358 Prof. H. E. Armstrong on Loiv- Temperature Research 



Professors Tait and Dewar, in 1874-5, were the first to recognise and 

 take advantage of the property it has of retaining them even under 

 very low pressures. A brief account of their work was pubhshed, 

 under the title " On a New Method of obtaining very Perfect Vacua," 

 in the ' Proceedings of the Royal Society of Edinburgh ' (1875, viii. 

 348, 628) and also in 'Natui;e' (1875, xii. 217). The method 

 consisted in heating charcoal to redness in a tube attached to a 

 mercury pump while the exhaustion was proceeding and to seal the 

 vessel when this was completed. When the charcoal was cold the 

 vacuum was found to be so complete that, even when a powerful coil 

 was used, no spark would pass between platinum wires sealed into the 

 tube only one-fourth of an inch apart. The account referred to is 

 of special interest also as containing a noteworthy contribution to 

 the theory of that most wonderful of instruments the Radiometer, 

 the discovery of which by Sir William Crookes has been the point 

 of departure for all modern research on the phenomena of high vacua. 

 In closing their communication to the Royal Society of Edin- 

 burgh, Professors Tait and Dewar remark : — 



" We need hardly say that this easy means of obtaining high vacua 

 will be of importance in spectroscopic observations and we intend shortly 

 to communicate observations in this direction." 



Nevertheless the method remained undeveloped until 11)04 and 

 was then resuscitated almost by accident — which is but another 

 illustration of the manner in which the obvious is neglected until, 

 for some reason, it compels attention. Sir James Dewar had made 

 experiments with finely divided platinum and palladium, which are 

 known to occlude gases, in particular hydrogen and oxygen, even at 

 ordinary temperatures and to take up larger quantities when heated. 

 Finding that their absorptive power was but little affected by cooling, 

 he bethought himself of charcoal and proceeded to contrast its 

 behaviour with that of the metals before mentioned. The momentous 

 discovery was then made that when cooled by liquid air charcoal has 

 an altogether extraordinary power of condensing gases. 



Those who have attended the lectures at the Royal Institution 

 during the period under consideration are aware that this has been 

 demonstrated in the most striking manner possible and in a great 

 variety of ways. 



All charcoals possess the property. The light variety used in 

 making gunpowder or that prepared by carbonising blood both act ; 

 that made from cocoanut, however, is the most effective variety for 

 general use. 



The difference in the behaviour of various gases with cocoanut 

 charcoal at 0° and at —185° is well shown in the following table, in 

 which the results are recorded of a series of experiments made with 

 one and the same portion of charcoal, the volume absorbed being 



