52 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



two distinct phases in the solution, one solid and the other liquid, and 

 between the two, because of the great subdivision of the original par- 

 ticle, is an enormous surface of contact. The solution is heterogeneous, 

 and at the interface between the two " phases" the physical forces which 

 depend on surface e. g., surface tension (see page 65) are enormously 

 developed, and are responsible for the peculiar properties of colloidal 

 solutions as compared with those of molecular solutions, which may, 

 therefore, be styled homogeneous. The solutions of crystalline substances 

 which we have hitherto been concerned with, are homogeneous. 



Between these two groups of solutions is an intermediate one namely, 

 suspensions (as suspensions of quartz or carbon, or oil emulsions). Be- 

 sides being turbid in transmitted light, the solutions may be seen by 

 means of the ultramicroscope to contain particles. These can be sepa- 

 rated by filtration from the fluid they are suspended in, except in the 

 case of many emulsions in which the particles can squeeze their way 

 through the filter pores by changing their shape.' On standing or being 

 centrifuged suspensions may also separate into their constituents, al- 

 though this can be greatly hindered by the addition of a suspending' 

 substance such as gelatin or certain bodies having a so-called protec- 

 tive action (as peptone, proteose, etc.). 



True Colloidal Solutions 



1. The Solution Is More or Less Turbid. Frequently this can be recog- 

 nized by holding the solution in a thin-walled glass vessel against a 

 dark background, but the turbidity may be so slight that it requires 

 for its detection the use of the Tyndall phenomenon. This is familiar 

 to all in the effect of a beam of sunlight let in through a small aperture 

 into an otherwise darkened room. In the course of the beam suspended 

 dust particles, which are invisible in an equally illuminated room, be- 

 come visible, and thus render very 'distinct the pathway of the beam. 

 If a colloidal solution contained in a glass vessel, preferably with paral- 

 lel sides, is held in the course of such a beam, the Tyndall phenomenon 

 will be seen in the liquid, which is not the case with molecular solutions. 

 Focused artificial light may be employed for intensifying the effect. 

 The light that is sent out at right angles to the beam is plane-polarized, 

 which means that the particles reflecting the light must be smaller than 

 the mean wave length of the light forming the beam. It should be under- 

 stood that the individual particles themselves may not be rendered 

 visible to the naked eye by the beam, although in such cases they can 

 often be seen by using intense illumination and a dark-field (ultramicro- 

 scope) combined with suitable magnification (Fig. 12). 



2. Colloids Do Not Readily Diffuse. To demonstrate this, test tubes 



