1914] on The Coming of Age of tne Vacuum Flask 



245 



high vacua, by placing a small quantity of charcoal in a globular recess 

 in the vacuous space of the flask. When such a vessel was filled with 

 liquid air, the charcoal was also cooled and thus absorbed remaining 

 traces of air, producing thereby a vessel with very high heat insulating 

 qualities. 



The employment of charcoal made it possible to use metals such 

 as brass, copper, or nickel, instead of glass, for the construction of 

 vacuum vessels, with obvious advantages. Metals irive off small 

 quantities of gas for a long time, which diminish the vacuum in 

 such flasks, if ic were not maintained by some charcoal, cooled to the 

 low temperature of liquid air, or other liquefied gas, placed in the 

 vessel. Many forms of such vessels have found a ready application to 

 low temperature work, not only for storage and transport, l3ut in cases 

 where rough usage was unavoidable, or where Ihe fractures to which 



Fig. 4. 



glass is liable might be dangerous : also when high pressures are to 

 be employed along with low temperatures metallic construction is 

 necessary, and only by the addition of charcoal condensation can the 

 necessary heat insulation be secured. 



The most recent achievement in the attainment of a high 

 exhaust is Dr. Gaede's invention of the Molecular pump, which is 

 capable of producing a vacuum of • 000001 mm. of mercury. 



The working principle of this pump is shown in Fig. 4. A is a 

 revolving cyhnder fixed on a shaft a enclosed in an outer casing B. 

 In the inner side of B a groove n to m is cut, communicating at 

 either end through the casing with a mercury gauge M. When the 

 cylinder rotates h'om left to right, or clockwise, the friction set up 

 amongst the molecules of the gas or air contained in the groove 



