Adeps. (Axungia, Ed. Adeps Suillus Preparatus, Dub.)
Lard. The prepared fat of the sus scrofa or common hog, free from salt.
History. — Good lard is white, inodorous, granular in appearance, of a sweetish taste, smooth to the touch, of a soft consistency at ordinary temperatures, fusible at about 100° F., insoluble in water, sparingly soluble in alcohol, more so in ether and the volatile oils, is dissolved and decomposed by the stronger acids, and is converted into soap when boiled with caustic alkaline solutions. When melted it readily unites with resins and wax, on which account it is extensively used in making ointments and liniments. When heated in close vessels, it undergoes a species of destructive distillation, by which margaric, oleic, acetic, and probably benzoic acids are formed, together with other less important modifications of its constituent fatty principles. It consists of three neutral fatty principles, called olein, stearin and margarin, which are found in most animal-oils and fats, and upon the relative proportions of which depends their consistence respectively.
Olein, is the liquid principle of oils ; when pure it is colorless, has a sweetish taste, little odor, is insoluble in water, soluble in boiling alcohol, readily soluble in ether, and congeals at 20° F. It is convertible by saponification into glycerin, margaric acid, and oleic acid, and consists of carbon, hydrogen, and oxygen. Lard contains 62 per cent, of olein, which is extensively used for burning in lamps.
Stearin is white, concrete, of a crystalline appearance like spermaceti, pulverizable, fusible at 144°, soluble in boiling alcohol or ether, insoluble in either of these fluids when cold, and convertible by saponification into a peculiar fatty acid, named stearic acid, and glycerin. It consists also of carbon, hydrogen, and oxygen, and may be separated from the concrete matter of lard, by treating this with cold ether so long as anything is dissolved ; the stearin is left behind, and the ethereal solution yields margarin by evaporation.
Margarin very closely resembles stearin, but is more fusible ; one variety, obtained from animal fats melting at 118°, and being soluble in cold ether ; another variety, from vegetable oils, melts at 82°. Lard should be kept in well closed vessels, as if exposed to the air it absorbs oxygen, and becomes rancid, and consequently unfit for medicinal use. When pure it consists of 62 per cent, of olein, and 38 per cent, of stearin and margarin together.
Properties and Uses. — Lard is emollient. It is sometimes used alone in frictions, and is frequently added to laxative enemata ; but its chief use is as an ingredient for cerates and ointments. When applied to blistered surfaces, care must be taken that it be not rancid, as it is then apt to cause ulceration.