658 



Professor The Hon. R. J. Strut! 



[Feb. 28 



When oxygen and hydrogen unite, the union may occur in two 

 distinct ways. It may occur with luminosity throughout the volume 

 of the mixture, as when the gases are exploded, or, again, it may 

 occur at the surface of a solid such as clean platinum. In the latter 

 case there is no luminosity. 



Similarly, active nitrogen atoms may reunite in the volume of 

 the gas with luminosity — this we have seen already — or the com- 



LIQUID 



AIR 



Pig. 2. 



bination may occur without luminosity at a suitable surface. Oxidized 

 copper affords such a surface. This bulb (Fig. 3) can be made to 

 glow like those you have seen before, by inserting it into a coil ; and 

 if the copper wire is situated in the side tube the glow lasts a long 

 time, for the gas has as yet no access to it. ;' But if I excite the gas 

 again, and turn the bulb round so as to drop the oxidized wire into 

 it, you see that the luminosity is extinguished in a fraction of a 

 second. Combination of the nitrogen atoms ^occurs much more 

 quickly at the surface, so that the whole quantity of active nitrogen 



