A. B. Plowman— Electrotropism of Boots. 229 



other negatively galvano tropic. It should be stated, however, 

 that in the earlier part of his paper Elf ving observes that gal- 

 vanotropism is not to be considered a biological phenomenon 

 of the same order as heliotropism, geotropism, and most other 

 common paratonic responses. In accounting for the invariably 

 fatal effects of a long-continued current, Elfving offers the sug- 

 gestion that the electrolyte is probably rendered toxic by the 

 formation of poisonous compounds at the surface of the 

 electrode. 



While Elfving's study of this subject was quite complete and 

 his methods were in general above question, it has seemed de- 

 sirable not only to repeat his work, but to extend it in a number 

 of lines, in order, if possible, to arrive at some more satisfactory 

 explanation of the results obtained. 



The electrical equipment used for these and related studies 

 consists of a battery of " Excelo " cells, a small direct-current 

 dynamo, the 500-volt city power current, and various resist- 

 ances, rheostats, and measuring instruments. The " Excelo " 

 cell is a combination of the principles of the Daniell cell and 

 the "gravity" cell. This cell is very satisfactory for closed- 

 circuit work, since it maintains a practically constant potential 

 for days or even weeks, especially when only a fraction of the 

 available amperage is used. The dynamo is of the General 

 Electric type, driven by a General Electric 500-volt direct- 

 current motor. The dynamo is rated at 2*5 kw. capacity, at a 

 voltage ranging from 75 to 125 at normal speed, controlled by 

 a field rheostat. A speed-rheostat on the motor permits a con- 

 siderably wider range of voltage in the dynamo. The 500-volt 

 power-current itself is available for experimental work, through 

 a series of resistances and rheostats. Among the measuring 

 instruments are included a watt-meter, volt-meters, ammeters 

 and galvanometers, one of which is sensitive to 10" 6 volt. It 

 is possible to obtain from this equipment a fairly complete 

 gradation of measured initial potentials from 500 volts down 

 to 10- 6 volt. 



However, it is not always sufficiently explicit to state merely 

 the difference in potential between the terminal electrodes, 

 since the amount of the current, which is the essential thing, 

 depends quite as much upon the form and conductivity of the 

 electrolyte as upon the initial potential-difference between the 

 electrodes. In other words, it is the density of the current 

 which is of importance in our present investigation. Current 

 density has been variously defined by different authorities,* 

 but it is sufficiently definite for our purpose to consider current 



*A. Gray, " Absolute Measurements in Electricity and Magnetism," p. 7, 

 1888. Oliver J. Lodge, " Modern Views of Electricity," p. 69. 1889. W. E. 

 Ayrton, "Practical Electricity," p. 117, 1887. Clerk Maxwell, "Electricity 

 and Magnetism," §64, 1881. 



Am. Jour. Sci.— Fourth Series, Vol. XVIII, No. 105.— September, 1904. 

 16 



