Latent Heat of Evaporation of Benzene. 23 



swing might be announced as + 8 or 4 10, and before the 

 next announcement could be made it would be found that it 

 had shot up to 300 or 400. After the preliminary experiment, 

 it was of course possible to roughly estimate (knowing the 

 weight of the dropper when filled) the probable duration of 

 an experiment and, by adding it to the observed time of 

 establishing the current, to predict approximately the time 

 when evaporation would cease. Owing, however, to differences 

 in the size of the air-bubbles necessarily left in the droppers, 

 to the different rates of stirring, &c, and to errors in calcu- 

 lations made while all the attention of the observers was 

 needed elsewhere, the current was very often switched off too 

 late, thus increasing the value of the correction Hg un- 

 necessarily. If any mistake was made, the final value of Q x 

 was in consequence higher than the initial one, for in those 

 cases where the current was switched off too soon, it was 

 always possible to bring l up to its initial value by re-estab- 

 lishing the current for a second or two : if, however, 6 X was 

 too high there was no means of diminishing it, all the benzene 

 having been exhausted. Observer I. endeavoured, if possible, 

 to arrange so that the swing at the close of an experiment 

 should be about — 150, for the following reason. Evaporation 

 having ceased and the current being switched off, the tap 

 between the evaporating-fiask and condenser A was closed, 

 and a tap (also immersed in the tank-water) was slowly 

 opened so that air (dried by having passed through sulphuric 

 acid and phosphoric anhydride) was gradually admitted into 

 the evaporating-flask through a 30 ft. copper coil immersed 

 in the tank. Thus the air was at the temperature of the 

 calorimeter. The heat liberated by its compression caused, 

 however, a rise in # x equivalent to a swing of about 120 ; thus 

 no heat was, on the whole, gained by the calorimeter, for a 

 corresponding loss had been experienced during the exhaustion. 

 On this account we preferred to commence with a small 

 positive swing, as it was not then necessary to reduce 0j_ so 

 far at the close of an experiment to allow for this final 

 increase. When Observer II. found that 1 had again 

 become steady three final swings were taken, the chronograph- 

 key being pressed at the middle one, thus giving the ter- 

 mination of the time t s during which the stirring supply had 

 to be estimated. Thermometer II. was read at the same time, 

 and gave 6'^, the final tank temperature. Throughout the 

 experiment, every thousand revolutions of the stirrer had 

 been automatically recorded on the chronograph-tape. 



