96 PRINCIPLES OF GENERAL PHYSIOLOGY 



from a given substance. The word " emulsoid " indicates the liquid nature of the 

 dispersed phase, but, since this phase may contain a greater or less percentage of 

 water, or other solvent, with the same composition of the actual solid matter 

 itself, as especially seen in proteins, it is clear that all degrees may exist between 

 solid and liquid. When the dispersed phase consists of an immiscible liquid, say 

 petroleum, the system exhibits properties approximating to those of the lyophobe 

 class, for example, comparative sensibility to electrolytes (Lewis, 1909, i. p. 493). 

 The fact that very minute drops of liquid have rigidity has already been pointed 

 out, and the further fact that they are retained by the ultra-filter shows that they 

 cannot be sufficiently distorted to be forced through apertures less than of certain 

 dimensions, large in comparison to molecular dimensions. 



Again, silicic acid is lyophile, but, after evaporation to dryness, does not again 

 go into solution on addition of water, as gum does. It is then irreversible, 

 contrary to most of the members of the class, which are reversible, in the sense 

 indicated. 



The designation, "stable," refers to the fact that the sensitiveness to 

 electrolytes is much less than that of the suspended solid particles of the lyophobe 

 class. This, again, is a matter of degree, as facts already given in the previous 

 section (page 92) are sufficient to show. One may also refer to the fact that egg- 

 white, a typical emulsoid, is precipitated by La--- ion in a concentration of about 

 0'002 molar, whereas arsenious sulphide, as we have seen, reacts to the same ion in 

 0-00005 molar. Remembering the ratio of activity of ions of different valency, it 

 is not surprising that univalent ions are practically inactive on emulsoid colloids, 

 that is, so far as their effect as charged ions is concerned. 



Mines (1912, p. 211) has found a useful test for the emulsoid state. This 

 consists in the reaction to complex trivalent ions as compared with that to simple 

 trivalent ions. Cobalt, and some other metals, form complex salts with ammonia 

 and an acid ; these are electrolytically dissociated with the formation of a large 

 trivalent cation, such as Co(NH 3 ) g ---(luteo-cobalt) ion ; emulsoids, such as egg- 

 white, are not precipitated, even by comparatively high concentrations of this ion, 

 up to 0'02 molar, whereas suspensoids are nearly as sensitive to it as to the simple, 

 La-", ion. 



Egg-white, coagulated by boiling, behaved in this respect as a suspensoid. As Mines 

 points out, tea infusion contains suspensoid colloids, whereas cream is an emulsoid , so that 

 opportunity for testing the different behaviour is ready to hand. Silicic acid seems to 

 be an exception ; although showing most of the characters of emulsoids, it is as sensitive to 

 the complex trivalent ion as to the simple one. A fine emulsion of olive oil behaves as an 

 emulsoid to trivalent ions. 



The facts of the preceding paragraph show that valency is not the only factor 

 concerned in the action of electrolytes, even in that aspect of their action 

 connected with the electrical charge. There are two ways in which the complex 

 ion differs from the simple one, viz., its slow rate of movement, and the less density 

 of the charge on its greater surface. Mines (1912, p. 235) calculates the relative 

 density on the lanthanum and luteo-cobalt ions as being in the ratio of 1'37 to 

 0-26, and suggests that the power of adhesion to the particle to be discharged is 

 in relation to this fact. 



The two phases of which hydrophile colloids consist differ only in the relative 

 amount of water and solid in each. It will readily be seen, therefore, how the 

 properties can be altered by agencies capable of changing this distribution of water. 

 This point has been especially insisted on by Hatschek (1913, p. 46). If the water 

 content of the internal phase is diminished far enough, this phase will become solid, 

 and the system will be a suspensoid one. With large water content of the internal 

 phase, its properties will approach to those of a liquid, and the system will be an 

 emulsoid one. The "salting out" of proteins, etc., by high concentiations of 

 electrolytes is due to removal of water from the internal phase, and consequent 

 precipitation of this latter. The way in which water is present in emulsoids is 

 regarded by Hatschek as similar to imbibition, which will be discussed later, although 

 the possibility must not be lost sight of that more strictly chemical affinities may 

 play a part. 



