A manual of materia medica and therapeutics : including the preparations of the British pharmacopoeia (1867) and many other approved medicines / by J. Forbes Royle and Frederick W. Headland.

Date:
1868
Catalogue details

Licence: Public Domain Mark

Credit: A manual of materia medica and therapeutics : including the preparations of the British pharmacopoeia (1867) and many other approved medicines / by J. Forbes Royle and Frederick W. Headland. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    differ much in weight, as, for instance, Lead and Cork, in conse- quence of the former containing more matter in the same space than the latter ; or, in other words, lead has a greater density than cork. Bodies are, therefore, weighed under two points of view : first, with respect to their absolute weights, and secondly, with reference to that which is peculiar to each species, and is hence called Specific Gravity. This refers to the comparative weights of different bodies occupying the same space and referred to a common standard. As the quantity of matter within the same space differs very much according as it is more or less expanded by heat, so the Sp. Gr. of bodies is always referred to one temperature, that is 60°; in the L. P. it was 62° of Fahr. The quantity of a medicinal substance, as of an acid, an alkali, or a spirit, may, moreover, vary very much in the same bulk according as it is more or less diluted with water. And as this fluid will be either lighter or heavier than water, its solution will thus be heavier or lighter, exactly according to the degree of dilution. The strength, therefore, which is the same thing as the Sp. Gr., requires to be ascertained, both for Pharma- ceutic and for Medicinal purposes. Water is, for convenience, taken as the standard to which the comparative weights of other bodies are referred ; but its Sp. Gr., though usually reckoned as 1, is by some taken at 1000, to avoid fractional parts. The Sp. Gr. of a liquid may easily be ascertained by weighing it in a bottle which holds exactly 1000 grains of water at 60°. Solids are weighed first in air, and then, when suspended by a hair, in water. In this case, they displace a quantity of water equal to their own bulk, and weigh less than in air, because they are supported by the surrounding water with a force equal to the weight of water which has been displaced. So that the loss in the weight will denote the weight of an equal bulk of water. This then is the rule : find the difference between the weight of the body in air, and when weighed in water ; take this difference to divide the weight of the body in air, and the quotient will be the specific gravity. The Sp. Gr. of aeriform bodies is ascertained by weighing certain measured quantities when passed into a vessel exhausted of air, and of which the weight has been previously ascertained. The different gases vary very much in their Sp. Gr., but they are all referred to Atmo- spheric air as a standard. By careful experiment, it has been found that 100 cubic inches of air weigh 31-0177 grains at 60° of Tem- perature and 30 inches of Barometrical pressure. Professor Daniel] has given the following table of the Sp. Gr. of the lightest gas, of air, of steam, and of water :— Cubic Inches. Weights, Grains. Sp. Gr. Air 1. Sp. Gr. Water 1. Hydrogen 100 Air . . . 100 Steam . . 100 Water. . 100 2-130 31-000 19-220 25250-000 00694 1-0000 0-6240 814-0000 O-0000S46 0 1)012277 0- 0007611 1- 0000000