Latent Heat of Evaporation of Benzene. 13 



that, for several reasons, loss of this kind could not be dis- 

 regarded : — 



(1) The vapour-pressure of benzene so greatly exceeds that 



of water at corresponding temperatures that the loss 

 by diffusion through the capillary was appreciable. 



(2) The surface-tension of benzene is so great that the liquid 



crept up the sides of the capillary to the opening, and 

 the consequent loss by evaporation was increased. 



(3) In order to supply the air necessary for starting the 



boiling when the exterior pressure was removed, a 

 capillary tube, closed at one end, had been sealed 

 within each dropper. It was found that this answered 

 very well during some preliminary trials, when the 

 dropper was placed within a glass tube connected with 

 an exhaust pump so that its manner of discharging 

 could be watched ; but on a second filling with our 

 purest sample (the first filling having thoroughly 

 cleaned the interior surface of the droppers) no action 

 took place even when the surrounding pressure was 

 reduced to a few millimetres. In order, therefore, 

 that the expulsion of the benzene from the dropper 

 should commence as soon as the external pressure 

 was reduced to the right amount, it was found 

 necessary to leave a very considerable air-bubble 

 within the tube. Precautions had to be taken to 

 prevent the expulsion of the liquid by the alternate 

 contraction and expansion of the air -bubble when the 

 temperature was changed from that of the balance- 

 case to that of the tank. 

 These difficulties were surmounted in the following manner. 

 During the process of filling, as above described, the 

 droppers stood in a vertical position, with the doubled over 

 and open end uppermost, and were never inverted after 

 their removal from the filling-tube, at a temperature of about 

 65° C, until their insertion into the calorimeter. The co- 

 efficient of expansion of benzene is very large, and, on cooling 

 to the room-temperature, the whole of the upper bend together 

 with a couple of centimetres of the neck between it and the 

 body of the dropper was found to be free from benzene. 

 The dropper was then placed in its case and left in a vertical 

 position in the balance-case for, as a rule, some hours, when 

 the small air-space in the enclosing-case no doubt became 

 saturated with the vapour and thus further evaporation ceased. 

 After being weighed, the dropper and case were lowered 

 into a large tube placed within the tank and left until they 

 acquired the tank temperature O ; they were then rapidly 



