244 



Scientific Proceedings, Royal Dublin Society. 



surface. The burner was lighted, the fan stai-ted, and the burner regulated 

 until the water came to the steadj^ temperature required, when the observations 

 of the float were made as before. By this means the temperature of the water 

 could be maintained at the boiling point even in a draught of 1,000 feet per 

 minute. 



The results of these experiments are shown plotted as a graph in fig. 3. 

 The effect of the vapour pressure is shown by the upward sweep of the curve 

 in the region 70° to 100° C. Thus the increase in rate of evaporation due to 

 the rise from 90° to 100° is equal to that due to the rise from 30° to 90°, so that, 

 roughly speaking, each degree rise in the higher region is six times as effective as 

 a degree in the lower region. 



This is further emphasised in the graph fig. 4, where the rate of evaporation 

 is plotted against vapour pressure. It will be seen that the rate is proportional 

 to the vapour pressure up to 90° C, but above this it increases more rapidly. 



480 

 Vapour Pressure 



Fig. 4. 



960 



The region 90° to 100° is therefore particularly effective in producing rapid 

 evaporation, and wherever possible the water should be kept within this tem- 

 perature range. 



It is also interesting to note how the results for evaporation in still air 



compare with figures calculated by means of an approximate formula put 



forward by Hinchlev," namely, rate of evaporation in kilograms per sq. metre per 



fP. - Pd\'^ 

 hour from water surface = I — ^?j — I where Pc = vapour pressure of the liquid 



in mm. of mercury P,i = pressure of the water vapour in the air. 



'"The General Problem of Evaporation," J. W. Hinchley. Jour. Soc. Chem. Ind., 41, 

 242 T., 1922. 



