234 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
without danger and easy to use. The earlier form of burner in which 
the gases were allowed to mix before burning gave rise to many 
explosions. In the safety burners, the gases only come together at the 
moment of burning. They give a little less light than the first, but 
the loss is more than compensated for by the absolute freedom from 
danger. 
All oxygen burners make use of the Drummond principle, which 
consists in raising the temperature of burning gas by the introduction of 
a stream of oxygen, so that a piece of incombustible material placed in 
the flame is raised to incandescence. The brightness of the light so 
produced is dependent on the heat of the flame and the emissive power 
of the incandescent body. As the latter, lime, zirconia, and metallic 
iridium have been used. Iridium possesses many properties which 
Fig. 37. 
should recommend it for such a purpose. It does not oxidise at the 
highest temperatures, fuses with much more difficulty than platinum, and 
has a good emissive power. Zirconia is less to be commended owing 
to its high price and to the fact that it becomes useless after a few 
hours’ burning. Lime as dense as possible and free from sand is in" most 
general use. 
As gas for the source of heat, coal gas, hydrogen or the vapour of 
some volatile inflammable liquid can be employed. Coal gas is usually 
most convenient. For places without coal gas, petroleum is the usual 
source of light for projection purposes, but is quite inadequate for 
demonstrations before large audiences. 
The spirit-blowpipe burner, in which the light was obtained by the 
