118 



COLLOIDS IN BIOLOGY AND MEDICINE 



It is necessary to test especially, whether true flocculation has occurred, i.e., 

 whether the particles have really gathered in flocks or whether they have only 

 sunk to the bottom under the influence of gravity. 



In the case of fine measurements, Jena glass that has been steamed is used, 

 because the leaching out of alkali from the glass may give rise to errors. 



This method gives information not only concerning flocculability, 

 but is at the same time quantitative; it informs us concerning "thres- 

 hold values" and, under some circumstances, concerning the rate of 

 flocculation. 



In order to work with very small quantities, test tubes are used 

 which are narrowed at their lower ends. If there is insufficient fluid 

 even for this determination we may employ a "hanging drop," in 

 other words, drops are mixed on a cover glass, and this is placed up- 

 side down on a slide with a depression ground into it, so that the 

 drop will hang. The cover glass is ringed with vaseline to hold it 

 to the slide. This method is more qualitative and is especially 

 adapted for the agglutination of bacteria. 



It is desirable in all cases to prepare series of experiments. It 

 may occur as the result of "irregular series" that high and low con- 

 centrations result in flocculation, whereas flocculation may not occur 

 in medium concentration. 



Electric Migration. 



Electric migration reveals the nature of the charge of a colloid. 



The most primitive arrangement for migration experiments is a 



beaker in which are suspended 

 two platinum electrodes that 

 are part of the circuit of a 

 direct 

 volts. 



current of at least 60 

 This is to be recom- 

 mended only for simple demon- 

 strations in the lecture room, 

 where migration must be exhib- 

 ited quickly. Because of the 

 changes in reaction due to elec- 

 trolysis, the results are very in- 

 exact. It is preferable to use a 

 U-shaped tube or an arrange- 

 ment such as is shown in Fig. 26a. The middle glass jar contains 

 the colloid to be tested, and is united by the U-shaped return bends 

 filled with water to the two outer beakers which also contain water 

 and into which dip the electrodes EE. For research work I employ 



Simple apparatus for elec- 

 tric migration. 



