394 INCANDESCENT MANTLES. 



wonderful light omis.sivity to the exalted temperature l)rou«>"ht about 

 by local combination due to the power of the ceria in attracting 

 oxygen, and it matters })ut little Avhether this he ])roaght about by a 

 purely chemical or b}' a physicochcmical action. 



It is clear that if the light be due to the conversion of heat rays into 

 light, on heating the material out of contact Avith air, the same difier- 

 ences in their power of emitting light should be observed as exist in 

 the mantle, and several experiments have been made in order to find 

 if this were so or not. 



Dr. Bunte took a thick-walled tube of arc carbon, having the walls 

 of its middle portion reduced for a length of 4 inches to a thickness 

 of 0.059 inch, and heated it by an electric current until the middle por- 

 tion attained a most intense white heat far above 3,630^ F. In order 

 to prevent combustion and loss of heat, the middle of the tul)e was 

 embedded in magnesia, over which asbestus was wrapped. Small s(iuare 

 prisms — ()..5*,> inch long by 0.28 inch wide — of magnesia were coated 

 with the substances to })e examined, and each was cemented to a simi- 

 larly shaped piece of carbon or magnesia, so that the two adjoining 

 faces consisted of the two substances to })e compared. On placing these 

 dou])le prisms in the tul)e it was possil)h> with certain prin-autions to 

 observe the relative intensities of the radiation bv comparing the two 

 halves of th(^ surface visil)lc in the tu))e. These researches showed 

 very small diH'ei'cnces in the intensity of tiie radiation from cai'bon, 

 magnesia, ])ure oxides of thorium and cerium, and the Welsbach 

 mixtuiH\ 



Within the last few weeks eontirmation has been given of 1 )r. Hunte's 

 results by an (extremely interesting paper communicated to the Royal 

 Society by Mr. A. A. C. Swinton. in which he inclosed the luminous 

 materials in a vacuiun tube and subjected them to bombardment l)y 

 means of cathode rays which would raise them to a very high tempera- 

 ture, as it is possil)le by such a method to melt platinum and glass and 

 bring tinely divided carbon to l)right incandescence. The mantle to 

 be experimented on was mounted on a platiinnn wire frame and placed 

 between the two electrodes, so that as the electric current alternated 

 and each electrode became in turn the cathode the mantle was sul)- 

 jected on alternate sides to cathode-ray bombardment. Experiments 

 were made with mantles consisting entirely of ceria and thoria. l)oth 

 separate and mixed in diti'erent proportions, and in order to obtain 

 accurate comparisons between the pure oxides and different mixtures 

 the mantles were made in patchwork, each mantle being made up of 

 two or four sections separately impregnated with different solutions 

 and then sewed together with impregnated cotton before being burned. 



With a compound mantle prepared in this way, composed ojie half 

 of pure thoria and the other half of a mixture of 99 per cent thoria 

 with 1 per cent ceria, it was found that after exhaustion on starting 



