= distinguished from raw rubber by its greater indifference to alterations of temperature-it neither becoming soft and sticky on moderate heating nor rigid on slight cooling-and by the possession of greater "elasticity" and indifference to cold solvents, which, at most, cause it to swell, but do not take it into solution. The method of vulcanisation may be briefly described thus:—The raw rubber, being plastic, is capable both of being intimately mixed with the sulphur for its vulcanisation and with any mineral weighting materials that may be added, and of being moulded by any ordinary mechanical process-e.g., rolling_or squeezing through dies into sheet, cord, or tube form. The articles thus fashioned are kept in shape by appropriate moulds, which are heated, generally by high-pressure steam, to about 140° C. 284° F., at which temperature the process of vulcanising takes place. Instead of sulphur, "golden sulphide of antimony" (Sb,S,) is sometimes used, which decomposes, affording sulphur for vulcanisation and becoming Sb,S,, thus imparting a red colour to the finished goods. Various loading materialse.g., chalk, zinc oxide, barium sulphate and lead oxide-are often added, especially to common goods. Lamp black is used for black goods. The vulcanising of rubber can also be performed by the use of sulphur chloride, S,Cl, in dilute solution, CS, being the ordinary solvent. The solvent swells up the rubber and facilitates the penetration of the sulphur chloride. The process is largely used for waterproofing cloth and making thin rubber tubing; it cannot be used for thick goods, as the penetration is imperfect. But little is known of the nature of the action of sulphur in the dry process of vulcanisation, but the action of sulphur chloride has been studied by Weber, who is of opinion that the vulcanised rubber consists of two molecules of polyprene united by one or more double atoms of sulphur. The introduction of the sulphur is effected by the formation of polyprene sulphochlorides. Ebonite or vulcanite is rubber vulcanised with so much sulphur (25 per cent.) that it is converted into a hard, horny mass quite different in appearance and mechanical properties from ordinary vulcanised rubber. The deterioration of rubber goods appears to be a process of oxidation, resinous products being formed. Rubber is often largely adulterated with "surrogates" consisting of the products of the action of sulphur and sulphur chloride on oils-e.g., rape and cotton-seed oil. These surrogates are gelatinous substances nearly devoid of mechanical strength, and are simply diluents of the rubber with which they are mixed. The use of these surrogates makes the determination of the specific gravity of rubber as a means of detecting adulteration altogether illusory. Common rubber goods made by moulding (called "mechanicals ") are Elasticity" is not here used in its exact mechanical sense. also largely prepared from old vulcanised rubber re-worked. Indiarubber overshoes are examples of this class of rubber goods. Gutta percha is a substance, similar in many respects to caoutchouc, obtained from Isonandra gutta, a tree found in the Malay Peninsula. It is collected in a manner similar to that used for caoutchouc. It has lately been proposed to extract gutta percha from the leaves of the tree by means of a volatile solvent. Like caoutchouc, gutta percha consists of a hydrocarbon associated with resinous matter. It is chiefly used as an insulating material in electrical work and for moulding purposes. III. VARNISHES. Varnishes may be divided into two broad classes, namely oil and spirit varnishes. The former class consists of those varnishes which contain a drying oil, capable itself of forming a varnish-like film when exposed to the air in a thin layer. The latter comprises those varnishes which are made up of some resin dissolved in a volatile solvent. The chief resins in use for varnish making have been already described; most of the other constituents of varnishes-e.g., drying oils and volatile solvents— have been dealt with under appropriate headings; it remains to consider the methods of preparing varnishes, and such principles as are believed to underlie these methods. (1) Oil Varnishes.-The typical constituents of an oil varnish are a resin-e.g., copal; a drying oil-e.g., linseed oil; and a volatile solvent-e.g., spirits of turpentine. The first process in the manufacture usually consists in rendering the resin, or "gum," as it is technically termed, amenable to the solvent action of the oil. Many resins, prominent among which are copal, amber and animé, are almost insoluble in oil or spirits of turpentine, until they have been fused. Others, such as elemi and common rosin, can be dissolved without previous fusion. The operation of fusing or "running" is carried out by simply heating the resin in a large copper pot over a direct fire, the heating being continued until frothing from the escape of moisture or of gases from the decomposition of the resin, has ceased. Ordinary linseed oil of good quality is boiled in a manner similar to that adopted when boiled oil is the product sought (p. 234), and the two materials are then mixed, and heated together at about 500° F. 260° C. for an hour or two, until the mixture becomes so viscous as to form strings, when a sample is drawn out for testing. The product thus obtained is thinned by the addition of spirits of turpentine, and the finished varnish is allowed to settle and run off into storage vessels. It appears that both the liuseed oil intended for varnish making and the finished varnish = are improved by keeping, though the nature of the changes which they undergo is unknown. Specific directions as to constituents and quantities must be looked for in compilations of trade recipes. The process of drying which an oil varnish undergoes, takes place in two stages. In the first instance, the volatile solvent (spirits of turpentine) evaporates spontaneously, leaving a sticky film composed of the resin and drying oil, which have been used for making the varnish. The latter then dries by oxidation, and a film results, containing both the resin and the oxidised products of the drying oil. The quality of this film depends naturally both upon the nature of its materials, and the skill with which the varnish has been prepared. The resins must be chosen so that the film, though hard, is not brittle, but tough enough to resist a fair amount of wear and tear. (2) Spirit Varnishes.-These are of the simplest description, merely consisting of solutions of any suitable resin in any appropriate volatile solvent. The resins most commonly employed are shellac, sandarac, elemi and mastic, and the commonest solvents are methylated spirit, spirits of turpentine and its various substitutes, and occasionally acetone. The method of preparation consists merely in warming the resin with its solvent, in a vessel heated by steam and provided with an agitator. It is fitted with a still head and worm, to recover any of the solvent which may distil over during the process of dissolution. The drying of spirit varnishes is simply the volatilisation of the solvent, the quality of the film left being dependent on the nature of the resins used. They are greatly inferior to oil varnishes in durability. Recently, varnishes for special purposes, such as the slight or temporary protection of bright metal surfaces, have been made by dissolving the lower products of the nitration of cellulose (see Explosives, Vol. II., Chap. XVII.) in solvents such as alcohol-ether, amyl acetate or acetone. RAW MATERIALS FOR SOAP. 247 CHAPTER XI. SOAP AND CANDLES. I. SOAP. Soaps are the alkali salts of fatty and resin acids, soluble in water and capable of giving a lather. Inasmuch as they are made from vegetable and animal oils, they consist of the alkali salts of such acids as occur in these oils (see p. 223) together with those of the acids of common rosin (see p. 242). Caustic soda is generally employed in the process of soap making, usually yielding hard soaps, while caustic potash gives as a rule soft soaps. For the chemistry of the action of caustic alkalies on fats see p. 225. When free fatty acids are used for soap making, saponification can be effected by alkaline carbonates, which are also capable of acting on neutral fats at high temperatures, under pressure. RAW MATERIALS.-The fatty matter most commonly used for good soap is tallow (palm oil and rosin being frequently added). A soda soap from this fat is hard and comparatively insoluble, but complete saponification without excess of alkali is difficult. Non-drying oils-e.g., olive oil-also yield hard soda soaps. Semi-drying and drying oils-e.g., rape, linseed, and fish ofls-yield somewhat soft soda soaps, and are, therefore, generally used for making potash soap which is intentionally soft. Some oils containing lower fatty acids, notably coco-nut oil, are easily saponified, even in the cold, and nevertheless produce hard soaps. Waste fatty material from a large number of industries is generally worked up into soap, the product varying according to the character of the fat. The description of the manufacture of caustic soda and potash is to be found elsewhere (see Vol. II., p. 30, and Potash, Vol. II., Chapter XVIII.). On account of the fact that saponification is generally conducted with a comparatively weak lye, the solution of caustic soda produced by the action of lime upon sodium carbonate solution suffices for the need of the soap maker, who therefore frequently makes his own lye. It is believed that the detergent property of soap is increased by the addition of certain alkaline substances, such as sodium silicate and aluminate. No such plea can be urged for inert mineral matter-e.g., barium sulphate, kaolin and finely powdered pumice frequently used as filling for common soaps. Sodium sulphate is sometimes similarly used in the manufacture of cold water soap, and water itself is often present in enormous proportion-e.g., 60 to 70 per cent. Sugar is also used in making transparent soaps, and various colouring and scenting ingredients are employed, especially for toilet soap. PROCESSES OF MANUFACTURE.-However the details of soap making may vary with the class of soap to be produced, the first part of the process merely consists in heating the fatty material and running in alkali of about 10 per cent. strength, little by little, so as to first emulsify the fat and then gradually saponify it, the completion of the process being judged by testing for the presence of free alkali. Salt is added to the contents of the copper containing the soap, and the soap is thus thrown out of solution, it being insoluble in brine. It rises to the top, and the brine, containing the glycerol of the fat freed by saponification, is run off. Fresh lye is then run in and the heating repeated, any residual fat being thus saponified, and the soap, which remains undissolved in the caustic liquor, being washed free from adhering salt. The subsequent treatment varies according to whether "curd," "mottled," or "fitted" soap is to be produced. For curd the soap is taken into solution again and allowed to rest until the dirt has settled; the surplus water is then evaporated, and the soap is run into frames where it solidifies. The separation of the lye is not quite complete in this process, so that the finished soap is somewhat alkaline, but contains less water than most other varieties. Fitted soap is similarly treated, but is allowed a longer time for subsidence, and the boiling down is omitted. The finished product will obviously contain more water than curd soap, but be more nearly free from unsaturated alkali, the lye being given time to settle out. "Primrose" soap is the commonest example of a fitted soap, the raw materials saponified being about 7 parts of tallow and 1 of rosin. (Rosin soap alone cannot be precipitated with salt, but a mixed rosin and fatty soap is readily separated from its solution in this manner.) Mottled soap, if genuine, is manufactured similarly to curd, but is a second time boiled with lye until this has become strong enough to precipitate the soap again. It is then run directly into the frames after the surplus lye has settled. The frames are jacketed and the soap cools slowly, a circumstance which causes the total dirt of the materials to segregate in the lines which are last to solidify, producing a mottled appearance in the finished product. Since more than 20 per cent. of water prevents this segregation, by allowing too rapid subsidence of the impurities before the soap passes to the frames, mottling is generally regarded as an indication of freedom from excess of water. Seeing that now artificially mottled soaps are made in which the addition of sodium silicate solution is found to prevent subsidence even though much water |