Latent Heat of Evaporation of Benzene. 3 



" The calorimeter (which was filled to the roof with the oil, 

 and the equality of temperature maintained by rapid stirring) 

 was suspended by glass tubes within a steel 'chamber, whose 

 walls were maintained at a constant temperature. So long, 

 therefore, as the calorimeter and the surrounding walls were 

 at equal temperatures, there was no loss or gain by radiation, 

 &c. If during an experiment the temperature of the sur- 

 rounding walls changed, the method of experiment involved 

 a corresponding change in the temperature of the calorimeter, 

 and, therefore, some loss or gain of heat would be experienced. 

 The apparatus was so designed that any such change in tem- 

 perature was extremely small (in no case amounting to iqq°), 

 yet, in order to estimate the loss or gain, it was necessary to 

 know approximately the capacity for heat of the calorimeter 

 and contents. 



" Small differences between the temperature of the calori- 

 meter and the surrounding walls would, during an experi- 

 ment, be of no consequence provided that the oscillations 

 were of such a nature that the mean temperature of the 

 calorimeter was that of the surrounding space, and it will be 

 found that this condition was fulfilled. 



" In addition to the heat supplied by the electric current, 

 there is also a supply due to the work done by the stirrer, 

 and it was in the estimation of this ' stirring supply ' that 

 the greatest difficulties were encountered. Fortunately the 

 heat thus generated was only about -j-Jq of the heat supplied 

 by the current, and thus any small error in that portion of 

 the work becomes of little account. 



" Of the accuracy with which the electrical supply could 

 be measured there is no question ; and even if the value of 

 the E.M.F. of the Clark cells, or the absolute resistance of 

 the box-coils given by the standardizations performed during 

 the determinations of J, is in any way inaccurate, such 

 errors would now eliminate, since the value of J was deter- 

 mined by means of the same standards as those by which the 

 quantity of heat developed in these experiments was deter- 

 mined. Hence, by assuming the value of J obtained by the 

 use of these standards, we get the comparison in terms of a 

 thermal unit at 15° C, independently of the numerical value 

 assumed in the reductions. 



" One further correction remains to be noticed. The tem- 

 perature of the calorimeter has been referred to as oscillating 

 about the exterior temperature, and it might happen that at 

 the close of an experiment this difference was not the same as 

 that at the commencement — if any such difference existed. 

 The magnitude of this correction depended, of course, on the 



B 2 



