48 PHYSICAL CHEMISTRY IN MEDICINE. 



as electrolytes, and this in both directions. Urea inhibits 

 gelation, while dextrose favors it. 



If gelation consisted fundamentally in the formation 

 of a phase rich in gelatine, then one would expect to 

 find all precipitating salts among those substances which 

 favor gelation. This is true, however, of only a part of 

 the precipitating agents; the precipitating chlorides of 

 potassium and sodium exert even a liquefying action upon 

 gelatine to a considerable extent. 



The following fact also speaks in favor of a strict 

 separation between the two kinds of changes in the col- 

 loidal state. 



If the gelation-points are plotted as ordinates, the 

 molecular concentrations of the added crystalloids as 

 abscissas, one obtains curves which readily indicate the 

 dependence of the gelation upon the added crystalloid. 

 These curves show no irregularities throughout their 

 course, not even when the amounts added approximate 

 very closely those at which precipitation occurs. 



When several substances are allowed to act together, the 

 differences between the laws governing precipitation and 

 those governing gelation also evidence themselves. It could 

 be shown on a large series of crystalloids that when more 

 than one are allowed to act simultaneously upon a colloid 

 the effects of the separate crystalloids upon gelation add 

 themselves algebraically. This summation of the effects 

 of the individual crystalloids is not altered when electro- 

 lytes are combined with non-electrolytes, or these with 

 each other, nor by the fact that through the combination 

 of electrolytes having a common ion the degree of dis- 

 sociation is reduced, nor by the valency of the ions. 



But matters are entirely different when the gelatine 



