THE COLLOIDAL STATE 75 



of the beam is polarised, a fact which proves that the particles are of the same 

 order of dimensions as the mean wave length of the light used. 



A further proof that we have to do with suspended particles is given by Friedenthal (1913). 

 By powerful centrifugal force, he has separated several colloids from solution, caseinogen from 

 milk, for example. Iodised starch, mixed with non-iodised, could be separated from the 

 latter, owing to its greater weight. 



The colloidal state, then, is of the nature of a heterogeneous system, or a 

 system of more than one separate phase. The point of importance to be 

 remembered is that the phases of which the system consists are separated from 

 one another by surfaces, interfaces, of contact. The colloidal state differs from a 

 coarsely heterogeneous system, such as a mass of gold immersed in water, in that 

 it is, to ordinary observation, homogeneous, and only shows its micro-heterogeneous 

 character by special methods of investigation. On the other hand, it is dis- 

 tinguished from true solutions of small molecules or ions by the fact of the 

 possession of surfaces of contact, with all the phenomena implied by this. These 

 properties will naturally be especially marked on account of the great surface area 

 due to the minute state of subdivision. 



It is convenient to have names for the two phases of which a colloidal system 

 usually consists. If we refer back to Fig. 15 (page 14), we see the appropriateness 

 of Hardy's names (1900, 2, p. 256), of "external" and " internal " 'phases. Other 

 workers call Hardy's internal phase the " dispersed phase," and the external phase 

 the "continuous" one (Wo. Ostwald, 1907, p. 256). The names will be used here 

 indifferently. 



One essential condition for the production of a colloidal solution of a substance 

 is that it should be practically insoluble in the external phase, or " dispersing 

 medium," to use another expression of frequent usage. This statement, however, 

 needs some qualification, as we shall see later. It is especially insisted on by 

 von Weimarn (1911, p. 6) that, given appropriate conditions, all substances can 

 be brought into the colloidal state. 



It may be mentioned, as an illustration, that resinous substances like gamboge or mastic 

 form true solutions in alcohol, but when such solutions are poured into water, a colloidal 

 solution is produced. The same investigator gives strong evidence to show that, conversely, 

 all substances can, by appropriate manipulation, especially very slow deposition, be obtained 

 in the crystalline form (1912) ; although the crystals of such liquid or semi-liquid substances 

 as proteins are apt to be very minute and distorted in shape, rounded at the edges, by the 

 action of surface tension. 



Most of our knowledge of the fundamental properties of the colloidal state is 

 due to Thomas Graham, whose portrait will be seen in Fig. 35. 



Graham started from a different point of view from that of Faraday. He 

 noticed that certain substances are extremely slow to diffuse, and devoid of the 

 power to crystallise (1861, p. 183). They are also unable to pass through a 

 membrane of similar nature to themselves, such as sized paper or parchment 

 paper (unsized paper treated with sulphuric acid). Amongst these substances 

 are hydrated silicic acid, starch, albumin, gelatine, etc. He says (1861, p. 183): 

 "As gelatine (KoAX?/ = glue) appears to be its type, it is proposed to designate 

 substances of the class as colloids, and to speak of their peculiar form of aggrega- 

 tion as the colloidal condition of matter. Opposed to the colloidal is the crystalline 

 condition. Substances affecting the latter form will be classed as crystalloids 

 The distinction is no doubt one of intimate molecular constitution." It will be 

 noted that, although Graham speaks here of the " colloidal condition " of matter, 

 he appears to regard the class of colloids as quite distinct from that of crystalloids. 

 "They appear like different worlds of matter" (1861, p. 220). At the same time 

 he is aware that the same substance, silica for example, may be obtained in either 

 state, while on the page following that on which the above statement is found, he 

 suggests that the colloid molecule may be "constituted by the grouping together 

 of a number of smaller crystalloid molecules." Perhaps stress is intended to be 

 laid rather on the word "appear." In any case, it is better to speak of the 

 " colloidal state " and not of " colloids " as a class. 



One important characteristic of this state, that of instability, was clearly 

 recognised by Graham. After referring to the fact that colloidal solutions of 



