32 Proceedings of the Royal Society of Edinburgh. [Sess. 
Table II. — Pressure- Reduction Factors (/) for the 50-lbs. Container, 
THE Gas in the Envelope being Air. 
Absorbent Substance. 
Relative Apparent 
Density (Cocoanut 
Charcoal = 1 00). 
Pressure-Reduc- 
tion Factor at 
-190° C. 
Plumstone charcoal ..... 
79 
1413 
Cocoanut charcoal ...... 
100 
894 
50% cocoanut charcoal ; 50% silica (A) . 
118 
750 
Birch charcoal ...... 
33 
564 
German impregnated charcoal 
73 
517 
S.S. Mixture 
62 
382 
Silica (A) 
105 
334 
Silica (B) 
191 
266 
Activated anthracite ..... 
176 
244 
Common wood charcoal 
46 
147 
atmospheric pressure. The table shows that a silica can now be made of 
appreciable evacuating power, though at liquid air temperature over four 
times as much of silica (A) is required (by volume) as of plumstone 
charcoal to obtain the same reduction of pressure. Silica, moreover, does 
not act so rapidly as charcoal in adsorbing gas at low concentration. 
Extremely high vacua may be obtained by using a number of charcoal 
bulbs in succession, the bulbs being severally sealed to the evacuating 
system and fused off, one at a time, after use. Such a method of exhaust- 
ing in stages is, for example, useful in evacuating thermionic valves. If we 
have a mass of dry charcoal which, in accordance with (8), yields a pressure 
reduction factor of /, we shall get a very different factor if we divide the 
mass into n equal parts and employ them, in the manner just stated, in 
separate bulbs. The factor will then be /2 where 
<*> 
Two interesting corollaries follow : first, no advantage can result from 
dividing the adsorbent and using it in stages if is equal to or less than 
4; secondly , /2 attains a maximum when . For example, if = 8*2, 
the best results with a given mass of charcoal would be obtained when it 
was equally divided between (8*2/2*72 = ) 3 
reduction factors would be : — 
bulbs ; and the pressure- 
With 1 bulb, 
00 
„ 2 bulbs, 
16-8 
„ 3 bulbs, 
20-4 
„ 4 bulbs. 
17-7 
