THE PROPERTIES OF COLLOIDS 165 



percentage of dye left in solution, and the abscissae the original con- 

 centration of the solution, the curve only approaches the axis (i.e. 

 zero concentration) asymptotically. In other words, however dilute 

 the original solution may be, there will always be a certain amount 

 of the dye left unabsorbed by the paper. Similar relations are found 

 to exist between proteins and electrolytes. By continuously washing 

 a protein or gelatin with distilled water, 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, therefore, 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 recog- 

 nised 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 indeter- 

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

 being to cause a gradual conversion of an oqaque, 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 ampho- 

 teric substances, such as the amino- acids. An amino-acid, such as 

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



NH 2 NH 2 HC1 



CH/ + HC1 = CH<( 



X C0 2 H C0 2 H 



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 



