108 COLLOIDS IN BIOLOGY AND MEDICINE 



P. RONA determined that the entire sugar in the blood serum is in 

 free solution. In analogous ways these authors investigated the 

 conditions of union between calcium and the casein of milk. 



Surface Tension. 



Because of its sensitiveness, the measurement of surface tension 

 is of the greatest significance for colloid investigation. As far as I 

 know at present, there is no case in which the measurement of these 

 factors has led to the solution of any problem. This is due to 

 the fact that even traces of other substances, especially colloidal sub- 

 stances, markedly influence the surface tension because they are forced 

 to the interfaces. According to J. TRAUBE, HgCl 2 in a dilution of 

 one in three million may be detected in dye solutions. For this reason, 

 measurements of surface tension are excessively sensitive and are 

 subject to certain errors. 



There are two essentially different groups of methods: (a) static, 

 (6) dynamic. 1 



(a) Static Methods (the rise of a fluid in a capillary; the def- 

 ormation of an air bubble in a fluid) show the condition of the de- 

 veloped surface. (&) Dynamic methods (the weight or number of 

 falling drops; the pressure necessary to force air through a capillary 

 dipped in a fluid) show the condition of a nascent surface. 



These two methods, especially with colloids, give fundamentally 

 different values because the ulterior of a fluid has a very different 

 composition from the surface, and a considerable time always elapses 

 before the surface has assumed its normal properties. 



As yet, because they are the simplest to perform, only the capillary 

 ascent and the falling drop methods have been used for biological 

 studies. 



The measurement of the height ascended in filter paper which has 

 been used especially by GOPPELSRODER in his numerous investigations 

 on alkaloids, dyestuffs and other organic substances 2 may be counted 

 a dynamic, rather than a static method, since in this porous material 

 with the ascent and evaporation, new surfaces are continually formed. 

 Filter paper offers a very useful method for demonstration purposes. 

 Thus, it shows why alcoholic solutions and soap tinctures rather than 

 aqueous solutions are adapted to disinfection of the skin (according 

 to BECHHOLD) (see p. 404). 



1 Detailed descriptions of the methods are to be found in Ostwald-Luther's 

 Physico-Chemischen Messungen (Leipzig, 1910), and in G. Quincke, Poggen- 

 dorff s Annalen d. Physik, 139, 1-89 (1870). 



2 Kolloid Zeit., Vol. 4, pp. 41, 94, 191. 



