392 INCANDESCENT MANTLES. 



Another example which bears closely upon the case of the ceriu is to 

 be found in the action of platinum upon mixtures of hydroo-en or coal 

 gas and air. If a piece of platimun foil bo heated to redness in a 

 Bunsen flame, and the gas be then turned out and again turned on 

 before the platinum has had time to grow too cool, it again becomes 

 heated to redness in the stream of gas and air and continues in that 

 condition as long as the gases are passing over its surface, while if 

 brought to what would have been the outer envelope of the tiann^ had 

 the gaseous mixture been alight, the temperature is so increased that 

 the ignition point of the gas is reached. The action here is what is 

 known as catalytic or surface action, and is due to the fact that plati- 

 num has the power of condensing gases upon its surface, and in doing 

 so renders them so chemically active that combination between the 

 condensed gases is often induced, and in the case of the coal gas and 

 air enough heat is evolved on the surface of the platiimm to raise it to 

 redness. The result being due to surface action, it is manifest that an 

 increase of the surface nuist increase the activity of the substance, and 

 if, instead of taking metallic foil with its comparatively small area of 

 exposed surface, spongy platinum made by reduction from piatinuin 

 salt or asbestus coated with the reduced metal be employed, ignition 

 of the gaseous mixture without the preliminary heating takes place. 



The fact that the platinum foil remains red hot in the stream of 

 coal gas and air, which is only at atmospheric temperature, at once 

 shows that the heat evolved by such an action is localized in the bod}^ 

 starting the action, and it is manifestly possil)le, and indeed probable, 

 that the very minute particles of ceria scattered over the surface of th(^ 

 inert thoria and so subdivided as to be capable of easy and rapid heat- 

 ing to a very high temperature may give rise to an action of the same 

 kind. Indeed, one knows that to a certain extent this is so, because if 

 the mantle burner be turned off and again turned on while the mantle 

 is still hot, the mixture of gas and air is reignited. although the rod and 

 mantle material are manifestly l)c!ow the ignition point of the gas, and 

 the fact that this does not take place with a pure thoria mantle is a 

 fairly good ])roof that it is the ceria and not the thoria w^hich is acting. 



Dr. Bunte has found that thoria had no influence in bringing about 

 the combination of hydrogen and oxygen, and the temperature at which 

 the two combined to form water was the same whether thoria was 

 present or not. but that the presence of ceria caused them to combine 

 at 600'-^ F. instead of 1,200° F. This being so, the question arises as 

 to what the action can be that the ceria induces, the localization of the 

 heat from which causes its minute and subdivided particles to glow 

 with so high an incandescence. 



When in a Bunsen burner the admixture of gas and air has been 

 carried to the extreme limit, so that what would have been the inner 

 cone of the Bunsen flame settles down as a green and seething sheet 



