Lecture II. 



wards in all directions until it takes on a globular form and be- 

 comes tightly stretched. 



Another simple experiment shows that the pressures generated 

 in this way are very considerable : Here is a thistle funnel ; across 

 its opening I have fixed a piece of parchment To prevent 

 leakage the edge of the funnel was smeared with glue and while 

 the glue was soft the parchment was bound tightly on with string. 

 When the glue was set the funnel was in- _ 



verted and filled with a sugar solution and 

 immersed in water, while the tube of the 

 funnel was supported in a vertical position 

 projecting above the water. After a short 

 time it could be noticed that the liquid in 

 the funnel tube was rising and it soon 

 mounted above the level of the surround- 

 ing water. This rise may be ascribed to 

 the pressure of the diffusing sugar particles 

 expanding the confines of the sugar solu- 

 tion, and the process may be described in 

 terms of the hypothesis which attributes 

 the pressure to the bombardment of the 

 dissolved particles. The parchment and 

 glass of the funnel being fixed, resist this 

 pressure, but the particles hurling them- 

 selves from below against the surface of 

 the solution in the tube and being unable 

 to escape from it, press it upwards and so 

 make room for more water to enter through 

 the parchment into the tube. Thus we 

 may regard the weight of the column of 

 liquid as being raised above the surround- 

 ing water by the bombardment of the dis- 

 solved sugar particles. After a time, the 

 solution will have risen the full length of 

 the funnel tube, and, if we wish to continue the observations, we 

 must add to its length. The height of the column of liquid thus 

 raised may be very great. Its rise will only cease when the 

 diffusion-pressure of the solution or osmotic pressure as it is 

 usually called is balanced by this hydrostatic head. 



By careful measurements it has been found that the height to 

 which the osmotic pressure can raise a column of solution depends 

 upon the concentration of the solution : in other words, the os- 

 motic pressure of a solution is proportional to the number of 



funnel (/) 

 closed below with a 

 semi-permeable mem- 

 brane (m). In / is a 

 solution, the upper sur- 

 face of which is seen 

 at a. The beaker con- 

 tains water. 



