594 



Sir James Dewar 



[Jan. 17, 

 and less cumber- 



steel cylinders, in that it is nmch lighter to carry, 

 some. 



Three methods were devised for this purpose. They were in 

 principle as follows : — 



1. Syphon Evaporation. — A vacuum container of liquid oxygen 

 was provided with an air-tight screw stopper : a release valve in the 

 stopper maintained a constant internal pressure produced by the slow 

 evaporation of the liquid oxygen. A delivery tube passed from near 

 the bottom of the liquid in the vacuum container through the neck 

 to an external metal chamber, the evaporator. This had an 

 outlet tube carrying a regulating valve. By opening this valve the 



Fig. 1. 



pressure fell slightly in the evaporator and allowed some liquid to 

 flow out from the vacuum container into the evaporator, where it was 

 rapidly evaporised, the rate of flow and consequent evaporation being 

 thus under perfect control. 



2. Electrical Heat Evaporation. — A small heiiting coil was 

 secured on the bottom of the vacuum container, and thin leads passed 

 from the coil through an air-tight insulated connection in the neck. 

 All that was necessary to control the rate of eva])oration was a small 

 rheostat in circuit with the heating coil, and a siq)ply hattery. The 

 oxygen was delivered through a tube in the screwed stopper. A release 

 valve is only needed at reduced pressures, as, e.g., in the upper air, 

 when it is placed on the delivery tube. 



