296 



FORCES CONCERNED IN ABSORPTION. 



ents in the fluids occurs until both fluids have the same composition. This exchange of fluids 

 is termed endosmosis or diosmosis. 



Diffusion. If the two mixible fluids are placed in a vessel, the one fluid over the other, but 

 without being separated by a porous septum, an exchange of the particles of the fluids also 

 occurs, until the whole mixture is of uniform composition. This process is called diffusion. 



Conditions Influencing Diffusion. Graham's investigations showed that the rapidity of 

 diffusion is influenced by (1) The nature of the fluids themselves ; acids diffuse most rapidly; 

 the alkaline salts more slowly ; and most slowly, fluid albumin, gelatin, gum, dextrin. These 

 R last do not crystallise, and perhaps do not form true solutions. (2) The 



more concentrated the solutions, the greater the diffusion. (3) Heat accel- 

 erates, while cold retards, the process. (4) If a solution of a body which 

 diffuses with difficulty be mixed with an easily diffusible one, the former 

 diffuses with still greater difficulty. (5) Dilute solutions of several substances 

 diffuse into each other without any difficulty, but if concentrated solutions 

 are employed, the process is retarded. (6) Double salts, one constituent of 

 which diffuses more readily than the other, may be chemically separated by 

 . diffusion. 



Endo8mometer. The exchange of the fluid-particles takes place inde- 

 pendently of the hydrostatic pressure. An endosmometer (fig. 215) consists of 

 a glass cylinder filled with distilled water, and into this is placed a flask, J, 

 without a bottom, instead of which a membrane, m, is tied on. A glass tube, 

 R, is fixed firmly by means of a cork into the neck of the flask. The flask 

 is filled up to the lower end of the tube with a concentrated salt solution, 

 and is then placed in the cylindrical vessel until both fluids are on the same 

 level, x. The fluid in the tube, R, soon begins to rise, because water passes 

 through the membrane into the concentrated solution in the flask, and this 

 p independently of the hydrostatic pressure. Particles of the concentrated 

 salt solution pass into the cylinder and mix with the water, F. These out- 

 going and ingoing currents continue until the fluids without and within J 

 are of uniform composition, whereby the fluid in R always stands higher (e.g., 

 at y), while it is lowered in the cylinder. The circumstance of the level of 

 the fluid within the tube being so high, and remaining so, is due to the fact 

 that the pores in the membrane are too fine to allow the hydrostatic pressure 

 to act through them. 



Endosmotic Equivalent. Experiment has shown that equal weights of 



different soluble substances attract different amounts of distilled water 



^through the membrane, i.e., a known weight of a soluble substance (in the 



Fig. 215. flask) can be exchanged by endosmosis for a definite weight of water. The 



Endosmometer. term "endosmotic equivalent" indicates the weight of distilled water that 



passes into the flask of the endosmometer, in exchange for a known weight 



of the soluble substance (Jolly). For 1 grm. alcohol 4 '2 grms. water were exchanged; while 



for 1 grm. NaCl, 4*3 grms. water passed into the endosmometer. The following numbers give 



X- 



the endosmotic equivalent of 



Acid potassium sulphate, . =2*3 



Common salt, . . . =4*3 



Sugar, = 7'1 



Sodium sulphate, . . . =11*6 



Magnesium sulphate,. . = 11*7 



Potassium sulphate, . . =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 cylin- 

 der is proportional to the concentration of the solution. If the water in the cylinder, there- 

 fore, be repeatedly renewed, the endosmosis takes place more rapidly and the process of equili- 

 bration 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, e.g., the rapidity of sugar, sodium sulphate, common salt, and 

 urea is in the ratio of 1 : l'l : 5 : 9 '5. 



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 solu- 

 tions of the substances. 



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. 

 Colloids. There are many fluid-substances which, on account of the great size of their 



