612 



Sir James Dewar 



[Jau. 1' 



250 grams of cocoaiuit charcoal was as follows : — the operation 

 consisted in alternating periods of exhaust and gas accumulation 

 from the hot charcoal with the pump shut off. The vessel w^as 

 supported (inverted) inside a lagged sheet iron dome for the 

 purpose of being heated within and without by steam jets. The 

 successive stages of the alternated exhausts and accumulated gas 

 pressures were as follows : — 



The steady diminution of gas accumulation is the l)est proof of a 

 satisfactory \'essel. 



In order to seal off a metal vessel after exhausting, a pure lead 

 tube is employed to connect it to the vacuum circuit. This tube is 

 soldered at one end into a small brass bend brazed into the outer 

 shell of the vessel, and at the other end into a straight light brass or 

 iron tube, which slips into a slightly wider glass tube on the vacuum 

 circuit. (See Fig. 14.) The metal to glass joint is made by sealing 

 wax. The intermediate lead tube must be long enough to prevent this 

 joint softening by the heating of the vessel during exhaust. When 

 the exhaust is complete the lead tube is squashed together with a 

 corrugated surface for about 4 or 5 cm. The corrugations are 

 deepest in the centre and become less on either side. A clean cut is 

 then made through tlie centre of the constricted portion and tlie cut 

 end rapidly tinned. As an additional precaution melted bitumen or 

 (^liatterton compound is painted over the joint, and a. metal cap 

 secured over all. 



The thorough cleaning of the chtircoal is of the first importance 

 if the best results are to l)e obtained. Very good results can be 

 realised by a shorter treatment as follows : — Pure oxygen up to about 

 10 cm. pressure is admitted to the hot charcoal after, say, the second 

 exhaust period. This is allowed to remain for about half-an-hour 

 before again exhausting : after two or three repetitions the charcoal 

 is rendered sufficiently active to maintain the vacuum required in 



