284 COLLOIDS 



viscosity is well known from such a familiar example as cod- 

 liver oil emulsion. 



Furthermore, such a system of liquid particles with no 

 sharply defined surface of separation from the surrounding 

 medium would not be expected to exhibit any marked electri- 

 cal properties due to contact electrification, such as occur in 

 the case of solid particles. This accounts for their want of 

 response to an electric current, and their relative indifference, 

 as compared with suspensoids, to electrolytes when present in 

 small quantities. 



Whereas traces of acids or alkalis will at once precipitate 

 suspensoids, similar quantities added to emulsoids will only 

 have the effect of imparting to them slight electric properties 

 in which they were previously deficient. 



Thus, it has been shown by Hardy that whereas native 

 albumen, when free from electrolytes, is electrically neutral, 

 it acquires a negative charge on the addition of a little alkali, 

 and a positive charge on the addition of acid. 



According to Pauli,* this accounts for the fact that posi- 

 tively charged metallic hydroxides are unable to precipitate 

 electrically neutral albumen, but precipitate albumen which 

 has become negatively charged by the addition of a little 

 alkali ; and similarly negatively charged colloids, such as phos- 

 phomolybdic or phosphotungstic acid or certain negative dyes, 

 are only able to precipitate albumen after it has acquired a 

 positive charge by the addition of acid. 



The behaviour of emulsoids towards stronger solutions 

 of electrolytes is dealt with under the precipitation by elec- 

 trolytes. 



THE NATURE OF GELS. 



As already pointed out above, emulsoids are regarded as 

 two-phase systems in which the disperse phase is a more con- 

 centrated solution, and the continuous phase a relatively dilute 

 one. When such a solution gives a gel, the roles of the two 

 phases are assumed to be changed, and one then has a sort of 

 net- or sponge-like structure of concentrated solution repre- 

 senting the continuous phase, whereas the disperse phase is 

 represented by a dilute solution filling up the interstices. 



* PauIi : " Beitr, chem. Phys, u. Path.," 1906, 7, 531, 



