PHYSICO-CHEMICAL INTRODUCTION 23 



stance. In regard to temperature, it appears to be true, allowing for 

 experimental difficulties, that the osmotic pressure of d solution is pro- 

 portional to the absolute temperature (e.g., Gay-Lussac's law for gases). 

 Also it is true for dilute solutions (e.g., of su^ar) that the osmotic pres- 

 sure is equal to the pressure ivhich the molecular concentration of the 

 substance would exert if it were in the gaseous state at the same tempera- 

 ture, and occupied the same volume as the solution. 



If two solutions of different osmotic pressure be separated by a 

 semi -permeable membrane, osmotic exchange of water will take place 

 until the pressures are equal on the two sides of the membrane, the 

 water passing from the solution with the smaller osmotic pressure 

 to that with the greater. This can be well shown by the following 

 pretty experiment, the success of which depends upon choosing the 

 right strengths of solution: A little potassium ferrocyanide (nearly 

 saturated) is slowly run from a narrow glass tube the end of which dips 

 below a solution of copper sulphate (a gramme-molecular solution)* 

 contained in a tali glass jar. As the ferrocyanide runs out, a filmy bag 

 of copper ferrocyanide is formed at the end of the tube. When the 

 bag is about 1 to 2 cm. in diameter, a slight jerk will disengage it, 

 and it will sink slowly to the bottom of the vessel. Its content 

 having a greater osmotic pressure, water will enter the bag and 

 gradually distend it. The density of the bag is thus gradually diminished, 

 and eventually becomes less than that of the surrounding copper 

 sulphate solution, when the bag rises spontaneously to the top of the 

 jar. The experiment may be varied by fitting the top of the narrow 

 glass tube, containing the ferrocyanide solution, with a piece of rubber 

 tubing, and pushing a drop of ferrocyanide out by closing this tubing 

 with a clip. When the glass tube is now lowered into the copper 

 sulphate, a hanging membrane is formed at its bottom. Water passes 

 into the ferrocyanide, and the copper sulphate, concentrating in the 

 immediate neighbourhood of the membrane, becomes denser than 

 the rest of the solution and sinks. This can be easily seen by the 

 naked eye, owing to the difference in refractive power of the denser 

 solution. If the experiment be reversed, and dilute ferrooyanide, in a 

 tube with an upturned end, be placed in strong copper sulphate 

 solution, the copper sulphate in the neighbourhood of the membrane 

 is diluted, and a steady ascending stream of the diluted liquid can 

 be seen. 



Interesting experiments on osmosis have been done with plants. 

 For example, in the epidermis of the leaf of the plant Tradescantia 

 discolor the fluid coloured contents of the cells are normally in close 

 contact with the rigid cell wall, which behaves as a semi -permeable 

 membrane (Fig. 14, A). If it be immersed in a solution containing 

 0-22 of a gramme-molecule of cane-sugar per litre, the coloured contents 

 detach themselves from the wall at one or more places. '" Plas- 

 molysis," as it is termed, has taken place (Fig. 14, B). Owing to the 

 withdrawal of water, there has been a decrease in the bulk of the cell 



* The molecular weight in grammes dissolved in 1 litre. 



