Bkckeu — Improved Methods of Evaporation in the Laboratory. 245 



The two series of values agree fairly closely up to 90°, but above this the 

 experimental values are higher than the calculated. From both these facts it 

 appears that, as the boiling point is approached, the rate of evaporation increases 

 more rapidly than it would if it were proportional to the vapour pressure. 

 This may be due to the fact that at the higher temperatures the convection 

 currents in the vapour over the surface of the liquid become much more rapid, 

 thus leading to a more effective removal of the vapour. 



The effect of a current of air in hastening evaporation is also shown by the 

 two upper curves (b) and (c) in fig. 3. The effect is most marked at low dish 

 temperatures, and falls off as the temperature rises. Thus at 50° C. the rate 

 is 28 times as great in a current of 500 feet per minute, and 38 times in 

 1,000 feet per mimite ; at 80° C. the figures are 20 and 25 respectively, while 

 at 100° C. they are 1-7 and 22. These figures also show that, while at low 

 temperatures the rate is approximately proi^ortional to the velocity of the 

 current, at high temperatures the moderate current is almost as effective as the 

 higher speed. 



The relatively high efficiency of the air current at low temperatures is 

 probably due to its action in preventing the vapour from accumulating over the 

 surface of the liquid, as it is liable to do, owing to the weakness of the convection 

 currents. On the other hand, the falling-off in efficiency of the current at high 

 temperatures is probably due to the fact that the draught removes the heat from 

 the surface layers faster than it can be supplied by conduction from the body 

 of the liquid ; this results in a surface cooling, which slightly reduces the rate of 

 evaporation. When the air is heated before being led over the surface of the 

 liquid this surface cooling is avoided, and slightly higher rates of evaporation are 

 obtained. Thus Aldrieh' has described an apjaaratus for evaporating solutions 

 at 20° to 30°, in which an average rate of evaporation of 01 c.c. per sq. em. per 

 minute is obtained in a draught of warm air, while the rate obtained with the 

 draught at room temperature is about 007. 



In most of the formulae put forward for rate of evaporation in a draught 

 the factor connecting it with evaporation in still air is assumed to be a constant, 

 but this probabl}^ arises from the fact that the experimental results were mostly 

 obtained at low temperatures. Thus Leonard Hill found that the evaporation 

 from a muslin surface was doubled in a wind velocity of 55 metre jjer second 

 (107 feet per minute) ; and Carrier arrived at a value' of 117 metre per second 

 (228 feet per mmute) for a similar doubling. The present experiments show 

 that the factor cannot be regarded as constant over the range 40° to 100° C. 



From a consideration of these results the best conditions for evaporation 

 from open dishes can be laid down. The rate of evaporation on the water bath 

 (dish temperature 70° C), as at present in use, is approximately 01 c.c. per 

 minute per sq. cm. Eaising the temperature of the water to 95° C. raises the 

 rate to -04, without leading to any increased risk of loss, while, if the water be 

 kept at the same temperature in a draught, the rates increase to -07 for a draught 

 of 500 feet per minute, and to 087 for 1,000 feet per minute. 



In order, therefore, to cut down the time required for evaporation to one- 

 seventh of its present value it is merely necessary to make arrangements to keep 

 the water at 95° C. in a moderate current of air.'' This may be done by means 



" J. Biol. Chem., 23, 255, 1915. 



^ See HincMey, loc. cit. 



^ It is important to note that it is not sufficient to heat the water to 95° C. and then 

 start the draught, as the cooling effect of the air then practically neutralises its efficacy in 

 removing the vapour; the heat supply must be sufficient to maintain the liquid at 95° C. in the 

 draught. 



