in others the lower-while the other revolves, the material being fed through an opening in the centre of the upper stone. By suitable approximation of the stone disks, powders of various degrees of fineness can be produced.

The portable Bogardus Eccentric Mill (Fig. 86) is a great favorite with drug-millers, as it can be driven at a high rate of speed without

FIG. 86.

becoming heated, and discharges the ground material promptly without danger of choking. Both grinding plates revolve in the same direction, on centres which are about one or two inches apart from each other, hence the name eccentric; this arrangement causes the material between the plates to be moved about in every conceivable manner, to be acted upon by the plates at every point, and subjected to a peculiar twisting, cutting, and grinding motion, whereby it is rapidly disintegrated, with large results in quantity ground and the expenditure of but little power. In mills with single revolving plates (the other being stationary), one plate continually describes the same circle on the other, so that material ground in these mills is subject to motion in one direction only, hence greater power and more time are necessary to accomplish the desired result than if the material were acted upon in various directions and by different motions. The rate of feeding the mill is controlled by an adjustable slide attached to the hopper, and the degree of fineness of powder is regulated by means of a screw and lever controlled by a weight.

Bogardus eccentric mill.

The so-called Chaser Mill is preferred when large quantities of material, such as cinnamon, ginger, pepper, mustard-seed, and the like, are to be reduced to impalpable powder. Fig. 87 shows a sectional view of a large chaser mill in use at the drug mills of Messrs. Gilpin, Langdon & Co., of this city. It consists of two large stone disks, or granite wheels, connected by a short metallic axle with a revolving shaft, which compels them to travel in fixed lines on a base of granite. The name chaser mill is derived from the motion of the disks-called chasers-which appear to chase each other in their travels over the stone base. The grinding of any material supplied to the mill is effected between the granite base and the outer edge of the chasers; by means of iron scrapers appropriately fastened to the revolving shaft, the material is continually brought under the grinding edges again. As seen in the illustration, the base is surrounded

MECHANICAL SUBDIVISION OF DRUGS.

101

by a curb, to prevent the coarsely-ground particles from mixing with the finer powder, which, by means of the draught created by the rapid revolution of the chasers, is carried upward and over the sides of the curb. The whole mill is enclosed in a dust-proof compartment, which is frequently provided with a series of shelves for the purpose of allowing the fine particles of powder to be deposited for subsequent convenient collection. The feeding of the mill is accomplished through the top of the box, by means of a long funnel delivering the material directly upon the stone base.

SIFTING. In order to produce powder of uniform fineness, the ground substance should be subjected to the separating action of some perforated medium, whereby division into coarser and finer particles is readily effected. The construction of ordinary sieves is too well known to require special description. The perforated material or netting used may be made of iron, brass, or tinned wire, hair-cloth for substances affected by metal, and silken cloth for very fine or dusted powders. Different degrees of fineness of powder are designated in the U. S. Pharmacopoeia by numbers, which refer to the number of meshes to the linear inch in the material of which the sieve is made; thus, very fine or No. 80 powder should pass through a sieve having 80 meshes to the linear inch (or 30 meshes to the centimeter); fine or No. 60 powder should pass through a sieve having 60 meshes to the linear inch (or 24 meshes to the centimeter); moderately fine or No. 50 powder should

pass through a sieve having 50 meshes to the linear inch (or 20 meshes to the centimeter); moderately coarse or No. 40 powder should pass through a sieve having 40 meshes to the linear inch (or 16 meshes to the centimeter); coarse or No. 20 powder should pass through a sieve having 20 meshes to the linear inch (or 8 meshes to the centimeter). While it is impossible to grind drugs entirely of the degree of fineness wanted for many purposes, the aim should be to keep the finer portion down to a low percentage by frequent sifting; as prescribed in the Pharmacopoeia, not more than one-fourth of the powder should pass through a sieve having 10 more meshes to the linear inch. It should also be borne in mind that some parts of the drug can be ground more readily than others; it is therefore necessary to mix the powder thoroughly, after the grinding and sifting have been completed. The proper handling of a sieve cannot be

FIG. 88.

definitely described, it must be taught practically; this much, however, can be said -that no effort should be made to force the material through the meshes of the sieve by persistent pressure of the hand, which will cause the meshes to open farther and allow coarser particles to pass through. In Fig. 88 is shown the wellknown Harris Sifting Machine, which some twentyfive years ago was extensively used by pharmacists;

its construction is very simple and readily understood. Of late years, sifters and mixers combined in one piece of apparatus have been greatly preferred; such a combination, admirably adapted to the wants of the pharmacist who manufactures on a small scale, is shown in Fig. 89. Its capacity is 50 pounds, and the mixer is provided with a galvanized double spiral agitator so arranged that when the sifted powders come in contact with it the inside spiral carries the material one way, while the outside spiral carries it the other; thus a most thorough mixture is effected in a short time. After the powders have been mixed, the contents may be withdrawn by means of a slide in the bottom of the circular mixer. Smaller and larger sizes of the Lightning Sifter and Mixer are manufactured, and can be supplied with sieves of different degrees of fineness. Fig. 90 represents Jones' Mixer and Sifter, in which the mixing is effected on a different principle, by means of paddles and brushes; its capacity is 10 pounds. These combined sifters and mixers are well adapted for the manufacture of Seidlitz mixture, tooth-powder, compound liquorice powder, etc., without the annoyance of dirt and dust.

Powdered drugs are frequently offered at prices lower than those asked for a good quality of the crude drug; yet it is well known that

Jones' mixer and sifter.

the cost is enhanced by loss in drying, expense of powdering (from 3 to 10 cents per pound), and other incidentals. There can be but one explanation for this anomaly: either an inferior quality of drug has been ground, or admixtures have been made to increase the yield of the powder. As detection of the fraud is not within the reach of all, powdered drugs should be purchased only from dealers whose sense of truth and honor is paramount to their cupidity. Owing to the largely increased surface exposed to light and air in the case of powdered drugs, they are, as a rule, more liable to deterioration than crude drugs, and should therefore be more carefully protected, particularly against moisture.

Among other methods for the mechanical subdivision of drugs may be mentioned trituration, which consists in reduction of a substance to very fine powder by continued attrition of the particles between the hard surface of a pestle and the sides and bottom of a mortar. Trituration is usually applied to saline and similar chemical substances, and the mortars best adapted to the process are those made of Wedgewood ware, of the shape shown in Fig. 91. A rotary motion of the pestle accompanied by pressure is productive of the best results in trituration, the circles described being gradually enlarged from the centre outward and back again to the centre. A thin layer of the material should be kept between the pestle and the sides of the mortar. When the powder begins to cake and fall toward the centre of the mortar, a spatula should be run around the sides so as to loosen up and mix the different portions. The term trituration is also sometimes employed to designate the thorough mixture of vegetable or other

powders by rubbing them well together in a mortar; in such cases little if any pressure is employed, and thorough blending of the mixture is facilitated by frequently scraping the powder down from both pestle and mortar with a spatula.

The reduction of substances to fine powder by triturating them in the presence of a liquid having no solvent effect upon them, is termed levigation. The process is usually conducted in broad, shallow mortars. Formerly, when a stone slab and muller were employed, this method was also known as porphyrization, from porphyry, a very hard stone, the material of which the slab was made. Water, alcohol,

or oil may be used as suitable media for levigation, the process consisting of the formation of a soft paste of the substance to be powdered and the liquid, this paste being then triturated or ground until perfectly smooth. Red mercuric oxide may thus be reduced to an impalpable powder by trituration with alcohol, and white paints, such as zinc oxide and lead carbonate, are ground smooth with oil in special paint mills.

Elutriation is a process intended for obtaining certain inorganic substances in a finely pulverulent condition, by diffusing them in water after they have been ground or crushed; the coarser particles then rapidly subside, owing to their higher specific gravity, while the water holding the fine powder in suspension is decanted and allowed to settle in another vessel, the decantation being repeated a second time if necessary. To facilitate drying of the elutriated powder, the magma or soft mass is drained as completely as possible, and then formed into small conical nodules, which are conveniently dried on warm porous tiles. The well-known soft prepared chalk, French bismuth subnitrate, and numerous lake colors, are obtained as fine powders by elutriation.

Other methods for the mechanical subdivision of drugs are precipitation, reduction and granulation.