1 8 THE MECHANISM OE LIFE 



At the same temperature, but under another pressure 

 P', the gas will have a different volume V. Since, accord- 

 ing to Boyle's law, PV is constant (P'V' = P V ), it will 



P V T P V 



still equal -2 — 2—. Therefore — ?— 2. is also constant. This 

 4 273 273 



quantity is called " the gaseous constant," and if we 



represent it by the symbol R, we obtain the general formula 



PV = RT for all gases, or ~ = R. 



Suppose, for instance, we have a gramme-molecule of a 

 gas at 0° C. in a space of 1 litre. It has a pressure of 

 22'35 atmospheres at 0°C, or 273° absolute temperature. 



Since PV = RT, R = £l= X ~*L =-0819. This number 

 T 273 



'0819 is the numerical value of the constant R for all gases, 



volume being measured in litres and pressure in atmospheres. 



Substances in solution behave exactly like gases, they 

 follow the same laws and have the same constants. All 

 the conceptions which have been acquired by the study of 

 gases are applicable to solutions, and therefore to the 

 phenomena of life. The osmotic pressure of a solution is 

 the force with which the molecules of the solute, like gaseous 

 molecules, strive to diffuse into space, and press on the limits 

 which confine them, the containing vessel being represented 

 by the surfaces of the solution. Osmotic pressure is measured 

 in exactly the same way as gaseous pressure. To measure 

 steam pressure we insert a manometer in the walls of the 

 boiler. In the same way we may use a manometer to measure 

 osmotic pressure. We attach the tube to the walls of the 

 porous vessel, allow the solvent to increase in volume under 

 the pressure of the solute, and measure the rise of the liquid 

 in the manometer tube. 



Pfeffer's Apparatus. — Pfeffer lias designed an apparatus 

 for the measurement of osmotic pressure. It consists of a 

 vessel of porous porcelain, the pores of which are filled with 

 a colloidal solution of ferrocyanide of copper. This forms a 

 semi-permeable membrane which permits the passage of water 

 into the vessel, but prevents the passage of sugar or of any 



