ABSORPTION. 137 



When one radical, as iron, Fe, exists in two sets of salts, the 

 positive ions of these salts may be distinguished from each other by 

 a prefix indicating electro-valency. Thus, diferrion, Fe" ; trif er- 

 rion, Fe'". 



The names of all negative radicals terminate in ate, ite, or ide. 

 Corresponding to these we have the terminations for the negative ion, 

 anion, osion, and idion, respectively. Thus we obtain : 



Sulphanion, S0 4 " Sulphosion, S0 3 " 



Sulphidion, S" Hydrosulphidion, HS' 



Carbonion, C0 3 " Hydroxion, OH' 



The function of ions, by their presence in definite proportion 

 in each tissue, is to preserve the "labile equilibrium" of the colloid 

 materials of the protoplasm on which its activities depend. 



Osmotic Pressure. 



If over a layer of distilled water we drop a layer of colored solu- 

 tion like copper sulphate, then the two solutions are sharply separated 

 from each other. But soon the line of separation between the liquids 

 vanishes; the colorless layer of water always becomes smaller arid at 

 last disappears, so that the whole mass is colored. As soon as the 

 color-solution comes into contact with the water, then the molecules 

 of the salt begin to wander in the water and color it. By a certain 

 force the molecules overcome the heavy colored particles and from 

 beneath are moved upward, and this continues until both fluids have 

 the same concentration. 



Diffusion. When two miscible crystallized solutions of different 

 concentration are placed on either side of a perfectly permeable mem- 

 brane, it will be found after some -time that both solutions have the 

 same concentration. If on one side of the membrane there was a 

 20-per-cent. solution of NaCl and on the other side a 10-per-cent. solu- 

 tion of Nad, then after a time both will be a 15-per-cent. solu- 

 tion, because of the exchange of water and salt on each side of the 

 membrane. Diffusion, or dialysis, is the passage of the molecules of 

 the substances in solution. 



Osmosis. If now we separate these two solutions, a colored be- 

 neath and the water above, by a partition which can be penetrated by 

 the water but not by the colored particles, then the partition will be 

 pressed upward. If the partition is weighted so that its pressing 

 upward movement does not take place, then the weight corresponds 

 to the pressure exerted by the particles of the colored salt. The pres- 



