n DIALYSIS proper care ami noting of conditions, the abso-
- comparative diffusibilitics of substan-
be determined. Hydrochloric acid and tin- allied hvdracids arc found to be the most diffusive rabeUnOM known; the solid chlorides are high in tin- M-ale, and of these apparently chloride of -odium highest. As an illustration results of scries of experiment*, the approximate times of equal diffusion of the fol- lowing sultanres were found as here given: nloric arid, 1 ; chloride of sodium, 2'33; sugar, and sulphate of magnesia, 7 ; albumen, irarael, 98. When two or more diffusi- ble substances are mixed, the difference in
- ates of diffusion is increased, and effec-
tive analysis by diffusion is thus favored. The rate of diffusion is much accelerated by eleva- tion of temperature of the liquid or mass, so that separations may be effected in less time at high temperatures; but the degree of sepa- ration is less, since at the same time the less ditfu.-ible substances gain in the higher ratio. The rate of diffusion of all substances is less in alcohol, and probably in most other liquids, than in water, or in semi-fluid masses rendered snch by water. The name "diffusate" has been given to any substance as diffused, or separated by dialysis. The relations and ap- plications of the new facts, and the principle which is their basis, are numerous, and some of them of high importance. The dialyser affords an advantageous method for completely purifying soluble colloids without risk of de- composition, by the readiness with which all crystalloid substances pass from them into water ; and Prof. Graham in his paper (" Phi- losophical Transactions," 1861, part i., p. 183) give- directions for the preparation and purifi- cation of many substances of this class. Be-
- he distinctions already referred to, it
will be observed that crystalloid bodies tend
- egato in plane films and with angular
outlines, and are hard and solid; while the more usual condition of the colloid is that showing rounded outlines, a homogeneous mass, with more or less softness and tough- ness of texture. The water of crystallization in tlie former is represented by water of gelat- i nation in the latter. The colloids are usually in>i|.id; the crystalloids more commonly have a marked taste. Chemically, the former are the inert bodies; the latter, usually active, or But as observed in their most "iiditions, the rigid crystalloids are al- l holly unsusceptible to external impres- u liil. tin- soft colloids have a wide sen- sibility to external auMicies, and thus great mutability of condition. Even the simply min- eral rolloids cannot l,,ng be kept without change- pure hydnted silicic acid, or soluble silica, sealed uj, tightly, undergoing change Hthtt ^or weeks; and the existence -f many of them is only in and during a con- tinued metamor|,ho,is. This is especially true of albumen, gelatine, mucus, and related sub- ttancea, as existing in the fluids and living tissues of the animal body. These colloids are plastic or nutritive, and apparently in good part because they are mutable or capable of those successive metamorphoses during which the conditions of vitality can be secured, and in turn vital force and action evolved and manifested. Thus, these elements stand phys- iologically in relations the reverse of those they show chemically ; and Prof. Graham ac- cordingly terms the crystalloid a statical, and the colloid a dynamical condition of matter. He suggests that the colloidal condition of matter may be looked upon as " the probable primary source of the force appearing in the phenomena of vitality;" while "to the gradual manner in which colloidal changes take place (for they always demand time as an element) may the characteristic protraction of chemico- organic changes also be referred;" in these intending to include, of course, the time re- quired for application of the power of the will, for exertion of muscular force, and the physical changes that underlie the phenomena of sensation and thought. The facts observed in connection with diffusion appear to lead to a new understanding of endosmose, as effect- ed, in part at least, by the circumstances that a colloid cannot abstract water from (or dehy- drate) another colloid or a crystalloid, while a crystalloid can readily dehydrate a colloid, and in so doing effect its own movement through the latter. The method of dialysis can be employed for the extraction of arsenic, tartar emetic, corrosive sublimate, strychnine, mor- phine, and other crystalline poisons in the stomach, blood, milk, or any organic mixture. The crystalloid poisons will pass through the septum into the outer vessel, where their pres- ence can be shown by the usual tests. By it soluble albumen may be obtained in a state of purity, by addition of acetic acid, and use of a colloidal septum. Nitrate of silver, from pho- tographer's waste, when put into the dialyser readily separates from the albumen and other organic impurities, and can thus be saved. As early as 1864 Mr. Whitelaw took out a patent. in England for the removal of chloride of so- dium and nitre from the brine of corned and salted meats by means of dialysis. Liebig has shown that the brine contains a large propor- tion of the nutritious constituents of the meat ; and if we could remove the salts and evaporate the residue, we should have all of the proper- ties of a good soup. This process is success- fully accomplished by Mr. Whitelaw's appara- tus, as the savory and valuable constituents of the meat are colloids, and will not therefore pass through a membrane. A further techni- cal application of the doctrine of dialysis is in the extraction of sugar from the beet ; and it has been proposed to apply the same method to the extraction of sugar from the cane. The contrivances employed by sugar refiners are called osmogenes, and they are now much used in Germany and France. Graham applied the principle of dialysis to the concentration of tho
