RICHARDS AND MATHEWS. — HEAT OF EVAPORATION. 531 



temperature must be taken in the calculation. Thus, if the experiment 

 began at 20° and ended at 24° and the barometer indicated 7 (JO mm., 

 the first drop of water was cooled through 80°, the last through only 

 1^^. Obviously here again the average value, 78°, should be taken in 

 the calculation. It is easy to show by means of the calculus that all 

 these compromises in the calculation are legitimate. 



The amount of water evaporating into the small air space around 

 the calorimeter during an experiment was negligible both in weight 

 and in thermal effect. 



There follows, as a typical example, the complete data and compu- 

 tation of a single case, the second given in the table, selected at ran- 

 dom from results. 



For five minutes preceding the experiment, the calorimeter gained 

 at a perfectly constant rate of 0-009° per minute. When this was 

 certain the vapor was turned into the condensing coil, and after five 

 minutes more the electric current of 14 amperes was stopped, and the 

 vapor prevented from gaining further access to the coil. In yet five 

 minutes the calorimeter had settled down once more to a rate of in- 

 crease in temperature corresponding to that observed at first, that is 

 to say, 0.0085° per minute, clearly due only to radiation and conden- 

 sation from the vaporizer. The heat from the actual condensation had 

 all been imparted to the calorimeter, hence the experiment was con- 

 sidered as concluded. The several data and the simple calculation 

 depending upon them are given below. The barometric pressure was 

 exactly 760.0, hence the steam entered the condenser at 100.0°. 



In calculating the heat given out in cooling a gram of water from 

 100° to the mean temperature of the calorimeter (21.42°) the figures 

 of Barnes 2^ were employed, because they probably represent most 

 nearly the present standard of temperature. By graphic integration, 

 the average specific heat of water over this range was found to be 

 1.0012 times the value at 21.4° ; hence is calculated the value 78.67 

 calories above, corresponding to a fall of 78.58°. The 21° calorie 

 is apparently about 0.9985 times the 15" calorie, hence in terms of 

 the latter the result would be 535.2. 



Six determinations were conducted in this way with this vaporizer, 

 two of them being run very slowly and the others with increasing 

 speed. For the slowest about 11 or 12 amperes were usually necessary, 

 and 17 or 18 amperes were required for the fastest. The results are 

 rearranged in the table and renumbered in order of speed, beginning 

 with the fastest, so as to make the dependence of the results upon the 



29 Landolt and Bornstein, Tabellen, pp. 393 and 810 (Berlin, 1905). 



