396 THE COLLOIDAL STATE 



feathers is caused by the same phenomenon. In common 

 with suspensoids, the emulsoids also exhibit the Tyndall 

 phenomenon. 



Examined under the ultramicroscope they also show 

 Brownian movement, but this is not so well defined as in 

 the case of suspensoids ; this is probably due to the fact 

 that there is not the same difference in refractive index 

 between the disperse and continuous phases in the case of the 

 emulsoids since, as will be seen below, the disperse phase 

 itself contains a considerable proportion of the dispersing 

 medium. 



According to Bayliss * the pseudopodia of Amoeba when 

 examined with intense dark ground illumination show numer- 

 ous minute particles in Brownian movement, which may be 

 taken as affording evidence of the colloidal nature of the 

 protoplasm. 



(b) Electrical Properties. — Compared with suspensoids, the 

 emulsoids are relatively stable towards electrolytes ; the former 

 are liable to be precipitated from their solutions by the merest 

 traces of electrolytes, and hence a number of precautions have 

 to be adopted in preparing them to exclude contamination 

 with such bodies. The emulsoids, on the other hand, are 

 frequently contaminated by considerable quantities of electro- 

 lytes without detriment to their solubility. 



The reason for their comparative indifference to electrolytes 

 is to be found in the absence of well-defined electrical charac- 

 teristics. Typical emulsoids, in fact, when pure have no 

 electric sign, and only acquire one on the addition of either 

 acid or alkali to their solutions. Thus, it has been shown by 

 Hardy that whereas native albumen, when free from electro- 

 lytes, 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 f this accounts for the fact that posi- 

 tively charged metallic hydroxides are unable to precipitate 

 electrically neutral albumen, but precipitate albumen which 



* Bayliss : " Proc. Roy. Soc," 1920. B, 91, 196. 

 t Pauli : " Beitr. chem. Phys. Path.," 1906, 7, 531. 



