Lecture II. 1 1 



of dissolved substances. This may be readily seen in cells where 

 a coloured substance is in solution in the vacuole. We often find 

 such coloured cells in the skin of Tradescantia ; but it is more 

 convenient to observe this effect in a tissue in which all the cells 

 are coloured, such, for example, as the fleshy part of the beetroot, 

 or the petal of a rhododendron. If a piece of one of these tissues 

 is mounted in water it may be observed that as long as the proto- 

 plasm is uninjured it confines the dissolved red pigment within 

 the vacuole and does not allow it to escape into the surrounding 

 water. 



This property of protoplasm of obstructing the passage of solutes 

 (substances in solution) is one of very great biological importance 

 and must be considered shortly in relation to the nature of diffu- 

 sion and of solution. 



When a crystal of a soluble substance, e.g. any soluble salt or 

 sugar, is placed in water it dissolves, and the dissolved substance 

 slowly spreads itself throughout the surrounding water. This 

 spreading or diffusion, as it is technically called, takes place quite 

 apart from the mingling of the solution with water due to currents, 

 and even when the crystal lies in the bottom of a vessel, diffusion 

 may be seen to overcome gravity and in time to secure the uniform 

 distribution of the solute throughout the water. 



A mental picture of diffusion may be formed by supposing that 

 in solution the bonds that bind the particles of the solute to one 

 another are broken, and they, by virtue of the energy they possess, 

 spring forth into the solvent : here they are free to move in the 

 direction of their first spring till they encounter some other particle 

 or the walls of the vessel. At each collision they rebound only 

 to be hurled back again when they strike another obstacle. In 

 the solution of the crystal we must imagine countless millions of 

 these particles launched, jostling each other, rebounding from the 

 impacts, and thus gradually, with a disorderly and turbulent 

 swarming, tending to distribute themselves uniformly at last 

 throughout the liquid. Thus regarded the particles of a dissolved 

 substance behave in a manner very similar to those of matter in 

 a gaseous state. 



The process of diffusion is seen in action in these two tall glass 

 cylinders of water into which a few crystals of potassium bichro- 

 mate and copper sulphate have been placed respectively. The 

 crystals which were put in a few days ago have almost disappeared 

 and a dense solution of each salt is formed in the bottom of each 

 jar. If you look carefully you will see that there is no definite sur- 

 face for the solutions, but a gradual transition from the densely 



