DIALYSIS. 609 



interchange. Moreover, the sapidity of crystalloids and the insipidity of col- 

 loids are associated respectively, with a high and a low diffusibility. It has 

 even been suggested, as indeed was hinted by Dutrochet, that rapid dialytic 

 action may take place, not only in vegetable movements, but also in the inti- 

 mate changes of condition of the muscular fibres, in the states of contraction 

 and relaxation, and that it may thus form a link in the transformation of 

 chemical into mechanical force, which is realized in animal motion. Lastly, 

 organization and living action are indissolubly associated with the existence 

 of one at least of these two great classes of substances, discriminated by their 

 different dialytic power ; for all the tissues of plants, and animals, from those 

 of the seed or germ, upwards, are colloidal in their nature. 



[This interchange will ordinarily continue until the specific gravity 

 of the two fluids is equal. 



In the instance given in the text, the more rapid current will be 

 from the rarer to the denser medium, and the less rapid from the 

 denser to the rarer ; and although this is the law in the majority of 

 instances, there are many exceptions. Thus, if alcohol, or ether, be 

 placed in the osmometer, and water in the exterior vessel, the latter 

 will pass through the membrane with greater rapidity to the former. 

 Again, a saturated solution of oxalic acid, sp. gr. 1.045, at 77 F., 

 placed on one side of an animal membrane with water on the other, 

 will pass to the latter, augmenting its bulk, and diminishing its own 

 specific gravity. Solutions of citric and tartaric acid at a sp. gr. 

 below 1.05 pass to the water, while at a sp. gr. above 1.05 the water 

 passes to the acid solution. Whence, it is plain that density, per se, 

 is not the cause of increase of bulk in osmotic movement. 



This interchange of fluids, separated by an animal membrane, will 

 be found to take place in all instances where the two fluids are not of 

 equal specific gravity, and not chemically incompatible or non-miscible ; 

 though in such varying degree, that while on the one hand an instan- 

 taneous response may be noted in the case of many fluids, as solutions 

 of common salt, and the potash salts, others, as oil and albumen, can 

 scarcely be said to be capable of osmosis in their natural state and 

 under ordinary circumstances. Many of these substances, however, 

 which do not osmose under ordinary circumstances, may be caused to 

 do so by pressure, which is one of the potent modifiers of the pro- 

 cess. Though the effect of equal degrees of pressure varies with the 

 thickness of the membrane, it has been found by Liebig, that to force 

 the fluids below named through ox-bladder T Lth of a line in thick- 

 ness, 



Water requires a pressure of but 12 inches of mercury, 



Saturated solution of salt, " 18 " 



While marrow oil requires, " 34 " " 



When the membrane used was the peritoneum of a calf ^th of a 

 line in thickness, 



Water required a pressure of 8.10 inches of mercury, 

 Solution of salt, " "12.16 " " 



Marrow oil, " " 22.24 " 



Alcohol, " 36.40 " 



According to which the "amount of pressure required is inversely 



39 



