Becker — Improved Methods of Evaporation in the Laboratory . 243 



As the oil was maintained at a steady temperature by the thermo-regulator, it 

 was possible to note the difference of temperature between the oil-bath and the 

 water in the dish during the evaporation. This is an important point, as it is 

 an indication of the rate at which heat is being supplied to the liquid. As the 

 loss of heat becomes greater at higher temperatures, this difference naturally 

 increases rapidly with increasing temperature. 



The temperature of the water for different bath temperatures is shown in 

 fig. 2, where it will be seen that a bath temperature of 100° C. gives a water 

 temperature of about 70° C. in still air (curve a), about 60° 0. in a moderate 

 draught (curve b), and 54° C. in a strong draught (curve c). These temperatures, 

 therefore, represent the maximum obtainable under the given conditions with a 

 water bath as source of heat. Howevei', by using a higher bath temperature it 

 is possible to maintain the water at, or near, its boiling point, and thus obtain 

 the great advantage of the higher vapour pressure. The graph shows that to 

 maintain water at 100° C. in stiU air the bath must be at 170° C, in a moderate 

 air current it must be at 197° C, and in a strong draught at 215° C. This 

 graph thus shows the approximate temperature at which any given bath should 

 be maintained in order that water may evaporate at a certahi temperature mider 

 the given conditions. 



20 



40 



60 



Temperature 



Fig. 3. 



Owing to the difficulty of maintaining the higher temperatures with the oil- 

 bath, the evaporation from 70° to 100° C. in air currents was measured in a 

 slightly different way. An aluminium dish was supported on a powerful ring 

 burner in close proximity to the fan, so that the air could be blown across its 



