practitioner, by enabling him, at once, to substitute one preparation for another, without the risk of altering the dose of its active ingredients. Suppose, for example, a patient had been taking 8 fluid-drachms of _Mist: Ferri Comp._ and that we wish to give the same quantity of Protoxide of Iron in the form of the _Pil: Ferri comp_. we have only to bring the mixture in question to 8, and we shall see 13½ grs. are equivalent, both these quantities of the respective preparations containing ⅔ds of a grain of protoxide, or a little more than a grain of the Proto-carbonate. Suppose again, that we have an acetic acid of sp. gr. 1.059, and that we wish to produce, by its dilution, two fluid-drachms, or any other quantity, of acid having the strength of distilled vinegar, the question is, what are the proportions of water and strong acid to be employed. We have only to bring the Acidum Aceticum of 1.059, to 120, i. e. to fʒij, and the number opposite to the strong acid, _viz._ 16, is its equivalent, if therefore we take 16 minims of it, and dilute it with 104 minims of water, we obtain the mixture required. 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. PHARMACOLOGIA. COMPREHENDING THE MEDICINAL HISTORY AND CHEMICAL HABITUDES OF THE DIFFERENT ARTICLES THAT CONSTITUTE THE MATERIA MEDICA. “_Omnium Simplicium Pharmacorum vires nosse oportet cum qui aliquot compositum est facturus._” _Ætius._ PHARMACOLOGIA. ABIETIS[318] RESINA. L. E. D. (Pinus Abies, _Resina concreta_.) _Resin of the Spruce Fir._ Olim, _Thus—Frankincense_.[319]