THE COLLOIDAL STATE 



77 



silica sooner or later become gelatinous and finally crystallise, he says (1861, 

 p. 184) : "The colloidal is, in fact, a dynamical state of matter; the crystalloidal 

 being the statical condition. The colloid possesses ENERGIA. It may be looked 

 upon as the probable primary source of the force appearing in the phenomena 

 of vitality. To the gradual manner in which colloidal changes take place (for 

 they always demand time as an element), may the characteristic protraction 

 of chemico-organic changes also be referred." This "energia" we know now as 

 "surface energy" of its various kinds. 



The two phases of which a colloidal solution consists may obviously be of 

 many various kinds. The table below will illustrate this: 



The most important systems for the physiologist are those consisting of 

 solids and liquids, Nos. 3, 4, and 6. The nature of the dispersed phase as 

 solid or liquid has been adopted as a basis of classification by Wo. Ostwald 

 (1907, p. 334). This system is in many ways a useful one, although it does 

 riot direct attention to what is perhaps the most important distinction between 

 different classes, that is, the affinity of the dispersed phase for water. When 

 the internal phase, although liquid, is in extremely minute droplets, its 

 mechanical properties closely resemble those of a solid the great pressure 

 due to the internal component of the surface tension confers rigidity on them. 

 The characteristic which carries with it most of the other differences in the 

 general behaviour of a colloidal system is the affinity of the internal phase for 

 water, or other solvent, constituting the external phase. It will be clear that 

 the more water the internal phase contains, and it may contain as much as 

 90 per cent., the less will be the difference between the properties of the two 

 components of the interface of contact between it and the external phase, and, 

 consequently, the less will be the surface energy. 



Hardy (1900, 1, p. 236) calls attention to the fact, also pointed out by Quincke (1902, 

 p. 1012), that, as a rule, the material of which the internal phase is composed is not absolutely 

 insoluble in the external phase, so that the two phases will be (1) a solid containing a certain 

 amount of the solvent, and (2) a very dilute true solution of the solid. The substance of whicli 

 the solid phase is composed will become more soluble, as a rule, as the temperature is raised. 

 This fact is sometimes of use as a means of indicating whether the external phase of a colloidal 

 solution does actually consist of a dilute true solution of the substance in suspension. The 

 most convenient way of detecting this is by measuring the electrical conductivity. How- 

 ever long a colloidal solution- has been dialysed (a means of purification to be described later, 

 and depending on the impermeability of certain membranes for colloids), it is almost 

 impossible to remove all traces of foreign electrolytes. Now, as the temperature is raised, 

 these foreign bodies will not increase in number ; since the impurity is in extremely low 

 concentration, it may be regarded as being completely dissociated electrolytically. The 

 increase in conductivity, so far as it depends on this impurity, will be due only to the 

 increased rate of movement of the ions already present, dependent on the diminished viscosity 

 of the solvent. The temperature coefficient of this is known, and lies between 2 and 2 - 4 per 

 cent, of the conductivity at 18 per degree rise of temperature. Suppose we take a solution, 

 saturated at 18, of an electrolyte, for convenience a somewhat insoluble one, such as 

 sulphanilic acid, and determine its conductivity at various temperatures, we find the tempera- 

 ture coefficient to be 2'6. But if excess of undissolved acid is present, more and more will go 

 into solution as the temperature is raised, the actual number of ions is increased, and the 

 temperature coefficient appears to be considerably higher, viz., 5 '9. Applying this fact to the 

 colloidal system, if the conductivity is due to foreign ions, the temperature coefficient will be 

 only 2 to 2'4, and if it is found experimentally to be higher than this, evidence is afforded that 

 more of the colloidal substance itself goes into true solution. The free acid of Congo-red is a 



