SECTION III 



THE PROPERTIES OF COLLOIDS 



ALTHOUGH the chemical changes involved in the various vital phenomena 

 occur between substances in watery solution, the solution in every cas; is 

 bound up within the meshes or adsorbed by the surfaces of a heterogeneous 

 mass of colloids. The complex chemical molecules which make up 

 protoplasm itself are all colloidal in character. The participation of 

 colloids in chemical reactions introduces conditions and modes of reaction 

 differing widely from those which have been studied in watery solutions. 

 Our knowledge of these conditions is still very imperfect, but the important 

 part played by colloids in the processes of life renders it necessary to discuss 

 in some detail their properties and modes of interaction. 



The term colloid, from *oXX?7, -glue, was first introduced by Thomas 

 Graham, Professor of Chemistry at University College from 1836 to 1855. 

 Graham divided all substances into two classes, viz. crystalloids, including 

 such substances as salt, sugar, urea, which could be crystallised with ease, 

 diffused rapidly through water, and were capable of diffusing through animal 

 membranes ; and colloids, which included substances such as gelatin or 

 glue, gum, egg-albumin, starch and dextrin, were non-crystallisable, formed 

 gummy masses when their solutions were evaporated to dryness, diffused 

 with extreme slowness through water, and would not pass through animal 

 membranes. The process of dialysis was therefore introduced by Graham 

 for the separation of crystalloids from colloids. Although the broad dis- 

 tinction drawn by Graham between colloids and crystalloids still holds good, 

 some of the criteria by which he distinguished the two classes are no longer 

 strictly applicable. For instance, it has been shown that many typical 

 colloidal substances, such as haemoglobin, can be obtained in a crystalline 

 form. On the other hand, all gradations exist between substances, such as 

 egg-albumin, which are practically indiffusible, and those, such as common 



It, which are very diffusible. Graham pointed out that colloids exist under 



TO conditions : 



(1) In a state of solution or pseudo-solution, in which they form sols, and 

 ire distinguished as hydrosols, when the solvent is water ; and 



(2) In a solid state, in which a relatively small amount of the colloid 

 its with a large amount of a fluid, such as water, to form a jelly. This 



solid form is known as a gel. The most familiar instance is the jelly which 



t ; s obtained on dissolving a little gelatin in hot water and allowing the mixture 

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