central knife-edge; they must all be in the same plane, and the edges absolutely parallel to each other,

It is very apparent that the conditions of a good prescription balance cannot be satisfied if there is inequality in the length of the arms of the

FIG. 2.

Illustration of equilibrium.

beam. In Fig. 3, BA should equal AC, otherwise unequal weights would be required to establish equilibrium, or the excess of weight of the longer arm must be subtracted at every weighing, or weighing by substitution resorted to, all manifestly impracticable. If the central knifeedge be placed either above or below a line drawn so that it connects the end knife-edges, the loading of the pans will either cause the beam to

FIG. 3.

A

Position of knife-edges.

cease oscillating, or diminish the sensibility in proportion to the load. If the knife-edges are not parallel, the weight of a body would not be constant upon every part of the pan, but would be greater if placed near the edge on one side, and correspondingly less at a point directly opposite.

3. The beam should be inflexible, but as light in weight as possible, and the knife-edges in fine balances should bear upon agate planes.

Rigidity of the beam is necessary, because any serious deflection caused by a loading of the pans would lower the end knife-edges, and thus accuracy in weighing would be impossible. The beam should not be heavier than necessary, because its sensibility (particularly when light weights were placed upon the pans) would be thereby lessened, and to diminish friction arising from the rusting of the steel knife-edges which constantly increases with the age and use of a balance, the bearings of the knife-edges should be agate planes. Formerly this condition could not be practically carried out, except in expensive balances, owing to the cost of polished agate; but since the introduction of automatic machines for grinding and polishing this very hard substance, the cost has been so reduced that not only the bearings, but the knife-edges themselves, are now made of agate and used upon fine prescription balances, and it is practical economy to employ exclusively those so mounted.

The Tests of a Balance.-Having stated the essential points in the construction of the balance, the tests, which should always be applied before accepting a balance, will now be described. The prescription balance, being one of the most delicate and important of the instruments

in use by the pharmacist, is selected for illustration. 1. A perfectly level counter or table is secured, the balance is placed in position, the beam elevated so that it is free to oscillate, and when the oscillations have ceased, the smallest weight to which the balance is sensitive is placed upon the right-hand pan,-it should at once respond to the weight; this tests its sensibility with unloaded pans. 2. Both pans are now loaded with the full weight that the balance is designed to carry, and then the smallest weight is placed upon the right-hand pan, the oscillation, although slower than in the first test, ought to be as decided; this shows its sensibility when loaded. 3. The pans should now be loaded to half the capacity of the balance, and equilibrium perfected by adding, if necessary, a piece of tin-foil. The weights on the right-hand pan must now be exchanged for those on the left-hand pan, and vice versa; if equilibrium is still maintained, the arms of the beam are equal. 4. The pans should be moderately loaded and balanced, and one of the larger weights shifted in different positions upon the edge of one of the pans, and any variation in equilibrium carefully noted. This variation, as previously mentioned, indicates a want of parallelism in the knife-edges. Forms of Single Beam, Equal Arm Balances.-There are good reasons for believing that this kind of balance has a very ancient origin; and at the present time it is used more frequently than any other.

Hand Scales.-The commonest form in which this principle is utilized in pharmacy is seen in the cheap hand scales now fast passing out of use; in the better qualities the beam is of steel, and the knife-edges are enclosed so as to protect them from injury. In those usually seen the

FIG. 4.

Manner of holding scales.

beam is of brass, with a steel central knife-edge, having a perforation in each end of the beam for the insertion of two wire hooks, to which are attached silken cords for supporting the pans. The manner of holding these scales is shown in Fig. 4. They are now most largely employed in

America by physicians and pharmacists having a very limited use for a balance. The silken tassel is held in the left hand, and care should always be taken to see that the beam oscillates freely and properly before the weight is placed upon the left-hand pan, as the hooks frequently become entangled in the cords and the adjustment is lost. The requisite quantity of the substance to be weighed is placed upon the right-hand pan, preferably with a spatula.

Alkaloids and very poisonous substances should never be weighed upon ordinary hand prescription scales, except when, by previous actual test at the time, they have been shown to be sensitive and accurate; for, although this form has been in use many years, as now found in commerce they are the most unreliable of all kinds of prescription balances, and, notwithstanding their merits of cheapness and portability, in the end they may prove (through some serious error) an expensive investment. In Fig. 5 is shown an improved form of hand scale in which a sliding weight is used; this may be suspended on a hook at the proper height as shown in the cut.

Prescription Balances.-The advantage of substituting a rigid metallic column for the usually unsteady human arm, as a support to the beam, would seem to be apparent at a glance, for, in addition to the increased stability which is gained, both hands are left free to perform the weighing; more time can usually be devoted to secure equilibrium, and the oscillations can be more readily noticed.

A cheap form of prescription balance, called, technically, the army prescription scales (Troemner), is seen in Fig. 6; the beam, column, supports, pan, etc., are of brass, the knife-edges of steel, whilst the

drawer beneath is large enough to hold conveniently all the parts; this is a very useful feature where a balance is not in constant use, enabling the operator to set the balance at will, or keep it in the drawer

FIG. 6.

protected from dust and corrosion. It may be made sensitive to the of a grain; but it soon loses this degree of delicacy, because of the rusting of the steel bearings and knife-edges. With careful handling, and by protecting it from the atmosphere by covering with a glass shade, it will remain in good order for years, and is vastly superior to the hand prescription scales.

A pharmacist who desires to make use of all the modern improvements and secure the best practical results, should have, for economy's sake, two prescription balances,-one for delicate weighings and the other for ordinary work. These need not have the most fashionable and costly cases, but the workmanship of the balance itself cannot be too fine, if by it are secured the absolute essentials of accuracy, sensibility, and durability.

Army prescription scale.

In addition to the theoretical requirements before noted, the finest prescription balances now made by Troemner are provided with solid silver pans and gold-plated beams (see Fig. 7). As the pans are subjected to more wear and tear than any other part of the balance, it is economical

FIG. 7.

to use solid rather than plated pans, for, as they have to be cleaned repeatedly, constant friction wears off the plating, and the additional cost for replating soon absorbs the difference in price. To secure the best results, when a showy appearance is not desired, a dead-black finish to the column is preferable, as the labor of polishing, with the danger to the adjustment that it involves, is dispensed with. If the supports and pans are of solid silver or nickel, and the knife-edges and the bearings of agate, a prescription balance is furnished which is durable and really cheap, because it is fully equal to the most exacting demands.

Fine prescription balance.

Analytical Balances.-The growing importance of analytical work, in connection with the quantitative tests introduced into the last revision of the U. S. Pharmacopoeia, renders the possession of an analytical balance by the pharmacist very desirable. Formerly these delicate objects of mechanical skill were exclusively imported, but for twenty years American manufacturers have devoted unremitting effort to excelling in this fine work, with gratifying success. Fig. 8 shows an analytical balance of recent pattern. The open, metallic beam is made of

aluminum, the bearings and knives of agate, the pans and hangings of aluminum, all other parts being plated with gold. The ends of the stirrups are provided with hooks to suspend a body in taking its specific gravity, and a

contrivance for arresting the motion of the beam and pans, together with one for elevating the beam entirely from contact with the agate planes when not in use, is provided.

Fig. 9 shows an enlarged view of the end of the beam with the beam locked; the dotted lines show the position of the lever when the beam is permitted to oscillate. The right arm of the beam is graduated so that each division represents one-tenth of a

FIG. 8.

Analytical balance.

milligramme if metric weights are used, or one-hundredth of a grain if apothecaries' weight is employed; an aluminum-wire rider is pushed along the arm by a sliding rod to any point upon the graduated beam to indicate these fractions. An

index needle traverses an ivory scale fixed on the base of the column, and the most delicate oscillations may be thus measured by the needle: if, for instance, one-tenth of a grain on one of the pans deflects the needle ten divisions on the scale, each one of these divisions would then represent one-tenth of the weight on the pan, or one-hundredth of a

FIG. 9.

End of the beam of analytical balance.

grain. With practice, the use of this means of weighing very minute quantities can be brought to great perfection.

pan

Counter Scales. For counter and dispensing purposes, the single beam, equal arm principle was formerly exclusively used (see Fig. 10). These scales were usually made of polished brass, and answered most purposes if kept in good order, but were objectionable because the supports were frequently obstructive, as they only permitted the weighing of bodies having a limited surface, and the excessive amount of polished brass-work about them required the expenditure of considerable labor to keep them bright, without any corresponding advantage. The form in which the pans are placed above the beam, thus getting rid of obstructive