86 EEPOET — 1891. 



essential to a description of the process are also equally- 

 applicable to both — namely, concentration, volume, and 

 pressure ; but in the last case it is desirable to avoid any 

 possible confusion between the pressure which characterizes 

 the dissolved salt and the totally different vapour-pressure of 

 the solution, and so the term osmotic ijressure is employed. 

 Every dissolved substance exercises, then, an osmotic pres- 

 sure, the magnitude of which depends in the first instance 

 on its concentration. 



Pfeffer's experiment makes this fact plain. A vessel of 

 porous material, such as terra cotta, can be so doctored by the 

 deposition of a colloid matter within its j)ores as to render it 

 impermeable to dissolved substances while it remains perme- 

 able to water. Such a "semi-permeable cell" maybe filled 

 with a solution of (say) sugar, closed, and immersed in pure 

 water ; and, if the interior of the cell be properly connected with 

 a manometer, this will indicate excess of pressure inside as 

 compared with outside — an excess which increases to a maxi- 

 mum and then remains steady. The manometer then indicates 

 the osmotic pressure of the sugar at the particular concentra- 

 tion which characterizes that solution. The theory of the 

 experiment is plain. As water is free to pass through the walls 

 of the cell either way, the pressure inside and out will become 

 equal so far as the water causes it, but inside the cell there is 

 also the pressure of the sugar, which is prevented from diffusing 

 outwards. A proper understanding of this experiment removes 

 any doubts one may start with as to the osmotic pressure 

 being really a characteristic of the dissolved substance and 

 independent of the solvent. 



But the experiment has done much more. It has given 

 a method of quantitatively studying the osmotic pressure 

 and its variation (1) with change of concentration, for the 

 same substance and temperature ; (2) with change of tem- 

 perature, for the same substance and concentration; (3) with 

 change of substance, for the same concentration and tempera- 

 ture. 



The general outcome of such experiments may be briefly 

 stated, and it will be seen to give a striking justification of the 

 title " the gaseous theory of solution." (1.) Boyle's law holds- 

 good for matter in solution. (2.) Charles's law holds good for 

 matter in solution. (3.) Avogadro's law holds good for matter 

 in solution. These are the three gaseous laws which, as you 

 know, may be compressed into the single formula ^j . v . M = E . T, 

 where j^ is pressure, v is the volume of unit weight of the gas, 

 M is its molecular weight, T is its absolute temperature, andR 

 is a constant— the same for all gases under all conditions. We 

 may now safely apply the same formula to the state of solu- 

 tion, using p for osmotic pressure, v for the volume of that 



