no PHYSICAL CHEMISTRY IN MEDICINE. 



particles the conditions for the formation of large aggre- 

 gates are especially favorable. 



We have yet to speak of a few typical observations on 

 the precipitation of colloids that have become of great 

 importance for certain questions in biology. Such, for 

 example, is the of ten- observed variability in colloids. This 

 variability often does not attain a stabile end state until 

 after a very long time. It has been found that this origi- 

 nal instability is dependent chiefly upon the presence of 

 impurities introduced during the preparation of the 

 colloid, which because of their slight amount do not 

 make themselves felt until a long time has passed. As 

 soon as this slow process of neutralization has come to 

 an end, the colloid is in a stabile condition. 



A further very important observation is the great 

 influence that time has upon the formation of a precipitate. 

 We usually require very different amounts of a precipi- 

 tating salt or a colloid, depending upon whether the 

 precipitate is to be brought down at once or more slowly. 

 Not rarely the amount of precipitating substance used 

 in the second case is larger than in the former. This 

 is dependent upon the following fact: If the precipitating 

 colloid A is at once added to the colloid B, the particles 

 are present everywhere in the mixture in the size and 

 with the electrical charge which they possess in the 

 unmixed individual colloids A and B. Things are 

 different, however, when small portions of B are added 

 one after the other to A. Under these circumstances, if 

 the reaction does not take place too rapidly, new aggre- 

 gates of B and A are formed upon the addition of the 

 first amount of colloid, which are not entirely neutral 

 and which differ in size and charge from the original 



