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gases could move away further from the nozzle before coolings 

 down. The views of Siemens are supported by the observa- 

 tions of E. Wiedemann and of Hittorf on the passage of 

 electricity through rarefied gases. E. AViedemann has found, 

 by a direct measurement of the heat developed that a gas may 

 become luminous under the influence of an electric discharge 

 whilst its temperature is considerably under the boiling point 

 of water. Hittorf, by means of a powerful bichromate 

 battery of 2,400 cells, obtained an apparently continuous 

 current through a glass tube containing rarefied gas. On some 

 occasions the gas was luminous whilst the tube was not too 

 warm to be touched by the hand without inconvenience. This 

 is, of course, quite inconsistent with a high temperature of the 

 gas. In another experiment the current was led by iridium 

 terminals into a tube, and a dark space was observed round the 

 positive terminal. This terminal became so hot that the metal 

 softened, glowing with an intense white light, whilst the gas 

 immediately in contact with it (and therefore at a very high 

 temperature) was quite dark. We have, then, very strong 

 evidence that gases can become luminous under the influence 

 of electrical processes, independently of a rise of temperature ^ 

 and considering the very close relations between electrical and 

 chemical actions, it is not surprising that the same should be 

 true of chemical processes, such as we meet with in combus- 

 tion. 



Professor Eennie then made some remarks on the cause of 

 the luminosity of the first class of flames mentioned by Pro- 

 fessor Lamb : — The usually accepted theory regarding the 

 luminosity of an ordinary candle or gas flame is that it is due 

 to the presence of solid particles of carbon at a white heat. 

 Some years ago Professor Prankland made some experiments 

 which he thought disproved this theory. He showed that 

 intense light can be produced under conditions which, as he 

 believes, excluded the possible presence of any solid particles 

 — for example, arsenic burning in oxygen. In this case it is 

 said that the temperature of the flame is such that the arsenic 

 itself and the products of its combustion must be in a state of 

 vapour. He showed also that certain non-luminous flames 

 become luminous under increased pressure. He ascribed the 

 luminosity of ordinary flames therefore to the presence of very 

 dense hydrocarbons at a very high temperature. He was con- 

 firmed in this belief by the transparency of such flames and by 

 the fact that the soot deposited on cold substances placed in 

 the flames was not pure carbon, but contained hydrogen. 

 Since Erankland's experiments were made other scientists, 

 chiefly in Grermany, have investigated the subject, with the 

 result that the original theory has been confirmed for the 



