A semi-solid substance, consisting of hydrocarbons, chiefly of the marsh-gas series, C16H34, etc, obtained by distilling off the lighter and more volatile portions from American petroleum, and purifying the residue. Melting-point about 40° C. to 51° C. (104° F. to 125° F.), the first constituting the softer, and the second the firmer variety.

When petrolatum is prescribed or ordered, without specifying its melting-point, the low-melting variety, which liquefies at about 40° C. (104° F.), is to be dispensed.

Characters. - A yellowish or yellow, fat-like mass, transparent in thin layers, more or less fluorescent, especially when melted, completely amorphous, tasteless and odourless, or giving off, at most, only a faint petroleum odour when heated, and having a neutral reaction. When gently heated, until the mass is almost entirely melted, the liquid portion has a specific gravity varying from 0.835 to 0.860.

Solubility. - It is insoluble in water, scarcely soluble in alcohol, or in cold absolute alcohol, but soluble in 64 parts of boiling absolute alcohol, and readily soluble in ether, chloroform, bisulphide of carbon, oil of turpentine, benzin, benzene, and in fixed or volatile oils.

Reactions. - When heated on platinum foil, it is completely volatilised without emitting the acrid vapours of burning fat or resin. If 5 gm. of petroleum ointment be digested, for half an hour, with 5 gm. of soda and 25 gm. of water, the aqueous layer separated, and supersaturated with dilute sulphuric acid, no oily substance should separate (absence of fixed oils or fats of vegetable or animal origin, or of resin). Liquefied petroleum ointment agitated with sulphuric acid of specific gravity 1.540 should not acquire a dark colour within two hours (absence of readily carbonised organic impurities).

B.P. Paraffinum Durum. Hard Paraffin. Synonyms: Paraffin; Paraffin Wax; Solid Paraffin.

A mixture of several of the harder members of the paraffin series of hydrocarbons; usually obtained by distillation from shale, separation of the liquid oils by refrigeration, and purification of the solid product.

Characters. - Colourless, semi-transparent, crystalline, inodorous and tasteless, slightly greasy to the touch. Specific gravity, 0.82 to 0.94.

Solubility. - Insoluble in water, slightly soluble in absolute alcohol, freely soluble in ether.

Reactions. - It melts at 110° to 145° F. (43.3° to 62.8° C), and burns with a bright flame, leaving no residue.

Preparations.

Unguentum Acidi Borici. ,, „ Carbolici.

,, „ Salicylici.

„ Eucalypti.

,, Glycerini Plumbi Subacetatis.

Unguentum Hydrargyri Oxidi Rubri. ,, Potassae Sulphuratae.

„ Sulphuris Iodidi.

,, Veratrinae.

B.P. Paraffinum Molle. Soft Paraffin. Synonyms: Petrolatum; Petroleine; Unguentum Paraffinum.

A semi-solid mixture containing some of the softer or more fluid members of the paraffin series of hydrocarbons; usually obtained by purifying the less volatile portions of petroleum. It is known in commerce by various fanciful names.

Characters. - White or yellowish, translucent, soft, greasy; free from acidity, alkalinity, or any unpleasant odour or flavour, even when warmed to 120° F. (48.9° C). Specific gravity at the melting-point, from about 0.840 to 0.870. Melts at 95° to 105°

F. (35° to 40°.5 C), or even somewhat higher, volatilises without giving acrid vapours, and burns with a bright flame, leaving no residue.

Solubility. - Insoluble in water, slightly soluble in absolute alcohol, freely soluble in ether, chloroform, benzene, etc.

Reactions. - It is not saponified by solutions of alkalis.

Preparations.

Unguentum Acidi Borici. ,, „ Carbolici.

„ ,, Salicylici.

„ Eucalypti.

„ Glycerini Plumbi Subacetatis.

Unguentum Hydrargyri Oxidi Rubri. „ „ Nitratis Diluturn.

„ Potassae Sulphuratae.

,, Sulphuris Iodidi.

„ Veratrinae.

„ Zinci Oleati.

Uses. - These hydrocarbons, which are not liable to become rancid, have been found very useful as a bland protection, and as a substitute for animal and vegetable fats in the preparation of ointments.