608 



SPECIAL PHYSIOLOGY. 



to intermix in inverse proportions to their density ; nor is it to be explained 

 by capillarity ; for the diffusion of different substances does not coincide with 

 their ascension in capillary tubes ; but this process appears rather to depend 

 on chemical action. The high diffusibility of crystalloids is explained by their 

 powerful attraction for their solvent, the mobility or volatility of which is 

 determined by their presence ; whilst the low diffusibility of colloids is referred 

 to their feeble combination with their solvent, on the volatility of which they 

 accordingly have little effect. 



Dialysis. The phenomena of the diffusion of liquids into each other are ren- 

 dered more definite by the interposition of permeable membranes between them. 

 If a gutta-percha hoop be closed on one side with vegetable parchment, the 

 tray thus formed will not allow water to pass through it by nitration. By 

 supporting, or suspending, such a tray in a large vessel of water, and pouring 

 a thin layer of the fluid or solution to be experimented upon, into it, dialysis, 

 or diffusion through the permeable membrane, takes place. Crystalloid bodies, 

 in solution, pass^ through the membrane, or dialyze, into the water, with great 

 rapidity ; whilst colloid bodies are almost absolutely prevented from passing. 

 Thus, in equal times, the proportion of common salt which dialyzes is 7.5, of 

 cane sugar 1.6, and of gum .029 ; or, again, the quantity of salt which dialyzes 

 being 5.2, that of dextrin is .034, of gum .013, of caramel .009, and of albumen 

 .005 ; whilst gelatin, extract of meat, and boiled starch, do not dialyze at all. 

 The rate of dialysis is influenced by the depth of the fluid in the tray, by the 

 area of the membrane, by the strength of the solution, and, to a certain degree, 

 though less than liquid diffusion, by temperature. The process is not me- 

 chanical, but chemical, the results being more definite than those of liquid dif- 

 fusion. Dialysis depends on the affinity of the substance experimented upon, 

 for the water in the moist permeable membrane. Crystalloids, which dialyze 

 rapidly, have an affinity for, or unite with, the water of the septum, and, by 

 successive combinations of their molecules with the water in that membrane, 

 they pass through to the water outside it, and thus a movement, otherwise 

 invisible, becomes apparent. On the other hand, colloids have little or. no af- 

 finity for the water of the septum, and, therefore, do not make their way 

 through it. The membranous septum is itself colloidal ; its molecules have, 

 therefore, only a slight affinity for water, and permit the stronger affinity of 

 the crystalloids, successively to displace them, and so to pass through ; whilst 

 colloids generally, are too feeble to accomplish this displacement. Thin layers 

 of any colloidal substance, such as films of gelatinized starch, albumen, jelly, 

 gum, and mucus, also act as dialyzers. 



Dialysis has been employed for the artificial separation of crystalloids and 

 colloids. Saline and earthy matters, rendered soluble by acetic or hydrochloric 

 acids, may be dialyzed from albumen, or lime from solutions of gum, pure 

 albumftn or gummic acid remaining. Morphia, strychnia, and other crystal- 

 lizable alkaloids, have been separated from organic fluids ; and even W ^TT P ai *t 

 of arsenious acid, mixed with porter, milk, defibrinated blood, or pieces of 

 intestine, has yielded 80 or 90 per cent, of that minute quantity, by dialysis, in 

 24 hours. These dialytic actions also explain more completely, the long known 

 phenomena of endosmosis andexosmosis. The force concerned in liquid diffusion 

 was at first named, by Graham, osmotic force ; and endosmosis and exosmosis 

 were regarded, by hyn, as due to the action of this force in opposite directions, 

 or to a positive and negative osmosis ; the direction of the chief visible current 

 appears to be always towards any substance having the properties of a base, 

 water flowing towards a salt, and from an acid. Subsequently, however, 

 Graham distinguished liquid diffusion from diffusion through membranes, or 

 dialysis. 



Dialysis must take place in the living body, in which compound and simple 

 permeable and colloidal membranes abound, such as the basement-membranes, 

 capillary walls, and cell walls, all of which are subject to the constant action 

 of solutions of crystalloid and colloid bodies, either acids, alkalies, and salts, or 

 albuminoid and extractive substances-. The process of absorption most ob- 

 viously suggests diffusive and dialytic actions ; but so also do those of nutri- 

 tion, secretion, and excretion, and even the interchange of the gases of the 

 blood and air, in respiration, for these gases are dissolved at the moment of 



