ELECTROKINETICS 385 



concentrations stabilize them. This may be due to the fact 

 that excess salt or acid confers a high potential of opposite sign 

 to that in low concentration, or the excess nutritive salt may 

 depress the cohesive force between the colliding bacteria. The 

 effectiveness of an electrolyte in reversing the charge of bacteria 

 depends on the nature of the suspension. Thus, sodium chloride 

 and sodium sulphate reverse the charge on the bacillus of rabbit 

 septicemia but will only reduce the charge on Bacterium typhosum. 



While the work of Joffe and Mudd minimizes the role of poten- 

 tial in stabilizing certain bacteria in general, it is yet true that 

 most bacteria agglutinate when the potential falls to less than 

 15 mv. Northrop and Freund find that sheep-blood cells are 

 not agglutinated by electrolytes until the potential is depressed 

 to 6 mv. (except in the case of magnesium chloride and calcium 

 chloride). 



Other examples of agglutination — the reverse of stability — 

 some of which rest upon electric forces, are considered under 

 coagulation (page 479). 



Kinship. — The discovery of the physiologist Landsteiner that 

 there are four distinct types of blood in man and that these types 

 can be determined by coagulation tests (page 503) led to the 

 thought that there might be differences in rate of cataphoretic 

 migration of the red cells. This did not prove to be true. 



Work that had shown a relationship to exist between the blood 

 of man and that of lower animals (apes and chimpanzees), as 

 determined by coagulation, led Mez to ascertain if there was a 

 protein relationship between plants. He found that there was 

 (page 504). These findings led L. Moyer to seek another prop- 

 erty which might indicate species relationship, viz., the isoelectric 

 points of particles suspended in the latex of rubber-bearing 

 plants. About the same time Svedberg discovered a similar 

 basis of kinship (page 505). 



Moyer studied the electric mobilities of the latex particles of 

 Euphorbias. Hevea braziliensis, which is the commercial source 

 of rubber, is not available in the greenhouses of temperate regions, 

 but an excellent substitute is to be found among those plants 

 known as the spurges which are species of Euphorbia. When 

 a stem of Euphorbia is cut, it exudes latex. Latex is a fine 

 suspension of hydrocarbon (rubber) in an aqueous medium of 

 salts, carbohydrates, and proteins (pages 135, 385). It can be 



