INCANDESCENT MANTLES. 393 



on the gauze of the burner top, being indeed onl}^ prevented from 

 flashing- back in the tube bj^ the conducting power of the gauze and 

 the accelerated rush of the gas and air mixture through its meshes, 

 the combustion is being completed in two stages, the first taking place 

 on the surface of the gauze where the hj-drocarbons of the coal gas 

 undergo incomplete combustion at the expense of the oxygen of the 

 admixed air 3ielding carbon monoxide and hydrogen, together with 

 small quantities of carbon dioxide and water vapor, and it is the com- 

 bustion of the carbon monoxide and hydrogen which gives the outer 

 flame in which the mantle is heated, this combustion entirely taking 

 place at the expense of the oxygen derived from the air surrounding 

 the flame and not from the air originally mixed with the gas in the 

 burner tube. It is a mistake to speak of this outer flame as a "solid 

 flame," an expression often used in connection with burners in which 

 sufficient air is introduced with the gas to flatten the inner zone onto 

 the gauze, as it is merely the structureless flame which is always ob- 

 tained on l)urning gases like carbon monoxide and hydrogen which 

 burn only in one stage. An}' user of mantles knows perfectly well 

 that it is only in the edge of this outer flame that the mantle material 

 acquires its true incandescence, this being due to the fact that it is only 

 when the maximum quantity of air and the carbon monoxide and 

 hj'drogen meet that the highest temperature is attained, and it is here 

 that combustion is at its fiercest and that the catalytic action of the 

 ceria creates points of high intensity. 



The view put forward by Moscheles and Killing may be shortly 

 stated as follows: Cerium is a metal which exists in two states of oxi- 

 dation, a lower or cerous oxide, CegOj, and a higher or eerie oxide, 

 CeOo, and it is easily conceivable that in its highly subdivided condi- 

 tion and at a high temperature, exposed as it is to the high reducing 

 action of hydrogen and car])on monoxide, which tend to take oxygen 

 from it and convert it into cerous oxide, and also to the oxidizing action 

 of the air, which tends to again build it up into eerie oxide, a contin- 

 ual oxidation and reduction is taking place, the abstraction of oxygen 

 from the air and its liberation to combine with reducing gases on the 

 surface of the ceria raising these attenuated particles far above the 

 temperature of the flame. 



It is quite clear that whether one accepts the Killing theory of dual 

 oxidation, or Bunte's of catalytic action of the ceria, one arrives at an 

 explanation of the fact that unless the mantle be just in the right posi- 

 tion so that both the air and flame gases have access to it, the lumi- 

 nosity is practically destroyed. 



It iuay be roughly stated that there now exist two classes of theory 



with regard to the mantle, the first being that the ceria or ceria and 



thoria exercise some occult power in converting heat rays into light, 



and the second, the theories of Killing and Bunte, which ascribe the 



SM 1900 28 



