This instrument is capable of showing, on mere inspection, the absolute as well as relative strengths of the different Officinal Preparations of the Pharmacopoeia. The active Principles, or Medicinal Bases, are distinguished by Capital Letters, placed in coloured compartments, and each Officinal preparation is marked by a line corresponding in colour with that of its active ingredient. Where a preparation contains two active ingredients it is distinguished by two coloured lines, as may be seen in the Pulvis IpecacuanhÆ comp. By this expedient the eye, at once, recognises the different classes of medicines; all those, for instance, coloured red are Opiates; those blue, Mercurials; green, Acetic acid; &c. The introduction of colours, moreover, immediately indicates the basis to which any preparation refers, and thus prevents the possibility of doubt or confusion.

If we wish to learn the actual quantity of active matter contained in any given proportion of a compound, we have only to turn the scale until the name of such compound coincides with the number in question, when the figure opposite to the basis solves the problem; thus—how much opium is contained in 5 grains of the Pulvis IpecacuanhÆ compositus? By having brought this preparation to 5, we shall see ½ opposite to Opium, denoting that such a number of grains contains half a grain; for, unless it be otherwise expressed, the figures denote grains for the solids, and minims for the liquids. And, since the number opposite to the Base denotes the absolute quantity of it contained in those several proportions of its Officinal compounds, which are expressed by the respective numbers opposite to each, it follows that these latter must all be medicinally equivalent to each other; thus we have seen that 5 grains of Pulvis IpecacuanhÆ compositus contains ½ grain of Opium, if we turn our eyes to the other Opiate preparations we shall perceive that each has a different number opposite to it, these figures show the number of grains of each which contains ½ grain of opium, and consequently those numbers must be all Equivalents; thus 2½ grains of Pil. Saponis comp: 5 grains of the Pulv: Corn: ust: cum Opio, 9½ minims of Tinctura Opii, 10 grains of Pulv: Kino comp: 18 grains of Confectio Opii, 20 grains of Pulv: CretÆ comp: cum Opio, and 120 minims, or two fluid-drachms of Tinctura Camphor: comp: contain half a grain of Opium, and are consequently all equivalent to each other. This system of Equivalents will be found of much practical value to the 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 ?ds 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 f?ij, 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.

“Omnium Simplicium Pharmacorum vires nosse oportet cum qui aliquot compositum est facturus.”