LIGHT OF COMBUSTION. 223 



iron is raised to a red heat, the oxygen of the 

 air combines with it, as in cases of ordinary 

 combustion, by which the process is kept up; 

 but if they are heated in vacuo, the ignition 

 ceases when the heat is withdrawn. 



It is necessary to distinguish the quantity of 

 light produced by combustion from its intensity. 

 When carbonate of lime and many other fixed 

 bodies are submitted to the high temperature of 

 an oxy-hydrogen blow-pipe, they are slowly 

 dissipated, with the disengagement of a most 

 dazzling and intense light, but in small quantity; 

 whereas, if aether, alcohol, naphtha, wax, tallow, 

 oils, resins, phosphorus, sulphur, cotton, paper, 

 silk, and many other inflammable compounds, 

 are submitted to one third of the above tempe- 

 rature, a far greater quantity of light is produced, 

 but of much less intensity : yet we are informed 

 by Dr. Lardner, " there is reason to believe that 

 all bodies begin to be luminous when heated, at 

 the same temperature." (Treatise on Heat, p. 

 24.) On the other hand, when charcoal is exposed 

 to the concentrated heat of a large voltaic bat- 

 tery, it is expanded into light that in splendour 

 almost rivals that of the solar beams ; yet not 

 in great quantity, as during the combustion of 

 more volatile bodies. The concentrated heat of 

 a forge while acting upon iron and other metals 

 that are difficult to volatilize, creates a light so 

 intense, that it strikes upon the eye-balls like 



