COLLOIDAL SUBSTANCES 283 



We can form, on the basis of what has been said, a more 

 definite picture of the difference between gel formation and pre- 

 cipitation. When gelatin is dissolved in pure water or in much 

 water with little alcohol, probably only one of the groups of the 

 gelatin molecule has a greater affinity for other like groups 

 than for the molecules of water, while all the other groups of the 

 gelatin molecules have much stronger affinities for water than 

 for each other. Hence in this state the gelatin molecules can 

 form networks by their "oily" group (i.e., the groups with little 

 affinity for water), the "oily" group of one molecule adhering to 

 an "oily" group in the neighboring molecule. The rest of the 

 groups of these molecules must, however, remain separated, 

 since their "watery" groups (i.e., the groups with strong affinity 

 for water) cannot adhere to each other. The result is a network 

 in which the distribution of the molecules in the water is not 

 disturbed, since the forces of attraction between the "watery" 

 groups of the protein molecule and the molecules of water will 

 prevent the molecules of gelatin from attracting each other 

 except at the one "oily" group. Since the "watery" groups 

 prevail in bulk over the oily group of the gelatin molecule or 

 ion, a solid network or a gel formation results instead of pre- 

 cipitation. Under these conditions we observe the gel form 

 of micellae. In a solution with much alcohol and little water 

 (i.e., on the alcohol side of the critical point) the situation becomes 

 reversed. The forces of attraction between the "watery" 

 groups and the solvent which is now mainly alcohol are weak, 

 and since they form the bulk of the gelatin molecules the latter 

 will attract each other in many points, thus causing a close con- 

 tact over a wide area and this gives rise to the precipitation form 

 of the micellae. On the other hand, the few "oily" groups of the 

 gelatin molecule may now be attracted by the alcohol and this 

 may aid in stabilizing the solution; but at the best these forces 

 must be weak. 



In the case of crystalline egg albumin the forces of attraction 

 between the watery group and the molecules of water are very 

 strong and the mutual attraction of the oily groups for each 

 other must be very feeble, since no gel formation occurs at ordin- 

 ary temperature, low concentration, or a pH above 1.2. When, 

 however, the temperature or the hydrogen ion concentration is 



