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PRINCIPLES OF GENERAL PHYSIOLOGY 



p. 396). The various ways in which this manifests itself are only to be explained 

 by the assumption that we have to deal with a diphasic system of two liquid 

 phases (see Hatschek, 1913, p. 43). In contact with a solid surface, drops of 

 the more tenacious phase adhere, and, as the fluid is forced past, these drops are 

 deformed and torn apart. Another interesting fact, which proves the origin of 

 the high viscosity in a two-phase nature of the system, is that mechanical deforma- 

 tion produces double refraction (Kundt, 1881, p. 110). In homogeneous viscous 

 liquids, such as glycerol, strong sugar solutions, etc., this is not to be detected, whereas 

 in gelatine, even of O'Ol per cent., double refraction can be produced by mechanical 

 stress, which places the dispersed phase in a state of asymmetrical tension. 



The viscosity of emulsoids has a high temperature coefficient. 



There are two conditions very frequently met with in emulsoids, the phenomena 

 of gelatinisation and those due to imbibition of water or other solvent. The 

 following two sections deal briefly with these. 



GELS 



When a gelatine solution, which is a freely-flowing liquid at temperatures 

 above 20-25, is cooled, it " sets " to a substance having the property of preserving 

 the shape into which it is trimmed. It has, also, elasticity of form, so that, 

 within limits, it returns to its original form after distortion. What has taken 

 place ? In speaking of the action of fixing solutions on protoplasm, the experiments 

 of Hardy (1900, 2) were referred to. This investigator showed that gelatine, when 

 simply cooled and unacted on by reagents, required an enormous pressure to 

 squeeze out any of the water which it contained. This fact means that the water 

 no longer forms a continuous phase, but must be enclosed in vesicles composed 

 of the more solid phase, so that, to escape, the water must pass through gelatine. 

 Fig. 15, B (p. 14), represents diagrammatically the state of affairs, if we regard 

 the black as gelatine (containing water), and the white the liquid phase, that is, 

 dilute solution of gelatine. From what we have learnt above as to the nature 

 of emulsoids, it is clear that the word " solid " phase, used in describing the 

 phenomena, must be understood as " relatively more solid " phase. 



From Hardy's work (1900, 2) it appears that the first sign of commencing 

 gelation is a change of the system from a micro-heterogeneous one to a more 

 coarsely heterogeneous one, so that drops of the dispersed phase separate. It is 

 interesting to note the proof afforded by this fact of the liquid nature of the 

 internal phase of an emulsoid, since only a liquid could form drops. The further 

 fate of the drops depends on the concentration of the solution. In very dilute 

 solution, the droplets remain sufficiently small to become a permanent dispersed 

 phase, freely movable and, in fact, showing Brownian movement. When the 

 solution is more concentrated, the droplets join together to form a network or 

 similar kind of structure, but the watery phase is still continuous. When still 

 more concentrated, the droplets which separate can be seen by their refraction to 

 consist of the watery phase, so that the more solid phase has now become the con- 

 tinuous or external one, while the more liquid one is the internal or dispersed phase. 

 The change described above is a reversible one, and is important as illustrating 

 the kind of phenomena which may occur isothermally in a complex system of 

 emulsoids, such as living protoplasm. 



The composition of the two phases of a colloidal system, as not being qualitatively, but 

 merely quantitatively, different, is well seen in the following figures (Hardy, 1900, 2, p. 257) 

 from a case of a ternary mixture of gelatine, alcohol, and water. The numbers represent grams 

 of gelatine per 100 c.c. of the gelatine solution at 15. 



