460 HARSHBERGER— INFLUENCE OF SEA WATER [April 22, 



silver nitrate volumetric solution.- This is the method adopted I 

 am told by the botanists at Johns Hopkins University in studying 

 the salt marsh vegetation of Maryland, the results of which investi- 

 gation have not yet been published. 



Method of the Electric Bridge. — The Bureau of Soils, United 

 States Department of Agriculture, adopted some years ago the prin- 

 ciple of the slide wire bridge to the measurement of the salt content 

 of soils.^ The earlier instruments have been described in various 

 bulletins and the results obtained with them are scattered through 

 various publications of the bureau. Since 1899, when the electric 

 bridge was put first into practical use, various improvements have 

 been made, so that the improved instrument is the result of the 

 experience gained by its use in the actual field study of soils. The 

 use of the electric methods for determining the soluble content of 

 a soil depends on the fact that the electric current is conducted by 

 the salt in solution and that the conduction of the solution, or con- 

 versely, its resistance to the passage of the current, is largely deter- 

 mined by its concentration. The magnitude of current that will pass 

 is increased by an increase of salt in solution ; or the resistance to 

 the passage of the current decreases with the increase of salt. The 

 experience gained by the use of the modified instrument is embodied 

 in the recent bulletin of the Bureau of Soils noted above and its 

 general utility in the study of alkali soils, the salt content of irriga- 

 tion and seepage waters is given. 



Method of Plasnwlysis. — It is a well-known physiologic fact that 

 dilute solutions of potassium nitrate, sodium chloride and cane sugar 

 cause a removal of water from living plant cells, so that the proto- 

 plasm contracts away from the inside of the cell wall. The per- 



" Consult Hare, Hohart A., Caspari, Charles, Rusby, H. H., " The 

 National Standard Dispensatory," 1905, 1684; Fresenius, C. Remigius, "A 

 System of Instruction in Quantitative Chemical Analysis," 1894, 430; Sutton, 

 Francis, "A Systemic Handbook of Volumetric Analysis,"' 1890, 124; Fraps, 

 G. S., see bibliography. 



^ Davis, R. O. E., and Bryan, H., "The Electrical Bridge for the De- 

 termination of Soluble Salts in Soils," Bull. 61, Bureau of Soils, 1910, where 

 reference is made to previous bulletins; Cannon, W. A., "On the Electrical 

 Resistance of Salt Plants and Solutions of Alkali Soils," The Plant World, 

 II, ia-14. 



