THE PROPERTIES OF COLLOIDS 147 



the removal of electrolytes becomes slower and slower, but it is practically 

 impossible within finite time to get rid in this way of the last traces of ash. 

 Although the chemical behaviour of colloids is largely determined by 

 surface phenomena, it presents at the same time analogies with more strictly 

 chemical reactions, since it is conditioned by the chemical structure of the 

 colloid molecule as well as by the charge carried by the latter. A good 

 example of these adsorption combinations is presented by globulin, the 

 behaviour of which has been studied by Hardy. This may be obtained 

 from diluted blood-serum by precipitation with acetic acid. Four states 

 can be recognised in both the solid condition and in solution, viz. globulin 

 itself, compounds with acid or with alkali, and compounds with neutral 

 salt. The amount of acid and alkali combining with the globulin is in- 

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

 being to cause a gradual conversion of an opaque, milky suspension into a 

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

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

 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 : 





NH 2 NH 2 HC1 



CH 2 / + HC1 - CH 



N C0 2 H 



or as an acid anhydride with bases : 



CH 2 .NH 2 CH 2 .NH 2 



| + NaHO = | +H 2 O 



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 carries 

 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 





