NOTES ON NEBULAE. ! 33 



oxygen with some other substance is in progress, and this 

 is the source of the heat and the light that flame gives 

 forth. We cannot regard the Great Nebula in Orion as 

 originating in anything resembling flame. 



We can, however, in our physical laboratories arrange 

 an experiment which seems to throw some light on the 

 composition of the nebula. Into a glass tube a small 

 quantity of hydrogen gas is admitted, the air having been 

 previously extracted. Then, by means of two wires, one 

 at each end of the tube, an electric current is transmitted 

 through the gas. Here there is no combustion ; the gas 

 is merely the vehicle by which the electricity flows from 

 one pole to the other. In doing so the gas instantly begins 

 to glow with an intense bluish light, and a very beautiful 

 effect is produced, which can be renewed or terminated at 

 will by simply making or breaking the electric current. 

 It would seem as if the gas we see in the nebula were in a 

 condition somewhat analogous to the gas in the tube. I 

 do not mean that the passage of electricity through the 

 nebula is the source of its luminosity. There is, indeed, 

 no ground for such a supposition. It is the property of 

 electricity when passing through a conductor to warm 

 that conductor ; thus we know that if a powerful current 

 be transmitted through a wire of the most infusible of all 

 metals, platinum, the wire will not only get warm, but it 

 may become red hot, white hot, and even melt under the 

 influence of the heat which is generated. In those beau- 

 tiful incandescent electric lamps which are now happily 

 coming into such extensive use a current of electricity 

 flows through a filament of carbon, and kindles that exqui- 

 site incandescence which is maintained while the current 



