THE HYDROCARBONS AND THEIR DERIVATIVES. FLAME 357 



lene, and liberating carbon, is not reached. The oxygen in 

 the air plays no part mixing carbon dioxide or pure nitrogen 

 with the gas has exactly the same effect. A flame of this kind 

 is non-luminous. 



Although the middle zone of the non-luminous flame is cooler 

 than that of the luminous flame, the average temperature of the 

 flame as a whole is higher. This is the case because the same 

 total amount of heat is liberated in both cases, but the non- 

 luminous flame as a whole is smaller in size. 



The Bunsen type of burner, placed in a horizontal position 

 (Fig. 91) is used in the ordinary gas cooking range. As with the 

 Bunsen burner, some care is required to get good results. The 

 holes which admit the air to the mixer must be kept clear of 

 obstructions, as otherwise luminous flames are produced, smoke 

 and soot are formed, and less heat is generated. The size of 

 the openings must also be adjusted so_that 

 the admission of too much air will not cause 

 the flame to flash down the burner, and set 

 fire to the gas within the mixer. 



Flames with Incandescent Mantles. 



When gas is burned in a Bunsen burner, 

 a bright light may still be obtained from 

 the flame. This is managed by suspending 

 in the flame a structure (" mantle ") made 

 of the oxides of thorium (99 per cent) and 

 of cerium (1 per cent). These oxides act as 

 a contact agent, hastening the combustion 



and liberation of heat close to their sur- * - * 



face, which thereby becomes incandescent. 



The light has about ten times the illuminating power of a flat 



flame burner using the same amount of gas. 



Exercises. 1. When vegetable matter decays in the air 

 the carbon it contains is finally all turned into carbon dioxide. 



