536 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



On extrapolating the steeper but fairly consistent straight line 

 drawn through these determinations, the value 539.3 (cal. 21°) is 

 obtained for the heat of vaporization of a gram of water, a value only 

 about 0.06 per cent lower than that found in the previous series. A 

 part of this difference is due to the slightly higher temperature of 

 boiling in the second series, caused by the greater pressure. 





.,._ X 



Figure 7. The Heat of Evaporation of Water; Second Series. 



Time in fractions of a minute is plotted in the direction of abscissae, and 

 heat of evaporation (in 21° calories) in the direction of ordinates. The dotted 

 line is an extrapolation, giving the value for a hj'pothetical instantaneous 

 experiment. 



The individual determinations may be corrected by adding to them 

 9.65 multiplied by the time needed for vaporization of 1 gram, because 

 the loss of heat is found through extrapolation to have been 9.65 small 

 calories per minute. Corrected in this way the five determinations 

 become respectively 538.8, 540.5, 539.0, 539.1, 539.3, on the average 

 539.3 as given before. The average of the results given in this series 

 and those given in the previous series is 539.45. As the second dec- 

 imal place has no significance, this may be rounded off to 539.5, giving 

 the first series preference, partly because the pressure was more nearly 

 normal, and partly because the apparatus was undoubtedly better and 

 the series more concordant. This value becomes 538.7 in terms of the 

 calorie at 15°. If the weight of the water is reduced to the vacuum 

 standard, both of these figures are diminished by 0. 1 1 per cent. The 

 resulting values may be given in a brief table, together with the same 

 quantity expressed in terms of the c. g. s. units : — ■ 



Final Result. 



Latent heat of evaporation at 100° of 1.0000 

 gram (in vacuum) of water equals 



538.9 cal.2i° or 0.5389 Cal.2i° 

 538.1 cal.i5° or 0.5381 Cal.i5° 

 2251 joules or 2.251 kilojoules 



