82 



BOTANICAL GAZETTE 



[august 



arable soils, and also for determining the temperature^ and the 

 soluble salt content^ of soils. Oker-Blom'^ has determined the 

 conductivity of blood, serum, and defibrinated blood for cattle 

 and hogs, but rather from the standpoint of the physical chemist 

 than from that of the physiologist. Recently the rate of flow of 

 underground water^ has been determined by conductivity experi- 

 ments. 



APPARATUS 



Since the apparatus used is probably not familiar to the 

 majority of botanists it will be described more in detail than 



Fig. I. — For explanation see text 



would Otherwise be justifiable. The general plan of the appa- 



/. 



It consists essentially of a Wheat- 



stone bridge in which the galvanometer is replaced by a telephone 



{T) 



The resistance is measured along the wire adb^ which is 



one meter long, and stretched along a board provided with a 

 millimeter scale. The resistance-box R contains resistances of 

 I, 10, 100, 1000, etc., ohms, and is used for putting into the cir- 

 cuit a resistance nearly equal to that of the electrolytic cell s. 

 In order to secure a uniform temperature for all measurements, 

 the electrolytic cell s is placed in a thermostat. The windmill 

 thermostat, such as is used in the laboratory of physical chem- 

 istry at Leipzig, is the most practical. According to Kohlrausch 



' Bulletin no. 7. 

 3 Bulletin no. 8. 



* Pfliiger's Archives 79: III-145 

 s Science 14 : 972. 1901, 



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