254 Annals of the Philosophical Club 



Railway, showed them to be very similar to these in character 

 and probably due to the same cause. This inference was 

 confirmed by experiments made at Chelsea (2\ miles from 

 the railway) and at a point about J a mile from it, the 

 disturbances at both places showing an increase over those 

 registered at South Kensington, and becoming larger as 

 the railway was approached. 



1899. April 27th, 467th meeting (anniversary). Sir 

 G. G. Stokes, on being asked to account for the difference 

 in luminosity of mantles consisting of pure thoria in com- 

 parison with those formed of 99 per cent, of thoria and 

 I per cent, of ceria, when heated by cathode rays or the 

 Bunsen flame, explained the physical conditions to which 

 such difference was due. These were that thoria, from its 

 molecular structure, is little disposed to vibrate with the 

 frequencies corresponding with the less refrangible end of 

 the spectrum and the invisible rays beyond. But, if it can 

 be thrown into a state of high molecular agitation, by a 

 cause permanently at work, so that the intake of energy 

 balances that given out by radiation, much less of the 

 output is lost (for illuminating purposes) by being in the 

 form of vibrations with low frequency. Thus a compara- 

 tively large part of the output is in the form of vibrations 

 of higher frequency, which are needed for illumination. 

 To explain how the thoria is to be thrown into a state of 

 high agitation, we must suppose that, when a mantle of 

 it is thus affected by the Bunsen burner, this is mainly 

 produced, not by direct contact with the products of com- 

 bustion, but by taking up from the ether the violent agita- 

 tion, produced in it by, and emanating from, the various 

 molecules born by the combustion. At any moment 

 only an extremely small fraction of the molecules that 

 have just been born can impinge on the mantle before 

 their agitation has to a great extent subsided by com- 

 munication to the ether and in part to the molecules 

 in their immediate neighbourhood. We have, therefore, 

 mainly to look to the agitation in the ether for getting up 

 agitation in the molecules of the thoria. That will be, ia 



