74 



SCIENCE PROGRESS 



For the examination of liquids, the form of cell shown in 

 fig. 6 is useful. On the slide a a circular plate of glass b is 

 placed. This is surrounded by a ring of glass c, of such a 

 thickness that when a coverslip d is placed on it, a layer of 

 liquid o*i mm. in thickness is enclosed between b and d. The 

 pieces of glass are ground very carefully. 



Fig. 6. 



Explanation of the Properties of Colloidal Liquids. — This 

 granular structure of colloidal liquids allows us to explain very 

 readily their well-known properties, which may be briefly 

 summarised : 



(i) They diffuse very slowly— much more slowly than 

 electrolytes in solution. 



(2) They are unable to pass through certain membranes, 



such as parchment (or only pass through very slowly), 

 which are permeable to electrolytes (crystalloids). 



(3) The osmotic pressure is very low, and consequently the 



changes produced in the boiling-point and freezing- 

 point of the solvent are small. 



(4) Such solutions do not give the ordinary chemical reactions 



of the metals, etc., which they contain. 



(5) Colloidal liquids frequently act as catalytic agents, 



greatly affecting the velocity of a chemical reaction. 

 The diffusion is connected with the movements of the 

 particles, and when these are so large, we should expect them 



