394 



ENDOSMOSIS AND FILTRATION. 



grains, water passed into the endosmometer. The following numbers give the 

 endosmotic equivalent of 



Acid Potassium Sulphate, . = 2*3 



Common Salt, . . . = 4'3 



Sugar, . . . . . = 7-1 



Sodium Sulphate, . .. . =11 '6 



Magnesium Sulphate, . . = 117 



Potassium ,, . . = 12*0 



Sulphuric Acid, . . . = 0*39 



Potassium Hydrate, . . = 215'0 



The amount of the substance which passes through the membrane into the water 

 of the cylinder is proportional to the concentration of the solution (Vierordt). If 

 the water in the cylinder, therefore, be repeatedly renewed, the endosmosis takes 

 place more rapidly, and the process of equilibration is accelerated. The larger the 

 pores of the membrane, and the smaller the molecules of the substance in solution, 

 the more rapid is the endosmosis. Hence, the rapidity of endosmosis of different 

 substances varies thus, the rapidity of sugar, sodium sulphate, common salt, and 

 urea is in the ratio of 1: 1*1: 5: 9*5 (Eckhard, Hoffmann). 



The endosmotic equivalent is not constant for each substance. It is influenced 

 by (1) The temperature, which as it increases, generally increases the endosmotic 

 equivalent. (2) It also varies with the degree of concentration of the osmotic 

 solutions, being greater for dilute solutions of the substances (C. Ludwig and 

 Cloetta). 



If a substance other than water be placed in the cylinder, an endosmotic current 

 occurs on both sides until complete equality is obtained. In this case, the currents 

 in opposite directions disturb each other. If two substances be dissolved in the 

 water in the flask at the same time, they diffuse into water without affecting each 

 other. (3) It also varies with membranes of varying porosity. Common salt, 

 which gives an endosmotic equivalent with a pig's bladder =4'3, gives 6 '4 when 

 an ox bladder is used; 2'9 with a swimming bladder; and 20*2 with a collodion 

 membrane (Harzer). 



Colloids. There is a number of fluid substances which, on account of the great 

 size of their molecules, do not pass, or pass only with difficulty, through the pores 

 of a membrane impregnated with gelatinous bodies, which diffuse slowly. These 

 substances are not actually in a true state of solution, but exist in a very dilute 

 condition of imbibition. Such substances are the fluid proteids, starches, dextrin, 

 gum, and gelatin. These diffuse when no septum is present, but diffuse with 

 difficulty or not at all through a porous septum. Graham called these substances 

 Colloids, because when concentrated, they present a glue-like or gelatinous appear- 

 ance; farther, they do not crystallise, while those substances which diffuse readily 

 are crystalline, and are called Crystalloids. Crystallisable substances may be 

 separated from non-crystallisable by this process, which Graham called Dialysis. 

 Mineral salts favour the passage of colloids through membranes (Baranetzky). 



That Endosmosis takes place in the intestinal canal tract, through the 

 mucous membrane and the delicate membranes of the blood- and 

 lymph-capillaries, cannot be denied. On the one side of the membrane, 

 within the intestine, are the highly diffusible peptones, sugar, and soaps, 

 and within the blood-vessels are the colloids which are scarcely diffusible, 

 e.g. 9 the proteids of blood and lymph. 



II. Filtration is the passage of fluids through the coarse intermolecular pores of 

 a membrane owing to pressure. The greater the pressure, and the larger and more 

 numerous the pores, the more rapidly does the fluid pass through the membrane ; 

 increase of temperature also accelerates it. Those substances which are imbibed 

 by the membrane filter most rapidly, so that the same substance filters through 



