Latent Heat of Evaporation of Benzene. 21 



anhydride, until it showed a perfectly constant boiling- 

 point. 



16. Description of an Experiment. 



The dropper was placed in position (as described in section 9) 

 and the contact-maker was then set, by means of a magnifying- 

 glass, to the null-point corresponding to the tank temperature 



K 



When the observer (Observer II.) at the thermometer- 

 galvanometer (Gr 2 ) announced that 1 had become steady, 

 three observations of the galvanometer-swing were taken, 

 and the chronograph-key, being pressed at the second of 

 those swings, recorded the time from which t s was estimated. 

 At the same moment Observer I. recorded the reading of 

 thermometer II., which when reduced to the nitrogen scale 

 gave the value of #</, — the initial tank temperature. 



As a rule the initial swings were somewhere between and 

 + 50 (a swing of +50 would correspond to about + "006° C), 

 and, for reasons which will appear later, we preferred to 

 have this initial swing (which gave the value of d') positive. 



The Wolff's bottle (condenser A) had previously been 

 exhausted down to the pressure of the large globe (condenser 

 B) already referred to, but the tap between the two condensers 

 had been closed. A tap (immersed in the tank-water) between 

 the silver flask and condenser A was then opened, and the 

 air expanding into A caused a fall in the manometer attached 

 to it. The tap connecting condensers A and B was then 

 gradually opened, and the pressure in the flask fell until 

 some benzene was expelled from the dropper. The instant 

 this occurred the calorimeter temperature 6 X commenced to 

 fall, and Observer I. was acquainted with this fact by 

 Observer II. who, throughout the whole experiment, was 

 engaged in calling aloud the galvanometer- swings which 

 resulted from the inversion of the battery connexions in the 

 differential- thermometer circuit by means of a swinging key. 

 The storage -cell current, which had been running for some 

 time through a platinum-silver coil immersed in oil in the 

 outer tank, was then switched on to the calorimeter-coil, the 

 action recording itself on the chronograph-tape, and the 

 potential balance then adjusted by means of the apparatus 

 described in Paper J, p. 283. This balance had previously 

 been approximately obtained while the current was running 

 through the tank-coil ; thus only a small additional adjust- 



