PRECIPITATION-MEMBRANES. 225 



water passes inwards. The change of level caused is of course 

 accompanied by an immediate change in the hydrostatic pres- 

 sure, and hence water should be added to or removed from the 

 outer vessel, to balance inequalities of height as fast as thev 

 occur. 



611. The proportional amounts of the substances inter- 

 changed have been determined by various observers. Jolly, 1 

 by an ingenious modification of the osmometer, obtained the 

 following results ; the figures representing the weight of water 

 which replaced in osmosis one part by weight of the substance : 



Sodic chloride 4.3 



Sugar 7.1 



Sodic sulphate 11.6 



Magnesic sulphate 11.6 



Potassic sulphate 12. 



Potassic hydrate 215.7 



612. These figures are known as the osmotic equivalents of 

 the respective substances, but they are by no means constant ; 

 since, as Ludwig 2 has shown, they depend partly on the de- 

 gree of concentration of the solution used, the duration of the 

 experiment, and the character of the membrane. 



613. If, however, a colloidal body is placed in the reservoir, 

 very little comparatively passes outwards, and in the case of 

 some colloids nothing. " Indeed, an insoluble colloid, such as 

 gum-tragacanth, placed in powder within the osmometer, was 

 found to indicate the rapid entrance of water, to convert the 

 gum into a bulky gelatinous hydrate. Here, no outward or 

 double movement is possible." 3 This very important fact must 

 be borne in mind in the application of the phenomena of os- 

 mose to those of absorption of liquids by the colloids in active 

 vegetable cells. 



614. Precipitation-membranes. Traube 4 (in 1867) discovered 

 that when a drop of a solution of copper-sulphate is placed in 

 a solution of potassic ferroc3 - anide, there is produced over its 

 whole surface a coherent membrane (of precipitated cupric ferro- 

 cyanide), known as a " precipitation-membrane." This at once 

 begins to increase in size, but somewhat irregularly, as if breaks 

 occurred at the upper part through which a portion of the liquid 



1 Zeitschrift fur rationelle Medicin, 1849, vii. p. 83. 



2 Poggendorff : Annalen der Physik und Chemie, Ixxviii. p. 307. 



3 Graham : Journ. Chem. Soc., 1862, p. 269. 



4 Archiv fur Anat. u. Physiol. du Bois-Reyuiond u. Reichert, 1867, p. 87. 



15 



