348 
Proceeding s of the Boy at Society 
3. On the Establishment of the Elementary Principles of 
Quaternions on an Analytical Basis. T>y G. Plarr, Esq. 
Communicated by Professor Tait. 
4. Preliminary Note “On a New Method of obtaining very 
perfect Vacua.” By Professor P. G. Tait and Mr James 
Dewar. 
Professor Andrews, in the “ Philosophical Magazine” for 1852, 
recalled the attention of physicists to the method originally devised 
by Davy of making a vacuum so perfect, that the residual gas 
exercised no appreciable pressure as registered by the depression 
of a barometric column. This he effected by filling the vessel to 
be exhausted with carbonic acid gas, having previously inserted a 
cup containing a concentrated solution of caustic potash. On 
rapidly exhausting with an air-pump, and leaving time for the 
absorption of the residual carbonic acid by the caustic potash, he 
obtained a vacuum as perfect as a Torricellian. Andrews’ method 
was afterwards employed by Gassiot in his well-known investiga- 
tions on the passage of electricity through attenuated media. By 
the use of stick potash in carbonic acid vacuum tubes, he succeeded 
in rendering the tubes so free from any traces of gas, that the 
electric discharge will not pass. Caustic potash for this purpose 
is unsatisfactory, from the fact of its requiring to be fused before 
rapid- absorption takes place, and also from the fact that aqueous 
vapour is apt to be left. This plan is besides entirely confined 
to carbonic acid tubes, although other chemical agents might be 
procured to effect absorption of traces of other gases. The method 
we have devised to absorb traces of gases is based on the remarkable 
power of absorption of cocoa-nut charcoal for gaseous bodies gene- 
rally. By placing a piece of this charcoal in a glass tube, having two 
platinum terminals for the purpose of passing an electric discharge 
and exhausting with a Sprengel pump, heating the charcoal to a low 
red heat during the exhaustion, when the tube is sealed the vacuum 
is so perfect that no spark will pass with a coil giving quarter of 
an inch sparks in air. On now heating the charcoal with a spirit 
lamp, sufficient gas is given out to allow the spark to pass; on cooE 
