245 



demand time as an element), may the chronic nature and periodicity 

 of vital phenomena be ultimately referred. 



For the separation of unequally diffusive crystalloids from each 

 other, jar-diffusion was had recourse to. The mixed solution was 

 conveyed by means of a pipette to the bottom of a column of water 

 contained in a cylindrical glass jar. A kind of cohobation takes 

 place, a portion of the most diffusive substance rising and separating 

 from the less diffusive substances, more and more completely, as it 

 ascends. 



The separation of a crystalloid from a colloid is more properly 

 effected by a combination of diffusion with the action of a septum 

 composed of an insoluble colloidal material. Animal membrane will 

 serve for the latter purpose, or a film of gelatinous starch, hydrated 

 gelatin itself, albumen or animal mucus. But by much the most 

 effective septum used was paper, as it is metamorphosed by sul- 

 phuric acid (Gaine). It is now supplied by Messrs. De la Rue, 

 and has become familiar under the name of " vegetable parchment " 

 or "parchment-paper." From sheet gutta percha a flat hoop is 

 formed, eight or ten inches in diameter by three inches in depth, and 

 one side is covered by a disc of parchment-paper, so as to form a 

 vessel like a sieve. A mixed solution, which may be supposed to 

 contain sugar and gum, is placed upon the septum to a depth of half 

 an inch, and the instrument then floated upon a considerable volume 

 of water contained in a basin. Three-fourths of the sugar diffuses 

 out in twenty-four hours, and so free from gum as to be scarcely 

 affected by subacetate of lead, and to crystallize on the evaporation 

 of the external water by the heat of a water-bath. 



The unequal action of the septum, which causes the separation 

 described, appears to depend upon this : The crystalloid sugar is 

 capable of taking water from the hydrated colloidal septum, and 

 thus obtains a medium for diffusion ; but the colloid gum has little 

 or no power to separate the combined water of the same septum, 

 and does not therefore open the door for its escape by diffusion, as 

 the sugar does. This separating action of the colloidal septum is 

 spoken of as dialysis. 



Dialysis was applied to the preparation of various colloids. The 

 mixed solution obtained by pouring silicate of soda into water acidu- 

 lated with hydrochloric acid, was placed upon a parchment-paper 



