RICHARDS AND MATHEWS. — HEAT OF EVAPORATION. 537 



In all the subsequent work the first vaporizer was used, even al- 

 though its vacuum had been destroyed ; and it continued to give ex- 

 cellent results with other substances. The communication of these 

 results must be left for a future publication, partly because the neces- 

 sary specific heats of the liquids are not yet well enough known. One 

 point, however, in connection with the results may advantageously be 

 mentioned here, because it strengthens highly one's faith in the method 

 of extrapolation employed to eliminate the effect of premature conden- 

 sation. The loss of heat per gram was 7 calories per minute with 

 steam in the best apparatus (Vaporizer I). "With liquids of lower boiling 

 point the loss should be less ; with liquids of higher boiling point the 

 loss should be more, if the inference is really justified. As a matter of 

 fact precisely this phenomenon was noticed with different liquids. For 

 example in the case of benzene, where the difference between the tem- 

 perature of the vaporizer and that of the calorimeter amounted to 60° 

 instead of to about 80°, the loss per minute amounted to about 5.2 

 calories per minute,^^ a figure strictly proportional to the number 7.0 

 found in the case of water. Again, with ethyl butyrate, boiling at 

 122°, the difference between the temperature of the vaporizer and that 

 of the calorimeter was 102° instead of 80°, a fall of temperature which 

 should correspond to a loss of heat of about 9 calories per minute, and 

 the actual loss was found to be about 9.2 calories per minute, an 

 amount as close as could be expected to the computed result. More- 

 over, in the case of methyl formate, boiling at 32°, the time of the ex- 

 periment made practically no difference at all in the observed value 

 for the heat of vaporization. Thus it seems perfectly clear that the 

 march in the results is really due to an illicit loss of heat, and that 

 the method employed for correcting the results is the best that can be 

 devised. In conclusion the remark may be made that unfortunate as 

 this inevitable difficulty w^ith the method is, it is no more unfortunate 

 than similar difficulties which come into any other method for deter- 

 mining the latent heat of vaporization. As has been more than once 

 pointed out, the very nature of the problem renders impossible a 

 method wholly free from some sort of correction. Even Henning's far 

 more complicated method had its own difficulties of a somewhat sim- 

 ilar kind, as a perusal of his paper will show. 



The comparison of our value for the heat of vaporization of a gram 

 of water weighed in vacuum, 538.9 cal.2i° (or 538.1 cal.150), with the 



2^ This figure applies only to Vaporizer I, not to the prehminarj^ form used 

 in the benzene series given on p. 13. In the early form the loss per minute 

 was over 9 calories with benzene. 



