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 case 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 active 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 con- 

 ditions is still very imperfect, but the important part played by col- 

 loids in the processes of life renders it necessary to discuss in some 

 detail their properties and modes of interaction. 



The term colloid, from KoXXri, 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 crystal- 

 lised 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 sepa- 

 ration of crystalloids from colloids. Although the broad distinction 

 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 salt, which are very diffusible. Graham 

 pointed out that colloids exist under two conditions : 



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

 sols, and are distinguished as hydrosols, when the solvent is water ; 

 and 



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



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