INCANDESCENT MANTLES. 397 



Thoria, as far as the life of the mantle goes, is perhaps the most 

 important constituent, as there is no other known oxide which will 

 stand heat for so long- a period without being aifected l\y it, and the 

 getting away from shrinkage in the mantle was one of the chief steps 

 which led from the failure of the early mantles to the success achieved 

 by the later ones. 



Moreover, thoria is a body having a very low specific heat, and 

 owing to its bulk when produced from the nitrate of thorium by the 

 action of heat is a good nonconductor, so that the temperature created 

 on the surface of the ceria particles is more readily localized there. 



Within the last few years attempts have been made to attack the 

 question of mantle making from a different standpoint. Admitting the 

 superiority of thoria with its 1 per cent of ceria over other mantle 

 mixtures, efforts have been made to o})tain mantles of filaments of this 

 mixtui'o upon the principle under which the old Clamond basket was 

 made. 



Clamond produced his incandescents by making a paste of magnesia 

 with acetate of magnesia in solution, and molded the mixture into 

 threads by squeezing through holes in a plate, the threads while still 

 moist and plastic being wound to the required shape on a mandrel or 

 mold, the still moist threads being pressed together and made to cohere 

 at the points where they crossed each other, and then on baking the 

 acetate luting burnt to oxide and the coherent magnesia hood or basket 

 remained. 



The reduction in size of the filaments to be rendered incandescent 

 was a great advance over the old lime light and enabled incandescence 

 to be produced by a burner instead of a blowpipe, while Welsbach's 

 discovery, or rather adaptation of the principle of saturating a fabric 

 and incinerating, created a new era in incandescence by giving a degree 

 of fineness not before approached. 



As was natural, attempts were then made to reduce still further the 

 size of the filaments in the Clamond basket, and on November 4, 1890, 

 Lungren patented a distinct advance. It was found that if you made 

 a plastic filament as Clamond had done, but finer in substance, it dried 

 very quickly, and on attempting to make the material cohere at the 

 crossing of the threads, those first wound on the mandrel, being drier 

 than those wound across them, were harder, and on pressure bemg 

 applied cut through the softer threads instead of welding with them. 

 Finding this, Lungren patented the idea of making a plastic mass of 

 some elastic material charged with refractory earths or metallic oxides, 

 expressing from some such mass fine wires or threads, weavmg or 

 interlacing the threads into a fabric from which the cone or mantle 

 could be made and then burning out the combustible clastic binder. In 

 making this binder he gives mixtures of glue with glycerin, india 

 rubber dissolved in naphtha, or boiled linseed oil as examples, but also 

 says that a variety of materials may be used. 



