APPARATUS 



95 



of a mercurial pump can never exceed the exhaustion which this 

 represents. Moreover for cases in which the best vacuum is 

 required it has always to be remembered that the surface of 

 glass retains a film of air and moisture with extraordinary 

 tenacity. To get rid of this from the interior of the bulb or 

 tube which it is desired to exhaust, the tube must be heated to 

 a temperature as high as it will bear, while the pump is kept at 

 work. 



A very valuable method of obtaining a high vacuum without 

 the use of a pump has been introduced by Sir James Dewar. 

 It has long been known that charcoal absorbs many gases very 

 freely, and this property has been turned to 

 account for many practical purposes in past 

 times. Thus a respirator, consisting of a 

 small metal case filled with coarsely crushed 

 wood charcoal, which could be held over the 

 mouth and nose was actually in use by 

 nurses and dressers before the days of anti- 

 septic surgery. Such an appliance also pro- 

 tects the wearer very completely for a short 

 time if exposed to an atmosphere containing 

 irritant fumes. The most dense varieties of 

 charcoal, obtained for example by heating 

 cocoanut shell, seem to be the most efficient. 

 If a glass vessel filled with charcoal of this 

 kind is first heated, and the moisture and 

 gases removed by means of a mercury pump, 

 and the vessel is then connected with the 

 space to be exhausted and immediately 

 plunged into a vacuum vessel containing 

 liquid air, the gases present are absorbed 

 with great rapidity and completeness. A 

 selection is made by the charcoal when ex- 

 posed to contact with mixed gases, air, oxygen and nitrogen are 

 absorbed more readily than hydrogen, while helium and neon 

 remain unabsorbed to the last. 



Pumps in which mercury is used generally require to be 

 worked by hand, but for many purposes a pump is required 

 which will produce a moderately good vacuum, and which can 

 be made automatic or can be driven by power. It is only possible 

 in this place to mention the names of some of the more important 



FIG. 45. 



CHARCOAL VACUUM 

 VESSEL. 



