Heat Ya/porization of Water. 85 



limited in number by expediency. It is apparent that methods 

 necessitating observations of the first class are, other things 

 being equal, inferior to those involving readings of the second 

 class only. The determinations of Regnault, Jonle, Rowland, 

 Moorby, Griffith and Barnes all involve observations of the 

 first class. In Joly's method of steam calorimetry, however, 

 the temperature readings are made while the substance is in a 

 state of thermal equilibrium which may be maintained almost 

 indefinitely.* In this respect his method is unsurpassed. An 

 absolute determination of L substituted for the Regnault value 

 used by Joly would enhance the value of his work many fold. 

 His differential method is unquestionably the best general 

 method of calorimetry yet devised, the use of an uncertain 

 constant being, as Joly himself pointed out, its w r eakest point. 



Barnes' curve for the heat capacity of water from 0° to 100° 

 will never be changed much except, perhaps, by shifting the 

 origin along the axis of specific heats. Rowland determined 

 only a small portion of this curve, which from 10° to 20° is 

 practically parallel to Barnes' but lower in value. Regnault 

 determined the portion of the curve between 40° and 100°. 

 It is also practically parallel to Barnes' but much higher. 

 This indicates the presence of constant errors — but where ? 

 In this particular work the men are to be given almost equal 

 weight. A constant error in Rowland's work, whose results 

 agree among themselves most perfectly but for which he only 

 claims an accuracy of two parts in a thousand, is hard to locate. 

 It is possibly due to the sensitiveness of his thermometers not 

 being great enough for observations on a substance in a ther- 

 modynamic condition. Regnault's constant error is likely due 

 to several causes, including radiation, while Barnes' is possibly 

 due to the position of his thermometers, as this is a source of 

 error common to all continuous methods and very hard to elim- 

 inate or to correct for. In some preliminary work on the ratio 

 of L to C, an attempt was made to develop a continuous 

 method of steam calorimetry. It was abandoned for a time at 

 least because the results, while agreeing very well among them- 

 selves, were found to be a function of the position of the ther- 

 mometers placed in the ingoing and outcoming water. It 

 would not be safe then to decide which of these curves, agree- 

 ing so well in all but their positions, is nearest to the true one. 

 The substitution of the true value of the lieat of vaporization 

 of water in Joly's determination of the mean specific heat of 

 water from 12° to 100° in terms of the calorie at 20°, would 

 give a value by which Barnes' curve could be shifted. In this 

 way much of the work of previous investigators in calorimetry 



* Joly, Proc. Boy. Soc, vol. xlvii, 1889. 



