40 GENERAL PROPERTIES OF LIVING TISSUES 



against breakage. Either tube may at once be re- 

 moved from its holder. The platinum wires extend 

 to the binding post, and are not simply short pieces 

 soldered to copper wire. The wire to the capillary 

 tube extends to the bottom of the tube, thus main- 

 taining the contact until all the mercury in the tube 

 is used. 



About one cubic centimetre of paraffin oil should 

 be placed above the piston. Only absolutely clean 

 double-distilled mercury should be used. 



As the mercury in the capillary is kept from 

 falling by the surface tension, it is obvious that 

 whatever increases or diminishes the surface 

 tension, for example an electric current, will 

 raise or lower in corresponding measure the 

 mercury in the capillary. The alteration in sur- 

 face tension is accompanied by the movement 

 of ions between the meniscus and the remaining 

 electrode of the electrometer (the mercury in 

 the acid reservoir). In practice it is found that 

 this movement can be neither very rapid nor 

 long continued, without injuring the sensitiveness 

 of the instrument. The potential difference 

 from even a single element (Daniell or dry cell) 

 is far too large to be used safely. It is advisable 

 to employ a potential divider, or rheochord, which 

 shall permit only a fraction of the original 

 potential (not more than 0.1 volt) to reach the 



