﻿Surface- Tension and Osmotic Pressure of Solutions. 281 



the equation into a more convenient system of units, it 

 becomes T= 72*17 + 1*382 n, where T is the surface-tension 

 in dynes per centimetre, 72*17 the surface-tension of water 

 expressed in the same units, and n the number of gram- 

 molecules dissolved per litre. 



Taking n equal to 1, the equation gives for the difference 

 in surface-tension between a normal solution of sodium chlo- 

 ride and pure water, a value of 1*382 dynes per centimetre. 



It is also easy to calculate approximately the osmotic pressure 

 in a normal solution of the same salt, the amount of its dis- 

 sociation at this concentration being known from Kohlrausch's 

 conductivity determinations. Such a calculation gives for 

 the osmotic pressure in such a solution the value of 21 x 10 6 

 dynes per square centimetre. Substituting these values in 



2T 

 the equation r= -p, the value obtained for the radius of the 



capillary opening is r = 18 X 10~ 7 centimetres. 



Now the limits of the radius of capillary action have been 

 determined by various observers and by different methods, 

 and are found to be much greater than this. Thus Plateau's 

 determinations give, for /, the radius of capillary attraction, 



I > nn millim. = 6 X 10 -(} centim. ; and the mean of four 



determinations of Quincke's for different substances give 

 I > 6*1 X 10 -6 centimetres. 



These results show that the radius of the intercommuni- 

 cating capillaries necessary to give to the osmotic pressures 

 of solutions the values which they possess by means of capil- 

 lary action, lies well within the radius of capillary attraction : 

 so that the whole section of liquid in the capillary would be 

 urged by difference of surface-tension in the direction of the 

 liquid of greater surface-tension, that is from the solvent 

 towards the solution, and thus give rise to a difference of 

 pressure, in other words cause osmotic pressure. 



Consider, again, for a moment the relative values of the 

 capillary openings here calculated, and that found as a mini- 

 mum for the radius of capillary action. The former is 18 X 10~ 7 

 and the latter 6 X 10~ 6 centimetres ; and it follows that, under 

 the given conditions, the capillary opening is smaller than 

 the radius of capillary action, and hence only a portion of the 

 surface-tension will come into action, viz. that acting up to 

 the radius of the capillary instead of to the radius of capillary 



2T 

 action. So that the value of T in the equation r— -p will be 



diminished, and r will have a still smaller value than that 

 Phil. Mag. S. 5. Vol. 38. No. 232. Sept. 1894. U 



