METHODS OF ELECTRICAL STIMULATION 17 



hydrogen gas will pass through the metal palla- 

 dium when the metal is heated to 200 C. Nitro- 

 gen will not pass through. Palladium at 200 C. 

 is therefore said to be semi-permeable, i. e. it is per- 

 meable to one gas, but not to another. If a tube 

 of palladium at 200 containing nitrogen gas at 

 a pressure of one half atmosphere be placed in a 

 vessel containing hydrogen gas at a pressure of 

 one atmosphere, the hydrogen will pass through 

 the palladium into the tube until the pressure 

 of the hydrogen inside equals that outside the 

 tube, namely, one atmosphere. The pressure in 

 the tube will now be almost one and one half 

 atmospheres, which is the sum of the partial 

 pressure of the nitrogen (one half atmosphere) 

 and the partial pressure of the hydrogen (one 

 atmosphere), less an error due chiefly to the im- 

 perfect permeability of the palladium. Thus the 

 pressure on one side of the palladium will be 

 higher than that on the other side, as may be 

 shown by connecting the tube with a manometer. 



Substances in solution may exert a force like 

 the partial pressure of a gas. This force is called 

 osmotic pressure. 



Osmotic pressure is measured most readily by 

 the aid of semi-permeable membranes, first used 

 for this purpose by Pfeffer. For demonstration 

 they may be made as follows. Wash a porcelain 



