CLARK. — SURFACE TENSION BETWEEN LIQUIDS AND VAPORS. 369 



y 1 ^ C. at a temperature as high as 200°. By cutting off only a small 

 portion of the current the regulation to within y^° is very simple. 



The arrangement of the circuit is shown in Figure 4. A A are the 

 mains, R a large resistance adjustable through a wide range, H is the 

 heater, D the ammeter. The gap BB is bridged over either by the relay 

 S or the key L. When the relay is operated by the thermostat, the 

 circuit is broken and the current sent through the variable resistance 

 provided the key K is closed, otherwise the circuit remains open. 



Figure 5. 



Experimental Tubes. 



Considerable difficulty was experienced in making a style of tube 

 which would do the work required of it, be easy to insert in the appara- 

 tus and easy to fill. After many months of trial and failure and almost 

 innumerable accidents, the form shown in Figure 1 was adopted. Since 

 the pressures are considerable, the glass tubing must be selected with 

 great care. The tubing finally adopted was obtained from the Bausch & 

 Lomb Optical Company, and with its purchase ended a great many 

 difficulties. 



The tubing was carefully washed with aqua regia and caustic potash 

 and finally with distilled water, and dried with a current of air. 



VOL. XI.T. — 24 



