tion will be completed more quickly if only a small amount of drug is placed in the mortar at a time.

Grinding or powdering is the reduction of a substance to a powder by friction and pressure. There is no distinct line between the two, but we often refer to a substance in coarse particles as having been ground or granulated, and that in a fine condition as having been powdered. In powdering anything the fine particles should be sifted out occasionally so that they will not surround and protect the coarser ones. After a substance has been powdered, all of the siftings should be thoroughly mixed, because some parts of the drug are more easily powdered than others, and may contain more or less active matter.

Mills. Grinding or powdering vegetable drugs is done in mills, of which there are many forms and modifications, depending upon the fineness of product desired, and on the nature of the drug. A pebble mill is good if the drug is resinous or liable to cake. It consists of a cylinder revolving on its longitudinal axis and containing different sized balls of flint, iron, or porcelain. These grind by friction against one another and against the cylinder, having a sliding as well as rolling motion.

The term chaser mill is applied to those in which two large wheels, generally of stone and resting on their edges, are fastened on a short axle, and made to revolve around a central point in the axle-one wheel chasing around after the other, the two running on the flat surface of a third stone. Both a crushing and a rubbing effect is produced.

A buhrstone mill is one in which corrugated surfaces of two plates run together, one plate stationary and the other revolving, or both revolving but in opposite directions, the plates being in a horizontal or vertical position.

In the Bogardus eccentric mill the grinding surfaces

run in the same direction, but the axis about which one runs is not in line with the axis of the other. This mill can be run at a high rate of speed and causes a rolling, grinding motion which quickly reduces the material to a fine powder.

Meade's disintegrator consists of a disk on the side of which, near the edge, are fastened steel projections. The disk revolves very rapidly, the projections run close to a corrugated surface and beat the drug against the corrugations until it is sufficiently fine to pass through the openings.

In the retail store a small hand mill is sometimes convenient. After grinding a drug, the mill should be thoroughly cleaned. This can often be done by running sawdust through it, but if the drug has caked it may be necessary to take the grinding surfaces apart and wash with warm water. In grinding a drug, time and labor can be saved by reducing it to a coarse powder, then to a finer one, but not to the desired degree of fineness the first time it is passed through the mill.

Trituration is the reduction of a solid to a fine powder by rubbing in a mortar. Crystals and chemicals can generally be quite readily powdered, but with certain compounds it is easier if a volatile solvent is used in just sufficient amount to dampen it, as in powdering camphor with alcohol or ether. Some drugs give trouble by becoming electrified, as resin or acetanilid; dampening with a volatile liquid overcomes the difficulty. Drugs which are tough may best be powdered by the addition of some gritty solid, as sugar of milk in making Dover's powder, or sand in tincture of lactucarium.

Mortars are most commonly made of earthenware known as Wedgwood, sometimes of porcelain or glass. Glass is objectionable because it is so smooth and easily

broken. Wedgwood mortars after long usage become smooth and can be roughened by triturating with sharp sand. The solid or one-piece pestle is the most satisfactory, but breaks easily on being dropped. With the wooden handle Wedgwood-tipped pestle the handle becomes loose and the cementing material is likely to get into the drug being triturated. The convex surface of the head of the pestle should have a slightly smaller curvature than that of the concave surface of the mortar.

Levigation is the powdering of a substance in the presence of a liquid in which it is not soluble. Some chemicals can be more finely and easily powdered in this way. An example is found in the method of making ointment of yellow oxide of mercury. Levigation is carried on in a mortar and porphyrization is practically the same thing carried on with a slab and muller.

Precipitation is the rendering insoluble of anything in solution. This may be done by changing the solvent, as adding water to spirit of camphor; by adding another solid without causing chemical change (salting out), as adding sodium chloride to a solution of pepsin; by chemical action and the formation of a new compound. The object of precipitation may be to purify the material, to get it into a finer powder, or to get a new compound. If chemical reaction takes place and a dense heavy precipitate is desired, the solutions of agents reacting should be hot and concentrated and if a light precipitate is wanted the solutions should be cold and dilute. The best form of a precipitating jar is one in which the diameter at the bottom is greater than that at the top; the precipitate in settling does not rest on the sides of the jar. The liquid from which the precipitate has settled is called the supernatant liquid.

Granulation of chemicals is generally effected by evap

orating a solution and stirring while the solid is separating. Sometimes a powder is dampened with alcohol or alcohol and water and passed through a coarse sieve. A metal may be granulated by melting and pouring into water or by heating to near its melting-point and then rubbing it. Sometimes we speak of granulated vegetable drugs, meaning coarsely ground powders.

Elutriation is a method of separating coarse particles from fine ones. The powder is mixed with a liquid in which it is insoluble, the coarser particles, being heavier, are allowed to settle. The liquid holding the finer powder in suspension is decanted and allowed to rest and again decanted and allowed to stand, thus separating the powder into any number of lots of varying fineness. If the damp powder obtained in this way is allowed to drop from a funnelshaped vessel so as to make masses like drop chalk, it is called trochiscation.

Separation of fine from coarse particles is generally effected by sifting. Powders should not be forced through a sieve. The sieve is known by the number of the meshes to the linear inch. One having eighty meshes to the linear inch is known as a No. 80 sieve, and the powder which passes through it as a No. 80 powder. The U. S. P. specifies the maximum size of the particles passing through a sieve. Copper wire is generally used in making a sieve, and it should have the correct gauge, as a coarse wire would make the openings too small and a fine wire would make them too large. Of a powder of any given number, not more than 20 per cent of it should pass through a sieve having ten meshes more to the inch. A No. 80 or 100 sieve is about as fine as is generally used, and when a finer powder is wanted it is passed through a bolting cloth.

SOLUTION

Definition. Solution may be defined as the molecular subdivision of a substance by means of another substance, the result being a homogeneous liquid. In dissolving a solid in a liquid the cohesion of the molecules is overcome by adhesion. The product of solution is called solution; the agent producing solution, the solvent; the agent dissolved the solute.

Kinds. It is common to speak of simple solution and of chemical solution. In simple solution the agent dissolved and the solvent do not lose their chemical properties, and can be separated into the same conditions as they were before being dissolved, as salt in water. By chemical solution (sometimes called complex solution) is meant a solution in which chemical reaction takes place; the substance dissolved and the solvent form a new compound, as iron dissolved in hydrochloric acid gives chloride of iron.

A solution may be made by mixing a solid (as sugar), a liquid (as glycerin), or a gas (as hydrochloric acid), with a liquid (as water). Or it may be made by mixing two solids together (as menthol and camphor), but in this case chemical reaction may take place.

A saturated solution of anything is one in which no more of that substance will dissolve in the liquid, the adhesion of the molecules of the solvent for the solute being satisfied. Though the liquid is saturated with one substance it is not necessarily saturated for others.

A supersaturated solution is one in which, under certain conditions, the liquid is made to hold in solution more of the substance than it will do ordinarily. A hot saturated solution of certain salts, as magnesium sulphate, if allowed to cool without any disturbance, may retain the