TEE COMBUSTIBLE MATERIALS EMPLOYED.—These are chiefly tallow and vegetable fats, and 6ubstance6 prepared therefrom by complicated chemical processes ; also wax, spermaceti, .paraffin, ozokerit, &c.
Tallow.—Tallow is simply beef or mutton fat, or a mixture of both, prepared by being heated in contact with water and under alight 6team pressure until the membranous matter6 in which it is enveloped aggregate into lumps, and collect in a layer between the tallovg and the water, when the steam i6 turned off. For candles which are to be moulded from tallow alone, mutton suet is employed, while the commoner or coarser tallow is kept for those which are to be dipped, and aleo for the preparation of 6tearic and other futty-acida.
Whcn melted tallow is allowed to coal very slowly, and without disturbance, it separates into two portions, one much harder, and the other much softer, than the original tallow ; and if the temperature of the whole mass does not fall below 24°-27°(75°-80° F.), it will consist of hard, round nodules, suspended in a liquid oil. This process is technically called secding," and the idea of separating the solid from the liquid constituents of fats, by exposing them to pressure while in this condition, originated with the French chemist Chevreul, in 1823.
The re6earches of Chcvreul and others demonstrated the following facts with reference to the composition of fatty bodies generally ; and it is to a clear comprehension of these, and of the bearing of other scientific facts and phenomena upon them, that are due the various manufacturing processes which have resulted in the elegant and useful commercial products called " candles," the varieties of which range from the softest and cheapest English "cottage composite," to the alabaster-like stearic acid " bougie" of the continental salons.
It will be desirable, therefore, to coneider the important fact, demonstrated by Chevreul, that all the ordinary neutral fats ef commerce are, chemically speaking, " salts," in which the base is glycerine, and the acid is a mixture of various fatty-acids, which may be separated from each other, and prepared in a greateror lees degree of purity. Hence
Salt Acid Base Neuti al fat (e.g. tallow) = stearic and oleic acids + glyceiine, 0,115(OH), The fatty-acids of commercial 6olid fats belong chiefly to the series known as the "Adipie,'' of which Formic aoid, CH202, and Acetic acid, 02H402, are the lowed terms. The two membere of moat frequent occurrence are Stearic acid, C,211„02, which is a large constituent of tallow, and Palmitic acid, 0,2113202, which occurs in sirailarly large proportions in palm oil. Beeswax (to be referred to again presently)contains one of the highest kuown members of the series, Cerotic acid, C2,112402. The fatty-acid of the fluid constituents of most natural fats, and esp_cially of the non drying oils, is called Oleic acid, 0,2112,0„ and belongs' t,o another series', knawn as the " Acrylic." Each of the above-mentioned fatty-acide is capable of forming three salts with glycerine, tLe glycerides in natural fats being the third term in each series, tallow, for example, being a mixture of tri-stearine and tri-oleinc. Chevreul's researches materially assisted in developing the theory of " Saponification"; but ae this will be fully discussed in the article on " Soap," it need not be alluded to here, further than ie necessary to explain the principles of the proceas by which stearic acid was at first entirely, and is still very largely, manufactured.
When neutral fate are boiled in open vessels, with a solution of a strong caustic alkali, as soda, or with Hine mechanically 6uspended in water in a thin cream, the glycerine is replaced by the alkali, and a salt or soap ie formed hy the union of the fatty-acids with the alkali, thus:— Neutral fat 4- Alkali = (Fatty acids + Alkali) i. e. " Soap " Glycerine.
'When the " soap " is dissolved in water, and a strong mineral acid is added to the solution, the fatty-acid is liberated— Soap + Sulphuric acid =- Fatty acids + Sulphate of aoda (or lime).