Those who are acquainted with the sliding rule of Gunter, or the chymical scale of Dr. Wollaston, will immediately perceive that the present circular scale is divided upon the same logometric principle, and that the mechanical addition and subtraction of ratios here performed by juxta-position, corresponds in effect to the multiplication and division of the numbers by which the ratios are expressed in common arithmetical notation. It is not necessary that I should trouble the reader with the numerous difficulties and embarrassments which have opposed themselves to the practical success of this instrument. They have, after repeated failures, been at length overcome, except perhaps with regard to a slight central error, which as it is found in the most accurate brass instruments, could not be avoided where pasteboard alone had been employed. The error, however, is not of the slightest practical moment, not occasioning the difference of a hundredth part of a grain.
The proportions of active matter, in the several preparations of each class, have been, in general, derived from the best authorities, although in many cases they have been deduced from experiments expressly instituted for the occasion.
THE MEDICINAL HISTORY AND CHEMICAL HABITUDES
“_Omnium Simplicium Pharmacorum vires nosse oportet cum qui aliquot compositum est facturus._”
ABIETIS[318] RESINA. L. E. D. (Pinus Abies, _Resina concreta_.) _Resin of the Spruce Fir._
QUALITIES. _Form_, tears or small brittle masses: _Odour_, very fragrant when burning. It has all the chemical properties of a _Resin_, and is used only for external purposes: see _Pix Arida_. OFFICINAL PREPARATIONS. _Empl: Aromatic:_ D. _Empl: Galban: comp:_ L. _Empl: Opii_ L. _Empl: Thuris._ D.
ABSINTHIUM. (Artemisia Absinthium) _Common Wormwood._
QUALITIES. _Odour_, strong and peculiar. _Taste_, intensely bitter, slightly pungent, and very unpleasant, as its name[320] implies. CHEMICAL COMPOSITION. Extractive, a small portion of resin, and a green essential oil; in the first of which its bitterness resides, in the last, a narcotic principle; hence the watery extract is not possessed of the nauseous flavour of the plant but retains its bitterness almost entire; the narcotic principle is therefore dissipated by decoction, but its tonic and anthelmintic properties are not impaired by that process. MEDICAL USES. The whole plant is powerfully antiseptic; and its bitterness renders it stomachic. Infused in ale it forms the beverage known by the name of _Purl_. Its powers as a vermifuge has bestowed upon it the name of _Wormwood._ DOSE, ℈j, ℈ij; and of the infusion, (made in the proportion of ℥j of the plant to oj of water,) f℥i-f℥iss. INCOMPATIBLE SUBSTANCES. Precipitates are produced in the decoction or infusion by _Sulphate of Iron_, _Acetate of Lead_, and some other metallic salts. Tartarized Antimony is not in the least affected by it. OFF: PREP: _Extract: Absinth:_ D.