2O Practical Plant Biology. 



taken to avoid splashing the sides of the jars with the coloured 

 mixtures. When the gelatine had set the jars were filled up 

 with water without fear of stirring up the coloured mixtures. 

 After a few days it could be seen that the blue of the copper 

 sulphate had diffused upwards into the water, its surface rising 

 above the gelatine was no longer clear-cut, but merged gradu- 

 ally into the water above. This condition of things is still 

 plainly visible. In contrast to this the colloid has not risen 

 or left the gelatine, and its surface remains as clearly defined 

 as it was at the beginning of the experiment. Colloids do 

 not diffuse in water, and hence they do not produce osmotic 

 pressure as crystalloids do. 



Another simple experiment brings out a very important differ- 

 ence which exists between substances in the crystalloidal and 

 colloidal state. Here are two rectangular glass vessels. In 

 one is a solution of common salt and into the other a mixture 

 of i per cent, gelatine in water, which will serve in this case 

 as an example of a colloid. 1 By suitable arrangements a beam 

 of light from an arc lamp is passed in succession through the 

 two vessels. You will see that while the path of the beam is 

 clearly marked out in the mixture of the colloidal gelatine, it 

 is practically invisible in the solution of the crystalloid. It is 

 in fact only visible in the latter when it is reflected from the 

 surface of minute specks and dust particles floating in the latter. 

 The distinct visibility in the colloid mixture, on the other hand, 

 is to be attributed to the reflection of light from the surface of 

 the excessively minute colloidal particles suspended in the water. 

 This shows that there is a difference in the manner in which 

 the colloid and the crystalloid are distributed. The particles 

 of the former, though still almost inconceivably small (they are 

 quite invisible to the highest powers of the microscope), are 

 sufficiently large to reflect light from their surfaces; whereas 

 in the case of the dissolved crystalloid, subdivision is so much 

 more minute in comparison with the wave length of light that 

 there are no surfaces between the dissolved substance and the 

 solvent capable of scattering light. Here then is a most im- 

 portant difference between matter in the colloidal and crystal- 

 loidal states. The colloidal particles are sufficiently large to 

 have a surface between the solvent and the colloid. In the 



1 The gelatine should first be swelled in cold water and afterwards hot 

 water added to make up the mixture. The mixture should be thoroughly 

 stirred and mixed. It will look then scarcely less clear than the solution 

 of salt. 



