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CONTRIBUTIONS TO THE KNOWLEDGE 



oxide possess no illuminating power, and that the light emit- 

 ted by coal gas is due to the light carburetted hydrogen, 

 defiant gas, and other hydrocarbons. I hope, however, to 

 prove by the experiments detailed below, that, for all practical 

 purposes, light carburetted hydrogen is also entirely devoid 

 of illuminating power, and that therefore, the whole of the 

 light-giving effect is due to the olefiant gas and hydrocarbons. 

 This is an important point, as we shall find that it much 

 simplifies the estimation of the illuminating power of any 

 sample of gas, and teaches us that the nature of the combus- 

 tible diluents of the olefiant gas and hydrocarbons, has no 

 effect whatever upon the quantity of light emitted by the 

 mixture. 



The constituents of coal and other gases may be divided 

 into two classes, viz., illuminating and non-illuminating con- 

 stituents ; to the first will belong olefiant gas and the other 

 hydrocarbons above mentioned, and to the second, light car- 

 buretted hydrogen, hydrogen and carbonic oxide. To the 

 first class alone the illuminating power of the gas is due, but 

 some member of the second class is also indispensable as a 

 diluent, without which we should find great difficulty in con- 

 suming the hydrocarbons, without the production of much 

 smoke and consequent loss of light. The members of the 

 first class are all decomposed instantaneously at a white heat, 

 at a red heat more slowly, depositing the whole or the greater 

 part of their carbon in the form of very fine particles, which 

 become so many centres for the radiation of light in the gas 

 flame, and the greater the number of particles existing in 

 any flame at the same moment, the greater will be the light 

 emitted by that flame. It is therefore evident that the value 

 of these hydrocarbons for the production of light, depends 

 directly upon the quantity of carbon contained in a given 

 volume, and is altogether independent of the hydrogen with 

 which this carbon is combined ; consequently, the densest or 

 most easily condensible of these gases and vapours of the first 



