METHODS AND APPARATUS. 



21 



temperature of the air in the oven, and a tube for compressed air. By means 

 of this tube compressed air, which has previously been passed through an 

 indicator outside of the oven, is conducted into a bottle of water placed in the 

 oven, and finally passes in a succession of bubbles up through the water in the 

 Dewar flask itself. 



By automatically keeping the air in the oven at the same temperature as the 

 water in the flask, any small heat loss from the flask is avoided. The outer 

 air-space and asbestos casing also aid in the temperature control of the oven, 

 while the metal bottom assists in the equalization of the heat from the burner. 



It was at first thought that, with the flask surrounded by ideal conditions, 



Fig. 5. Constant-temperature oven. Inside the oven F, in the center, is shown the constant-tempera- 

 ture flask D, with the mercury thermometer E. On the right is the thermostat G which 



rure nasK u, wim uie mercury uieriiiuinetei rj. v >n tiie rignt is tne tiieriiiustat <j which 

 supples gas to the burner H below the oven; at the left is the arrangement 1 1 supplying com- 

 pressed air for stirring the water in the flask D. 



sufficient stratification could hardly exist to produce a sensible error in the 

 reading. It was found, however, that appreciable temperature differences did 

 exist in the flask and, although the thermometer bulb and thermal junctions 

 were all at the same level, it was considered advisable to provide some method 

 of stirring. For this reason, compressed air was introduced, being selected as 

 a convenient method for stirring the bath, especially in view of the fact that 

 the space available was small. By passing the air first through a vessel of 

 water inside the oven it is brought to the temperature of the water in the flask 

 and, still more important, is saturated with moisture at the same tempera- 

 ture, so that no heat is lost either by absorption or by evaporation. 



