pressed being fed directly into the hopper communicating with a tapering cylinder containing a large screw, the thread of which gradually diminishes in size toward the smaller end; the cylinder is provided with a perforated plate in the bottom, and the material is compressed by means of the tapering screw, which is turned with a

crank. The dry residue is discharged through an opening in the small end of the cylinder, and the liquid expressed flows out through the perforated plate.

Another method of separation is that effected by means of centrif ugal machines, which are extensively employed in manufacturing establishments for washing and drying crystals as well as for the rapid withdrawal of moisture in the drying of certain precipitates and fabrics. The apparatus used consists of a metal drum or cylinder having a solid bottom but open at the top, and provided with perforated sides, which revolves on its own axis inside of a larger stationary cylinder supplied with a cover to keep out dust, and an outlet tube at the bottom, through which the liquid coming from the inner cylinder is allowed to flow out; sometimes the perforated sides of the inner cylinder are covered with bolting-cloth, according to the substance to be operated upon, and the rotary motion is imparted to the cylinder from below by means of steam power. The value of centrifugal machines depends upon the velocity with which the material to be centrifugalized is hurled around and against the perforated sides, the revolutions usually running as high as 2000 or 3000 and even more per minute; the strong draft of air created between the walls of the inner and outer cylinders by such rapid revolution effects drying of the material more thoroughly than is possible by expres sion or other means. The use of centrifugal machines is based on the well-known laws of motion and inertia, according to which a body put in motion continues in a straight line unless turned from its

path by some external force, and thus liquids can readily be separated from solids when a mixture of the two is dashed against a finely

FIG. 145.

perforated surface. In sugar refineries, centrifugalizing is the only suitable method known for separating the granulated sugar from the viscid mother liquor or molasses. For special use in the pharmacist's laboratory, small centrifugal machines, to be operated by hand, have been devised; the outer evlinder is usually made of enamelled iron while the inner perforated cylinder is made of porcelain ;

Centrifugal separator (for hand use).

Centrifugal separator with cover (for hand power).

those in which motion is supplied from above are frequently provided with a cover for the inner cylinder, while in those operated from below a cover is fitted to the outer cylinder. In Figs. 145 and 146 are shown two styles of hand-power machines.

Dialysis.

Dialysis is a process of separation which differs entirely from those considered thus far, in not aiming at the removal of insoluble matter suspended in a liquid, but at bringing about a separation between solvents and matter held by them in solution; also between different kinds of matter held in solution by the same solvent. It is a practical application of the principle of osmosis, and is due solely to surface action and the difference in diffusibility of various substances. The word dialysis is derived from the Greek verb da, to part

asunder, to loose one from another, and was applied by Prof. Graham, of England, to the method of separation discovered by him in 1861. The process consists in placing a solution of the substances to be separated on a porous diaphragm and suspending this in pure water; osmosis is established, and certain substances will pass through the diaphragm into solution in the water, while others will remain on the diaphragm, the rapidity of diffusion being in proportion to the strength of the solution and increasing with the rise in temperature. Prof. Graham discovered that crystallizable substances passed through the diaphragm freely, while amorphous bodies, such as gums, starch, gelatin, etc., either did not diffuse at all, or only very slowly; he applied the name crystalloid (resembling crystals) to all substances thus capable of diffusion through a septum, and the name colloid (resembling glue or jelly) to those substances remaining on the diaphragm. All colloids are amorphous or non-crystallizable, but all crystalloids are not necessarily capable of crytallization, as, for instance, hydrochloric acid, the most highly diffusible body, and many others. By means of dialysis, sugar can be readily separated from gum or starch, pepsin from peptones, iron salts from iron oxide, Thus the process has become most valuable to manufacturers, whilst the analyst often finds dialysis the only means for determining the presence of certain substances in complex vegetable solutions, as, for instance, arsenous acid, corrosive mercuric chloride, or potassium iodide in compound sarsaparilla mixtures and other proprietary medicines, where the dark color and complex nature of the solution preclude all other methods of separation.

etc.

The apparatus used for dialysis is of very simple construction, as shown in Figs. 147 and 148. It consists of a circular glass vessel, with flat bottom and of convenient size, also another smaller circular but bottomless vessel of hard rubber or glass, having a projecting

rim, over which is stretched a piece of bladder, parchment, or parchment paper (see page 139). The latter constitutes the dialyser proper, and into it is poured the solution to be dialysed, to the depth of about one-half or three-quarters of an inch, after which it is floated in distilled water contained in the other larger vessel. In Fig. 148 the glass dialyser is provided at the top with a broad rim which rests upon the edge of the outer vessel, and thus serves as a cover to protect the water against dust, etc. In place of the foregoing convenient appa

ratus, an ordinary clean hog or beef bladder may be used; the same should be three-fourths filled with the solution, and then suspended in a large vessel of water.

Diffusion in a dialyser will not take place unless the porous membrane or septum is in contact with water, and, moreover, its limit will be reached when the water on the outside becomes charged with such a quantity of crystalloids as to render the strength of the solution identical with that in the dialyser; hence it is necessary that the quantity of water in the outer vessel be much greater than that of the liquid in the dialyser, and that it be renewed from time to time. The crystalloids from a 10 per cent. solution of sugar, salt, or hydrochloric acid will readily diffuse through a septum if the latter is placed in contact with water, but no diffusion whatever will take place if the dialyser be floated in a 10 per cent. solution of the same substances. While the rate of diffusion varies greatly for different substances, it was found by Graham to be uniform for isomorphous bodies; that is, those having exactly the same cry talline form.

The colloidal residue remaining on the diaphragm is termed the dialysate, while the solution of the crystalloids that have passed through the membrane is known as the diffusate.

CHAPTER X.

SEPARATION OF VOLATILE MATTER.

ADVANTAGE may be taken of the volatility of some substances for the purpose of separation, and by their vaporization, either of the following objects may be attained :

1. The separation of a volatile liquid from a solid, with a view of retaining the solid substance, or of one liquid from another, to obtain the less volatile; in such cases the process is termed evaporation.

2. When the separation of liquid and solid substances, by means of evaporation, is carried to complete dryness, the process is more particularly designated as desiccation or exsiccation.

3. The separation of a volatile liquid from either a less volatile liquid or a solid, in order to obtain and preserve the volatilized liquid for future use; the process is then known as distillation.

4. The separation of a volatile solid from either a liquid or a solid which is more fixed, the object sought being the volatilized solid body; this process is termed sublimation.

Evaporation.

In the practice of pharmacy, evaporation is extensively resorted to for the concentration of vegetable and saline solutions, the latter with a special view to subsequent crystallization, and the laws which control the process should be well understood. Evaporation may be divided into four kinds; namely, evaporation over a naked fire, on a water-bath or steam-bath, in a vacuum apparatus, and spontaneous evaporation. Evaporation over a naked fire is effected by the direct radiation of heat from a fire, on the bottom of an uncovered dish or pan, and is available when the substance in solution is not injured by direct heat or high temperature; it is usually employed for the concentration of saline solutions for crystallization, but only when the liquid to be vaporized is water. When evaporation at temperatures below that of boiling water is desired, the low-power burner shown on page 73, may be used with advantage.

Evaporation on a water- or steam-bath is the method most frequently resorted to; the latter can also be employed for rapid concentration of solutions at a high temperature, without the danger of injury from direct heat of the fire. Evaporation at temperatures below 100° C. (212° F.) is effected on a water-bath, and is confined to the surface of the liquid; this is the method chosen for the concentration of vegetable and other solutions liable to be injured by