Species, Vulnerary. Syn. Species Vulnerariæ. Thé Siusse. Prep. Leaves and tops of wormwood, betony, bugle, calamint, germander, hyssop, ground ivy, millefoil, origanum, periwinkle, rosemary, self-heat, sage, hartstongue, water-germander, thyme, speedwell, flower of Armea, flower of catsfoot, flower of coltsfoot, of each equal parts. Cut and mixed.
SPECIF′IC GRAVITY is the comparative weights of equal bulks of different substances, the assumed standard being 1 and sometimes 1000. This standard is pure distilled water for liquids and solids, and atmospheric air for gaseous bodies and vapours.[178] In England the sp. gr., unless when otherwise expressed, is always taken at 60° Fahr.; but in France it is taken at 32° Fahr. (0° C), or the temperature of melting ice. In the ‘British Pharmacopœia,’ whenever specific gravity is mentioned, the substance spoken of is supposed to be of the temperature of 60° Fahr. In most cases, however, it is sufficient merely to note the temperature, and to apply a correction, depending on the known density of water, or air, at the different degrees of the thermometric scale.
[178] By many modern chemists hydrogen, the lightest substance in nature, is taken as the standard for the specific gravity of gases and vapours.
To determine the specific gravity as a solid, we weigh it, first in the air, and then in water. In the latter case it loses, of its weight, a quantity precisely equal to the weight of its own bulk of water; and hence, by comparing this weight with its total weight, we find its specific gravity. The rule is—Divide the total weight by the loss of weight in water; the quotient is the specific gravity.
The specific gravity of a substance lighter than water may be determined by attaching it to some substance, as a piece of lead, the sp. gr., &c., of which is known. In this way, by deducting the loss in weight of the two substances, when weighed in water, from the loss sustained by the lead alone, when so weighed, we obtain a difference (a) which, added to the weight of the substance taken in air (b), gives the respective densities. From these the sp. gr. is found by the rule of three:—
(a + b) : 1 :: b : sp. gr.
The specific gravities of substances soluble in water are taken in pure oil of turpentine, rectified spirit, olive oil, or some other liquid, the density of which is exactly known. Sometimes, for rough purposes, the article is covered with a coating of mastic varnish. This last method answers for mercurial pill.
The specific gravity of a substance in fragments, or in powder, may be found by putting a portion (say 100 gr.) into a sp. gr. bottle, filling the latter with distilled water, and then weighing it. The weight of water which it is found to contain, deducted from 1000 (the weight of the bottle when filled with distilled water), gives a difference (a) which bears the same relation to the sp. gr. of water (1·000) as the weight of the powder (b) put into the bottle does to the required sp. gr. Or—
a : 1·000 :: b : sp. gr.
The specific gravity of alloys and mixtures, when no condensation has occurred, is equal to the sum of the weights divided by the sum of the volumes, compared to water reckoned as unity; and is not merely the arithmetical mean between the two numbers denoting the two sp. gr., as is frequently taught. See Beads (Lovi’s), Hydrometer, Mixtures (Arithmetic of), &c. For the mode of determining the specific gravity of gases, the reader is referred to the works on chemistry of Miller and Fownes.