SOLUTIONS 1 9 



colloid. The stopper which hermetically closes the vessel is 

 pierced for the reception of a mercury manometer. The 

 vessel is filled with a solution of sugar and plunged in a bath 

 of water. The volume of the solution in the interior of the 

 vessel can vary, since water passes easilv in either direction 

 through the pores of the vessel. The boundary of the solvent 

 has become extensible, and its volume can increase or diminish 

 in accordance with the osmotic pressure of the solute. Under 

 the pressure of the sugar water is sucked into the vessel like 

 air into a bellows, the solution passes into the tube of the 

 manometer, and raises the column of mercury until its pressure 

 balances the osmotic pressure of the sugar molecules. 



Osmotic Pressure follows the Laws of Gaseous Pressure. — 

 This osmotic pressure is in fact gaseous pressure, and max 

 be measured in millimetres of mercury in just tiie same way. 

 We may thus show that osmotic pressure follows the laws 

 of gaseous pressure as defined by Boyle, Dalton, and Gay- 

 Lussac. The coefficient of pressure variation for change of 

 temperature is the same for a solute as for a gas. The 

 formula PV = RT is applicable to both. The numerical value 

 of the constant R is also the same for a solute as for a gas, 

 being '0819 for one gramme-molecule of either, when the 

 volume is expressed in litres and the pressure in atmospheres. 

 The formula PV = RT shows that for a given mass, with the 

 same volume, the pressure increases in proportion to the 

 absolute temperature. 



Osmotic Pressure of Sugar. — A normal solution of sugar., 

 containing 342 grammes of sugar per litre, has a pressure of 

 22 '35 atmospheres, and it may well be asked why such an 

 enormous pressure is not more evident. The reason will be 

 found in the immense frictional resistance to diffusion. 

 Frictional resistance is proportional to the area of the surfaces 

 in contact, and this area increases rapidly with each division 

 of the substance. When a solute is resolved into its com- 

 ponent molecules, its surface is enormously increased, and 

 therefore the friction between the molecules of the solute and 

 those of the solvent. 



Isotonic Solutions. — Two solutions which have the same 



