Heat Vaporization of Water. 83 



tigation, and therefore I need not dwell upon them. I would, 

 however, venture to add my expressions of astonishment to 

 those of Rowland, that so many enquirers attach so little im- 

 portance to this point : many investigators, whose methods 

 have otherwise been of a high order of accuracy, having satis- 

 fied themselves with the mercurial thermometer as a standard." 

 Rowland" rejects, as having no weight, previous determinations 

 in which the thermometer readings were not reduced to the 

 air scale. As to the difficulty of this reduction, and. to the 

 general uncertainty of the apparent readings of the ordinary 

 thermometer, a very instructive object lesson is to be found in 

 an article by Cole and Durgan,f entitled "An Example in 

 Thermometry." 



It is the record of a systematic calibration of a Gerhardt 

 thermometer, made in a concise and thorough manner. The 

 mere statement of the corrections made, the record of the 

 observations, and the results of the calculations, stated as 

 briefly as consistency with clearness would permit, occupy 

 twenty pages. In his determination of the mechanical equiva- 

 lent of heat, Rowland made the most involved and elaborate 

 corrections on his thermometer readings, and only brought his 

 results and those of Joule into agreement by making the same 

 kind of corrections for the latter's thermometers. Without 

 raising the question of the soundness of such corrections, it is 

 evident that a method for the determination of any fundamen- 

 tal heat constant independent of them is desirable if only to 

 serve as a check : for the only way to minimize their effect is 

 to extend the range of temperature, which is sure to increase 

 the errors due to radiation, conduction and the calorimeter con- 

 stant. 



The error due to the latter constant need not be large, 

 provided only that the water equivalent of the calorimeter be 

 small compared with that of the substance under observation, 

 and this can usually be accomplished without much difficulty. 



The error due to the water vapor in the steam is only 

 present in methods of steam calorimetry and is almost entirely 

 eliminated by the differential method. 



The errors due to radiation, convection and conduction are 

 more serious. Reynolds;}: remarks on Joule's determination 

 ;t that notwithstanding the greater facilities enjoyed by subse- 

 quent observers owing to the progress of physical appliances, 

 the inherent difficulties remained : the losses from conduction 

 and radiation could only be minimized by restricting the range 

 of temperature and this ensured thermometric difficulties, par- 



- Rowland, Proc. Am. Acad., vol. xv. 



f Cole and Durgan. Phys. Review, vol. iv, 1896. 



% Reynolds, Phil. Mag., 1897. 



