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CONTRTBUTIOXS TO THE KNOWLEDGE 



due to hydrocarbons and light carburetted hydrogen, and the 

 value of the former as being directly proportionate to the 

 quantity of oxygen required for their combustion. If we 

 leave the light carburetted hydrogen entirely out of the 

 calculation, as I shall prove that this gas has practically no 

 illuminating power, this method generally gives results not 

 far from the truth ; but they are, nevertheless, liable to 

 very considerable error from the fact that the amount of 

 oxj-gen consumed does not depend alone upon the lumini- 

 ferous ingredient — the carbon, but also upon the amount of 

 hydrogen combined with that element, and which is neces- 

 sarily a variable quantity, being in some of the hydrocarbons 

 in the ratio C:H=n:n, in others C:H=n:n — 6, and in 

 some C : H=n : n — 12. In order to avoid this source of 

 error, and obtain a correct expression for the illuminating 

 power, however much the composition of the hydrocarbon 

 may vary, I have estimated the volume of carbon vapour 

 contained in the luminiferous hydrocarbons, and made that 

 the basis of the calculation. I have already pointed out a 

 method for this estimation of the carbon vapour ;* and Mr. 

 Leigh, in a memoir lately read before this society,f also 

 describes a similar plan, which he employs for the determi- 

 nation of the consumption of oxygen by these bodies. The 

 following is the mode of procedure which I have employed in 

 the annexed determinations of the value of various hydro- 

 carbons. 



A known quantity of the gas, previous to the action of 

 fuming sulphuric acid, is exploded with an excess of oxygen, 

 and the volume of carbonic acid produced accurately noted. 

 Another known volume of the same gas, after the withdrawal 

 of the hydrocarbons by sulphuric acid, is then similarly 

 exploded with oxygen, and the carbonic acid formed also 

 estimated. Thus, there are determined — 1st, The per centage 



* Journal of the Chem. Soc, vol. ii. p. 272. 1849. 

 t Mem. of the Lit. and Phil. Soc. of Manchester, vol. ix., p. 303. 1851. 



