rapidity with which the material should be fed to the mill depends entirely upon the character of the drug, as some drugs will soften under the influence of heat and pressure, while others are not affected at all. Substances like vanilla, which cannot be heated before powdering, on account of the rapid loss of the aromatic principle, must be reduced in the soft condition; and, although the old method

Enterprise drug mill No. 450.

Cutter for vanilla.

of grinding with sugar or clean sand is still largely in use, it is decidedly inferior to the process of cutting. Grinding or powdering vanilla has a tendency to press out the soft pulp, which soon retards the reduction of the tough fibre and requires the expenditure of much time and labor. If vanilla be reduced to the requisite degree of fineness for percolation by means of a rapid-acting cutter, it retains practically its original condition, no pulp being expressed, and a powder is obtained far superior to that by grinding with sand or sugar. Fig. 91 represents the American mince-meat chopper, an apparatus admirably adapted to the cutting of vanilla, and first suggested for this purpose, many years ago, by the late N. H. Jennings, of Baltimore. The large knife-blade with which the cutting is effected must be kept well sharpened. As the cylinder revolves with each turn of the lever, fresh particles of the material are continually presented to the knife, and disintegration is rapidly achieved, while the aroma and virtue of the vanilla are kept

intact.

The grinding of drugs on a large scale, and particularly into very fine powder, is accomplished either in buhr-stone mills, iron mills, such as the Bogardus eccentric mill, or stone "chaser" mills. In the first-named mill grinding is effected between two large stone disks placed horizontally and provided with numerous furrows to facilitate the passage of the ground drug from the centre to the circumference; one of the disks is stationary-in some mills the upper, and

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.

Substances liable to become heated and to cake when ground in ordinary mills, such as vegetable extracts, pepsin, etc., can be reduced to an impalpable powder in the so-called pebble mills now in use by all the leading manufacturers of pharmaceutical preparations. These mills do not crush or cut the material, but grind principally by friction, the effect being produced by the sliding, tumbling, and rolling inside of a stone cylinder, encased in iron, of a large number of flint pebbles, of about the size of duck eggs, mixed with the substance to be ground; the movement is caused by revolving the cylinder at a regulated but slow rate of speed-from 25 to 45 revolutions per minute.

FIG. 92.

The portable Bogardus eccentric mill (Fig. 92) is a great favorite with drug-millers, as it can be driven at a high rate of speed without 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 plate (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 the 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. 93 shows a sec

tional view of a large chaser mill in use at the drug mills of Messrs. Gilpin, Langdon & Co., of Baltimore. 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 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 by numbers which indicate the number of meshes to the linear inch in the material of which the sieve is made, and since the diameter or gauge number of the wire used for making the sievecloth has an important bearing upon the size of the mesh, it should also be specified. The U. S. Pharmacopoeia recognizes very fine or No. 80 powder as one passing through a sieve made of No. 38 gauge wire and having 80 meshes to the linear inch; fine or No. 60 powder should pass through a sieve made of No. 36 gauge wire and having 60 meshes to the linear inch; moderately fine or No. 50 powder should pass through a sieve made of No. 35 gauge wire and having 50 meshes to the linear inch; moderately coarse or No. 40 powder should pass through a sieve made of No. 33 gauge wire and having 40 meshes to the linear inch; coarse or No. 20 powder should pass through a sieve made of

No. 28 gauge wire and having 20 meshes to the linear inch. While it is impossible to grind drugs entirely of the degree of fineness required for many purposes, the aim should be to keep the finer portion down to a low percentage by frequent sifting; not more than of the powder should pass through a sieve having 10 more meshes to the linear inch. It should also be borne in mind that

FIG. 94.

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 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. 94 is shown the well-known Harris sifting machine, which at one time 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 preferred; such a combination, admirably adapted to the wants of the pharmacist who manufactures on a small scale, is shown in Fig. 95. 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. 96 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 licorice powder, etc., without the annoyance of dirt and dust. Powdered drugs are frequently offered at prices lower than those

Jones' mixer and sifter.

asked for a good quality of the crude drug; yet it is well known that the cost is enhanced by loss in drying and powdering, expense of grinding (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. According to Squibb, the average loss by powdering (and subsequent drying) of the following drugs has been found to be for