390 INCANDESCENT MANTLES. 



percoiit;i}4*o of ccvia in tho fluid to 0.5 roducos this to 18 candles for 2.5 

 cul)ic feet, and when 1 per cent of ceria is added a further reduction 

 to 13.5 candles for the 2.5 cubic feet i.s found; but while the candle- 

 power lost 43.2 per cent in 1,000 hours with the 0.25 per cent of ceria, 

 it only lost 12. ♦> per cent in one experiment and 2S per cent in another 

 with 1 per cent of ceria present in the ori^'inal fluid. He also o-ives 

 in the same paper as a composition foi' a mantle "'iving a yellowish 

 light: Lanthana, 40 per cent; thoria, 28 per cent; zirconia, 80 ])er 

 cent; ceria, 2 pei- cent; showinj^- that at this period the use of ceria 

 in small quantities in mantles containinj'' thoria was by no means 

 unknown. 



Later on. in lsl)3, Mr. Moeller. havin«if clearly realized that commer- 

 cial thoi'ia contained traces of ceria and that it was owin<i" to this that 

 the earlier mantles gave any light, took out a patent in which he sought 

 protection for thoria in combination with very small traces, not ex(e(>d- 

 ii\g 1 or 2 per cent, of cei'tain other rare metals, such as uranium, 

 cerium. terl)ium, neodymium. samaiium. piaxeodymiuni. yttrium, 

 and lantliatuuu. and the mantles of to-day nearly all consist of W^ per 

 cent thoiia and 1 pi'r cent ceria; as although several of the oxides 

 mentioned in miiuite traces endow a nonliuninoiis thoi'ia mantle with 

 the power of emitting light, yet e(M'ia so far transcends tlie others, not 

 only in its power of exciting Imninosity. but of keeping u}) the illumi- 

 nating power over a long period, that as far as our knowledge goes it is 

 needless to look ))eyond it. 



An absolutely pure thoria mantle gives less than 1 candle per (•ul)ic 

 foot of gas consumed, a pure ceria mantle gives but little light and of 

 a red nature, so that for all practical purpcjses the ingredients when 

 used alone are valueless, but the smallest addition of ceria gives a 

 rapid leap up in its power of light emissivity, which reaches its maxi- 

 mum when from O.H to 1 per cent of the total oxide consists of ceria, 

 further additions causing a rapid falling otf in the light, while the 

 light emitted tirst assumes a yellowish and then a reddish tint as the 

 percentage increases. 



The following curve of candlepower and percentage of ceria shows 

 the average result obtained, but it must be borne in mind that certain 

 traces of impurities derived from the ash of the cotton and from the 

 salts used lower to a certain degree the absolute candlepower, although 

 they do not affect the general contour of the curve. (Fig. 1.) 



Several attempts have been made to explain the wonderful power of 

 light emissivity possessed by the thoria- ceria mantle. Dr. Drossbach 

 was of opinion that the rare earths had a special action in converting 

 the heat rays into light rays, and that the molecules of ceria acted by 

 bringing the heat vibrations of thoria to the most favoi'able state of 

 resonance with the vibration of the hot flame gases. Dr. Moscheles 

 and Dr. Killing both pointed out that cerium formed two oxides and 



