E. W. Hilgard on Luminous Flames. 221 
own definition, he nevertheless teaches that the blue cup is 
formed by the combustion chiefly of carbonic oxyd, produced 
by the “first and weakest effect of the heat on the fuel ”—an 
surrounding the blue cup as well as the higher portions of the 
flame, and is thus proved to be nothing more than a zone of 
glowing gas; which of course, however, cannot be strictly 
defined from the luminous envelop, the oxydation being a grad- 
burnt by itself from a jet kept at a white heat.* 
Part of the flame, and increases to 76 per cent at the point of 
the Inner cone, with from 10 to 15 per cent of carbonic acid. 
The products and educts of combustion therefore greatly ex- 
up their oxygen, before final combustion. 
* See Gmelin’s Handbook, art. Carbonic Oxide- 
