242 



Scientific Proceedings^ Koyal Duhlin Society. 



great as possible, and the heat supply so adjusted that it keeps the temperature 

 of the liquid just at its boiling point without any danger of superheating. The 

 first part of this work was therefore directed towards finding how far these 

 conditions could be realised for evaporation in open dishes. 



In order to observe the rate of evaporation up to the boiling point, it was 

 necessary to use an oil-bath for heating the dishes. The bath was made of 

 copper, in the form of a rectangular box, 10" square and 5" deep, and was built 

 into the lower surface of a copper wind-tunnel, of which the front could be 

 closed by glass plates. As shown in fig. 1, the whole was designed so that the 

 dish containing the water was almost completely immersed in the oil, and the 

 water surface formed a continuation of the bottom of the tunnel. The cross- 

 section of the tunnel was 10" by 8", and it projected 14" on each side of the 

 bath. The bath could accommodate dishes up to 8" diameter, and was provided 

 with a stirx-er, a thermo-regulator, and a thermometer. For the lower tempera- 

 tures one bunsen burner was used for heating, but for the higher temperatures 

 two were necessary. 



100 



9 80 



E 60 



b 40 



20 



60 



80 100 120 140 160 



Bath Temperature 



Fig. 2. 



180 200 



220 



The dishes used were glass crystallising dishes, with straight sides, and the 

 rate of evaporation was measured by the fall in level of the dish. The level was 

 observed by providing a small glass float drawn out to a fine tip and kept in a 

 vertical position by a small stand made of brass rod, so as to rise and fall with the 

 surface of the liquid, the position of the tip of the float being read by means of 

 a cathetometer. This method allowed of a great number of observations being 

 taken in a short time, so that the mean rate of evaporation could be obtained 

 from a graph of the observations. 



At the left-hand end of the wind-tunnel an aluminium fan, 8" diameter, was 

 fixed, which could be driven by an \ H.P. motor. By this means air-currents 

 up to 1,000 feet per minute could be produced, the speeds being measured by a 

 small anemometer. 



With this apparatus the rate of evaporation of distilled water was measured 

 over a range of 30^ to 98"^' C. in still air, and from 30° to 70° C. in air currents. 



