THE PROPERTIES OF COLLOIDS , ,,, 



can be recognised in both the solid condition and in solution. v i, ,,, ((lmlm 

 itself, compounds wih acid or with alkali, and compounds ,,thn, 

 salt. The amount of acid and alkali combining with" the ,lo,ul, 

 determinate, the effect of adding either acid or alkali to the neutral riobufc 

 being to cause a gradual conversion of an oqaque, milky suspension m t,> 

 hmpid transparent solution. On drying HC1 globulin, the dried solid is 

 found to contain all the chlorine used to dissolve it. The acid may the. . 

 be regarded as being in true combination. Both acid and alkali globulins 

 act as electrolytes, the globulin being electrically charged and taking part 

 in the transport of electricity. In order to produce the same extent of 

 solution, the concentration of the alkali added must be double that of the 

 acid. The relation of globulin to acids and alkalies is similar to that of the 

 so-called amphoteric substances, such as the amino-acids. An ammo-acid, 

 such as glycine, can react as a basic anhydride with other acids, thus : 



X C0 2 H 



HC1 = CH 

 X 



NH..HC1 



or as an acid anhydride with bases : 



CH 2 .NH 2 CH 2 .NH 2 



+ NaHO= | +H 2 



COOH COONa 



Like these too, globulin forms soluble compounds with neutral salts. An 

 amphoteric electrolyte thus acts as a base in the presence of a strong acid, 

 and as an acid in the presence of a strong base. 



From true electrolytes, colloidal solutions differ in the fact that their 

 particles are of varying size according to the conditions in which they exist 

 and carry varying charges of electricity, whereas an ion such as Na or Cl 

 has a mass which is constant for the ion in question, and always cai 

 the same electric charge. The charged particles of an acid- or alkali-globulin 

 may be distinguished therefore as pseudo-ions, 



In these adsorption combinations, although the chemical nature of the 

 colloidal molecules is concerned, there is an absence of definite equilibrium 

 points, such as we are accustomed to in most chemical reactions. The inertia 

 of the system and the large size of the molecules determine the occurrence 

 of false equilibria and of delayed reaction, so that the condition and behaviour 

 of a colloidal system at any moment are determined, not entirely by the 

 quantitative relations of its components, but also by the past history of tin- 

 system. 



COMBINATIONS BETWEEN COLLOIDS' 



Besides the compounds between colloids and electrolytes, comlMii'ition. 

 or at least interaction, takes place between different colloids. Ma n v << tioidfl 

 are precipitated by other colloidal solutions. This effect is si 1 ways found 

 occur when the colloidal solutions carry different charges. Thus ferric 

 hydrate in colloidal solution is precipitated by colloidal silica or colloidal 



