Dialysis

(di-ul'e-sis.) [From Gr. dia, through, and lyo, to loose.] (Chem.) During his experiments on the diffusion of liquids, Prof. Graham discovered that solutions of certain bodies pa through membranes with considerable facility, whilst others pa through very slowly. He soon found that the former cla (crystalloids) embraced bodies which were of a crystalline character, such as metallic salts, and organic bodies such as sugar, morphia, and oxalic acid; whilst the latter cla (colloids) consisted of bodies devoid of crystalline power, such as gum, albumen, gelatine, . The most convenient dividing film or septum, as the discoverer named it, is made of parchment paper. A sheet of this substance is stretched over a gutta-perch a hoop, and its edges are well drawn up and confined by an outer hoop; it is then allowed to float on a basin of pure water, and in it is poured a mixture of colloid and crystalloid. Diffusion commences at once; the crystalloid rapidly pa es through and di olves in the pure water beneath, whilst the colloid for the most part remains behind. Prof. Graham gave this proce of separation the name of dialysis, and it is now in constant use in chemical laboratories for effecting separations which would be extremely difficult, if not impo ible, by other proce es. Thus, gruel or broth, containing a very little arsenic (arsenious acid), di olved in it and submitted to dialysis, gives up the whole of the arsenic to the pure water, whilst scarcely a trace of its organic substances pa through. In cases of suspected poisoning the course now generally pursued is to pour the whole contents of the stomach, or other liquid which the analyst has to examine, upon a dialyzer, and after allowing it to stay there for 24 hours to examine the aqueous solution. Almost all the poisons in common use, such as arsenic, strychnine, corrosive sublimate, oxalic acid, acetate of lead, morphia (the active agent in laudanum and opium), being crystalloids, easily pa through, and the work of the toxicologist is very much simplified, as he has only an aqueous solution of a comparatively pure substance to deal with, instead of a highly com Diamagnet i plex mixture of sm, organic (-mag'net-ism.) substances. through, [Gr. dia, andEng. magnet.) (Phy.) Coulomb observed, in 1802, that magnets act upon all bodies in a more or le degree; this action was at first attributed to the presence of ferruginous particles. Brugmann also found that certain bodies, for instance, bars of bismuth, when suspended between the poles of a powerful magnet, do not set axially between the poles, that is, in the line joining the poles, but equatorially, or at right angles to that line. This phenomenon was explained by the a umption that the bodies were transversely magnetic. Faraday made the important discovery in 1845 that all solids and liquids are either attracted or repelled by a powerful electromagnet; and this double action was afterwards observed on gases by Bancalari. The bodies which are attracted are called magnetic or paramagnetic substances, and those which are repelled are diamagnetic bodies, and the phenomenon itself is called diamagnetism. [s. 283]