5-2 OPTICAL PROJECTION 



one-sixtieth of an inch his reservoir burst like a bomb. 

 Hemming packed a tube with fine wires, on Davy's principle; 

 and a series of layers of fine wire gauze was also tried ; 

 finally the gases were bubbled through a water-chamber. 

 Sooner or later the gases thus mixed exploded through them 

 all. 



As a matter of fact, the gases are, not burnt in a jet in 

 their combining proportions, when properly adjusted by the 

 taps for the most brilliant light ; nor is the available heat 

 developed what was once supposed. The heat-energy liberated 

 by the combination of H 2 and into water, can be calculated 

 easily ; but no sooner is water formed, than a considerable 

 portion of that energy is absorbed in again dissociating it. 

 A certain amount of mechanical current, conveying the gases 

 to a certain distance away from the orifice, may and does, 

 therefore, increase the effective heat, as does a certain amount 

 of external free hydrogen flame, which aids in the same 

 object of carrying the water away in vapour. 



Even now it is almost impossible to burn either pure 

 hydrogen or carburetted hydrogen (house-gas), and oxygen, 

 in their combining proportions, without explosion, though 

 with modern apparatus this latter will be only the * snap ' of 

 a small and harmless explosion in the jet-chamber itself. 

 Take any mixed jet, and gradually turn on the oxygen ; 

 almost invariably, just as the surplus flame of the hydrogen 

 is absorbed, the jet will snap and go out. Before this, the 

 light will have diminished considerably ; and it will be found 

 that the most brilliant light is always obtained with a con- 

 siderable surplus of the hydrogen. That surplus is greater 

 as the bore of the nipple is larger ; and those manuals are 

 wrong which state that the gases are burnt ' in equal volumes.' 

 With large orifices, nearly ten feet of house-gas may be 

 required for eight feet of oxygen, and it will be found that at 

 the best light there is always a loose flame of hydrogen 

 playing about the lime. With this increase of hydrogen from 



