466 Prof. V. B. Lewes. The Cause of Luminosity [Mar. 21, 



other hydrocarbons available for distribution amongst the products 

 of decomposition. 



25670 



These ratios must now be divided amongst the atoms liberated at 

 the moment of decomposition from the molecule, and we thus obtain 

 the ratio : 



1 1-31 2-86 

 864 



or 1 : 1-74 : 5'72 



The determination of the illuminating value of a gas becomes more 

 and more difficult the higher its illuminating value, owing to the 

 cooling effect of the small burners that must of necessity be used in 

 order to ensure complete combustion. Dr. Percy Frankland* assigned 

 the illuminating value of 35 candles to ethane as the mean of four 

 tests, which varied considerably amongst themselves, and, adopting 

 his figure, the calculated illuminating values for the ethane, ethylene, 

 and acetylene would be : 



Illuminating value. 



i --- * ---- \ 

 Calculated. Found. 



Ethane ............ 1 x 35 = 35 35 



Ethylene .......... T79 x 35 = 60'9 68'5 



Acetylene ........ 5'72 x 35 = 200'2 240 



figures which are far nearer the experimental ones than could have 

 been expected, considering the crude character of the calculation and 

 insufficient data, which leads to omitting altogether such important 

 *f actors as the amount of gas consumed to bring about the requisite 

 temperature of decomposition, the specific heat of the products, and 

 the thermal value of the change from gaseous to solid carbon, and are 

 of no value except as showing that a ratio does exist between heat of 

 formation and illuminating value. 



Methane is the only other gaseous hydrocarbon of which the heat 

 of formation is known, it being -f- 21750, and as the molecule contains 

 only 1 atom of carbon, 2 mols. have to be taken, and on calculating 

 the probable illuminating value by the same method as was applied 

 to the other hydrocarbons, we should have 



* ' Chem. Soc. Jour.,' vol. 47, p. 237. 



