at the Royal Institution, 1 900-1 907. 381 



at low pressures. In this, probably, is involved the whole question 

 as to the existence, as actual separate entities, of the mysterious units 

 termed electrons, to which so much is now attributed — but on the 

 basis of experiments that, for the most part, have never involved 

 an approach to the care taken in the inquiry now under discussion. 



In discussing the problems of the atmosphere in 1902, Sir James 

 drew attention to the auroral discharges in its upper regions and to 

 the partial identification of the lines in the spectrum of the discharge 

 with those given by some of the newly discovered gases, especially 

 neon ; at the same time he pointed out that we have still to account 

 for the appearance of some of the rays of these gases and for the 

 absence of others, particularly of all the rays of nitrogen. To quote 

 one of his statements : — 



" If we cannot give the reason of this, it is because we do not know 

 the mechanism of luminescence— nor even when the particles which 

 carry the electricity are themselves luminous or whether they only 

 produce stresses causing other particles which encounter them to vibrate ; 

 yet we are certain that an electric discharge in a highly rarefied mixture 

 of gases lights one element and not another, in a way which, to our 

 ignorance, seems capricious/' 



It may be that in the intensely cold upper regions of the atmo- 

 sphere precisely those substances are absent— such as water vapour — 

 which necessarily accompany the gases under ordinary la])oratory 

 conditions, a degree of purification being effected such as Sir James 

 himself has demonstrated to be an effective means of stopping the 

 discharge. 



It may be added that the results under discussion are in harmony 

 with those arrived at by Dr. H. B. Baker ; in fact, the refined 

 experiments on the influence of conducting moisture on the occur- 

 rence of chemical change in gases which Dr. Baker has carried out 

 with such exceptional skill afford a body of evidence which, taken 

 together with that ehcited in the laboratory of the Royal Institution, 

 should compel attention to the need of greater precaution in practice 

 and, meanwhile, of greater caution in speculation. 



Sir James Dewar has pointed out that it would be interesting to 

 repeat light repulsion experiments in the highest attainable charcoal 

 vacuum. Perhaps, in making this suggestion, he has in mind the 

 possibility that the measured effects may be in part attributable to 

 secondary causes such as are brought to hght in his radiometer 

 observations. 



A similar question may be asked also with reference to the differ- 

 ence of electric potential which is set up when two metals are brought 

 into contact — a subject of much controversy in the past. The one 



