Page:The American Cyclopædia (1879) Volume VI.djvu/79

 DIALYSIS masses. (See ENDOSMOSE, and GAS, vol. vii., p. 634.) An example will illustrate the nature of the processes and results. A sheet of thin paper being thoroughly moistened, and de- ' pressed at the middle to form a sort of cup, a solution containing 5 per cent, each of cane sugar and gum arabic is poured into it ; and the paper cup is then placed upon the surface of water in a deep basin narrow enough to keep its edges elevated, and left for 24 hours. The cup being removed at the end of the time, the quantity of liquid it contains is found in- creased by endosmose ; but while a little of the liquid in the vessel below, tested with acetate of lead, shows a mere trace of gum, upon evaporation of the remainder the sugar crys- tallizes from it, in quantity equal to three fourths of that placed within the paper cup. The sugar, therefore, has rapidly made its way through the septum used, while the passage of the gum has been almost perfectly resisted. The paper can be replaced by moist animal membrane, or by a thin layer or film of any substance having the character of a jelly, as hydrated gelatine, albumen, mucus, or gelati- nous starch ; but the most useful septum is parchment paper in thin sheets and without sensible flaw or porosity. It is found that, through moist films or partitions such as those here named, through masses of different sub- stances in the gelatinous state, or through liquids, very many and perhaps all substances capable of crystallization in definite forms make their way by diffusion at rates which, though differing for the different substances, are rapid in comparison with those at which any substance having itself the gelatinous or jelly-like condition can traverse or diffuse in the same media. These facts led Prof. Gra- ham to divide the great body of chemical sub- stances (especially compounds) into two class- es, which are readily characterized by the ten- dency of the former to crystallize definitely, either alone or in combination with water, and to dissolve rapidly and generally in solutions free from viscosity ; while the latter when dry incline to the vitreous structure, having little tendency to crystallize, dissolve slowly or only soften, and as a rule assume the viscid or ge- latinous state. To these classes, respectively, he gives the names of " crystalloids " and " col- loids 1 ' (the latter from the Gr. K^T?, glue). In experiments such as those already referred to, the paper sized with starch, or other film or membrane containing a jelly or viscid ma- terial of any sort, is a colloidal partition or body; and the colloid gum has very slight power to penetrate it. This is found true also of such substances as hydrated silicic acid, a number of hydrated metallic peroxides, starch, vegetable gums, dextrine, caramel, tannin, albumen, and vegetable and animal extrac- tive matters, all of which are colloids. The crystalloids, however, as cane sugar, and a large number of chlorides, sulphates, &c., of metallic bases, readily penetrate the colloidal partitions or media ; and the explanation given is, not that either class of substance is afforded or denied passage through any effect of capil- lary attraction as ordinarily understood, but that, the affinity of any colloid for water being of the feeblest character, one colloid cannot with any rapidity abstract molecule for mole- cule the water from another, by which process it could be conveyed through it; while the crystalloids brought in contact with a moist colloid, having a high affinity for water, can displace the colloid from solution particle by particle, and thus make their way through its mass. These results are beautifully shown by placing at the bottom of two glass jars respec- tively, in a little starch jelly and then sur- mounted with several inches depth of the same jelly, a colored crystalloid, as bichromate of potash, and a colored colloid, as caramel ; the gradual elevation of the former through the mass can be daily observed, while at the end of eight days the caramel has scarcely begun to discolor the jelly above its first position. The different rates of diffusion through such septa allow of the employment of the method thus discovered for separating, in degree or partially, one from among two or more crystal- loids existing in mixture, but more readily and satisfactorily a crystalloid from a colloid. To this peculiar mode of separation Graham gives the name dialysis. It is conveniently effected with a " hoop dialyser," a sheet of parchment paper stretched beneath a hoop, and secured about it in the manner of a sieve. The sheet being moistened, and receiving in it a very thin layer of the solution from which some sub- stance is to be separated (the separation being more rapid as the layer is thinner), is floated on a sufficient body of water in a larger vessel. To separate in degree two or more crystalloids, the simpler method of "jar diffusion" often suffices. The mixed solution of crystalloids is conveyed by use of a pipette, so quietly as to leave the superincumbent liquid quite undis- turbed, to the bottom of a jar of water or al- cohol, and left at rest ; the most diffusible sub- stance rises most rapidly, and is more entirely separated from the others as the time is great- er, and the height to which it ascends through the column increases. By carefully drawing off with a siphon, at the end of the experiment, successive strata of the liquid into separate vessels, and quantitatively analyzing their con- tents, the quantities of the "diffusates" in the strata from below upward, and so the diffusi- bilities of the substances, are determined. Thus, with 10 per cent, solutions in pure water, in- troduced to the bottom of separate vessels, beneath 4'38 in. of pure water, 1 per cent, of common salt in solution had at the end of 14 days reached the uppermost of 16 strata of equal depth in the column ; while in the same time sugar had barely appeared (-005 gramme) in the uppermost stratum, gum had diffused itself to the tenth stratum, and tannin to the ninth, from the bottom. By such means, with