12 Mr. E. H. Griffiths and Miss Marshall on the 



This, however, in no way affects the values of L in Paper W, 

 nor, except in the case of two experiments (viz.. Preliminary 

 and XIX. a., infra), would it, if left uncorrected, have affected 

 the values of L as given in this communication. 



We believe that the values of %q are correct to better than 

 1 in 100 ; and Table I. (p. 5) shows that an accuracy of 1 in 

 10 would have been sufficient. 



9, Measurement of Mass. 



The actual measurement of the mass of benzene evaporated 

 presented several difficulties which were not encountered when 

 working with water. A tube resembling a weight-thermo- 

 meter was filled with benzene, placed in an air-tight case just 

 large enough to contain it, and then weighed against a pre- 

 cisely similar case used as a tare. The weight-thermometer 

 (termed by us a a dropper ") narrowed at its open extremity 

 to a capillary tube, which was doubled back on itself for rather 

 .over 1 cm., and again bent near the open end, so that the last 

 1 or 2 mm. were horizontal. These droppers varied in capacity 

 from about 4 to 6% c.c, and were filled in the following 

 manner : — The dropper (point uppermost) was lowered by a 

 fine wire to the bottom of a tube about 4J ft. long, of which 

 the lower 10 inches or so were filled with benzene, while the 

 upper 3 ft. were surrounded by a " condenser-tube " through 

 which tap-water was continually passing. The lower end, 

 containing the benzene, was transferred at regular intervals 

 from a vessel of water at about 86° C. to a vessel of cold 

 water ; thus the benzene was alternately boiled and cooled 

 without any escape of vapour into the room. Five or six such 

 transferences were generally required to completely fill the 

 dropper. The containing tube was then placed in a bath at 

 about 65° C. until the temperature of the benzene was steady. 

 The dropper was now removed and allowed to stand in the 

 open air for some time, in order to get rid of any benzene 

 adhering to its surface. Although simple and effective, these 

 operations occupied a considerable time, and, as a rule, the 

 whole of the morning had to be devoted to the filling of the 

 droppers required for the experiments, which were usually 

 performed at night. 



Before an experiment the dropper was lowered, by means of 

 a thread passed through a platinum wire sealed into the closed 

 end, into the calorimeter, where it stood in a vertical position. 

 In Paper W (p. 307) it was shown that, when filled with water, 

 the evaporation through the capillary opening between the time 

 of weighing and the commencement of an experiment might 

 be neglected. In the case of benzene, however, it was found 



