178 SCIENCE PROGRESS. 



Moreover, since, according to experiment, osmotic 

 pressure is equal to the corresponding gaseous pressure, it 

 is again a plausible assumption that Avogadro's hypothesis 

 may be literally extended to dilute solutions, and, therefore 

 that the molecular condition of gaseous and dissolved sub- 

 stances is the same. This is in reality the fundamental 

 hypothesis of the new theory : in all dilute solutions to which 

 the gas-equation applies, the dissolved substance is assumed 

 to be present in the condition of simple molecules, and the 

 behaviour of solutions to which the equation is inapplicable 

 finds an explanation in the supposition that in them the 

 molecular state of the dissolved material is exceptional. 

 We are thus led to the conclusion that when the molecules 

 of a substance are widely separated from one another, as in 

 the case of a gas or a dilute solution, the pressure which 

 promotes their diffusion depends solely upon their number 

 and not upon their nature ; it is, further, independent 

 not only of the nature, but even of the presence of the 

 solvent. 



Osmotic pressure and diffusion. — Although this is so, 

 the actual process of diffusion is very different in the case 

 of a gas and a dissolved substance, on account of the 

 enormous frictional forces, due to the presence of the 

 solvent, which oppose the movement of dissolved molecules. 

 On the supposition that osmotic pressure is the cause of 

 diffusion (Nernst, 1888), this frictional force can be 

 calculated. The osmotic pressure-laws give the value of 

 the force promoting diffusion, actual observations give the 

 velocity of diffusion ; all the data are thus to hand for 

 evaluating the resistance opposing the movement. In this 

 way it has been found that in order to drive one gram- 

 molecule of sugar through water at 1 8 C with a velocity of 

 1 cm. per second, a force of no less than 47 x io 9 kilograms- 

 weight must be employed. That the force should be so 

 large finds an explanation in the extreme smallness of the 

 moving particles — the molecules. Hence, although gaseous 

 and osmotic pressure are equal in magnitude, the equalisa- 

 tion of the pressure of a quantity of gas is almost instan- 

 taneous, whereas the equalisation of the concentration and 



