INCANDESCENT MANTLES. 395 



the cathode discharg-e the thoria plus ceria heated up to incandescence 

 more rapidly, and on stopping the discharge cooled more rapidly than 

 the pure thoria. Further, when at full incandescence and observed 

 through a dark glass the thoria plus ceria was slightly more luminous 

 than the pure thoria, though the difference was very small, probably 

 not more than 5 per cent. Owing to the difficulty of obtaining a con- 

 stant vacuum, accurate photometric measurements were not possible 

 but the amount of ligfht under favorable conditions was roughly esti- 

 mated as at least 150 candlepower per square inch of incandescent sur- 

 face, this being obtained with an expenditure of electrical energy in 

 the secondary circuit of about 8,000 volts pressure of approximately 

 1 watt per candle. Tiie amount of exhaustion suited to give the best 

 results varied with the dimensions of the tube and the conditions men- 

 tioned in the paper, but was approximately about 0.00005 atmosphere, 

 the maximum luminosity being obtained when the dark spaces of the 

 two cathodes just crossed at the center of the bulb. Owing to the 

 large amount of gas occluded by the mantle, a proper degree of per- 

 manent exhaustion was very difficult to arrive at, and required con- 

 tinuous pumping for many hours, with the cathode raj's turned on at 

 intervals. 



Even then the conditions of maximum luminosit}" were exceedinglj^ 

 imstable, owing to the further liberation of occluded gas on the one 

 hand and on the other to the rapid increase in the degree of exhaus- 

 tion, owing to absorption of the residual gas ])v the electrodes. That 

 such absorption probably took place in the aluminum electrodes and 

 not in the mantle Avas demonstrated by other experiments with a tube 

 in which there was no mantle, but only two electrodes of aluminum 

 wire. 



These experiments all point to the fact that the idea of a mixture of 

 99 per cent thoria and 1 per cent ceria having the peculiar power of 

 converting heat rays into light, while thoria and ceria alone have not 

 this power, or at any rate only have it to a veiy limited extent, is not 

 tenable, as otherwise the same diti'erence would have been noticed when 

 the materials were heated either in the carbon-tube furnace or in the 

 vacuum tube, and this undoubtedly gives great support to the second 

 theory. 



It lias been pointed out that the amount of ceria in the mantle is so 

 extremely small that it seems hardly credible that any surface ac-tion 

 that it possessed would play an importan<^ part in the production of 

 luminosity, but Dr. Bunte answers this objection by saying that 

 ^'according to Davy\s theory, the illuminating power of an ordi- 

 nary gas flame is due to particles of carbon which are separated 

 from the gas and raised to a white heat. The carbon arises chiefly 

 from the decomposition of the heavy hydro carbons ethylene and ben- 

 zene, which form together about 5 per cent of the volume of the gas. 



