength. Pure gum arabic is in roundish or irregular pieces of various sizes, more or less transparent, hard, brittle, and breaking with a shining fracture. It is usually white or yellowish white, but frequently presents various shades of red, and is sometimes of a deep oiange or brownish colour. In powder it is always more or less purely white. It is liable to adulteration both in powder and in masses. Much of the white gum arabic of the shops, consists of the cheaper and coarser gum Senegal, bleached by what is called "Picciotto's process." The gum is dissolved in water, and sulphurous acid gas passed through the solution. The liquid is afterwards boiled to expel the sulphurous acid, a little of which, however, still remains behind. The product is very white, but lacks the peculiar toughness and adhesiveness of the best gum.

The powdered gum is frequently adulterated with dextrine, gum Senegal, starch, sugar, cherry-tree gum, etc. These substances are not difficult of detection, but where a good article is required for preparing a cement, it is best to purchase gum arabic in lump from a reliable dealer, taking care, in any case, to avoid the bleached article. Powdered gum has no advantage, except in the fact that it dissolves more quickly than when in lumps. It, therefore, forms, when in this state, a very convenient and portable cement, which may be made ready in an instant by the addition of a little water.

For preparing gummed surfaces which will adhere when moistened (such as gummed labels, &c), there is no material superior to gum arabic. The great difficulty with gum arabic, and, indeed, with other gums and pastes, lies in the fact that when thoroughly dry, they become brittle, so that the label or other object falls off. A simple remedy for this difficulty lies in the addition of 5 to 10 drops of glycerine to each fl. oz. of mucilage or paste. Gum arabic is used not only alone, but when mixed with other matters. The following formulae produce very good cements. (1) Rub together, in a mortar, 2 parts nitrate of lime, 25 of water, and 20 of powdered gum arabic. This forms a transparent cement of great strength, and applicable to wood, porcelain, glass, and stone. The surfaces to be united should be painted with the cement, and firmly bound together until the drying is complete. (2) A white paste, adhesive to most surfaces, is said to be made as follows: A solution of 2 1/2 oz. gum arabic in 2 qt. of warm water, is thickened with flour paste well boiled, and to this is added a solution of alum and sugar of lead, 720 gr. each, in water; the mixture is heated and stirred till about to boil, and then cooled. It may be thinned, if necessary, with the gum solution.

It will be seen that this mucilage consists of a solution of gum arabic and flour paste in acetate of alumina, coloured white with sulphate of lead. (3) To 250 grm. (9 oz.) of mucilage prepared by dissolving 2 parts of gum in 5 of water, add 2 grm. (30 gr.) of crystallized sulphate of aluminium dissolved in the least possible quantity of water. A solution of alum does not answer as well as the simple sulphate of alumina, which can be prepared from alum by precipitating the alumina with ammonia, washing thoroughly on a filter, and dissolving in sulphuric acid. The mucilage thus prepared does not sour or mould, and may be used as a cement for general purposes. (4) It is said that a mixture of 1 part dry chloride of calcium, or 2 parts of the same salt in the crystallized form, and 36 parts gum arabic, dissolved in water to a proper consistency, forms a mucilage which holds well, does not crack by drying, and yet does not attract sufficient moisture from the air to become wet in damp weather.