196 Prof. W. M. Thornton on the Relation of Oxygen to 



The observed limits for ethylene are 17*5 arid 57, their 

 ratio being 3*07. The other compounds agree well enough 

 with this to support the calculated limits. ! 



These values are given for the purpose of furnishing a 

 guide for the experimental examination of the limits of 

 inflammability, and as showing how any constituent hydro- 

 carbon in petrol or " paraffin " mixtures with air is likely to 

 affect their range of ignition. 



XV. The Relation of Oxygen to the Heat of Combustion of 

 Organic Compounds. By W. M. Thornton, D.Sc, D.Eng. y 

 Armstrong College, NeiccastLe-upon-Tyne *. 



1. rpHE heats of combustion of the hydrocarbons (C, H) 

 X can be expressed by the formula 



in which A and B are the heats of combustion of carbon and 

 hydrogen respectively to C0 2 and H 2 0, Xv being the internal 

 energy represented by the bonds. For the paraffins and 

 related compounds A = 105*92 large calories per gram mole- 

 cule, B = 52*42; for the aromatic hydrocarbons A' = 107*16, 

 B' = 51*78. In both cases the heat of combustion of carbon 

 is very nearly twice that of hydrogen. This numerical 

 relation, which may only be accidental, leads to a singularly 

 convenient law for the estimation of the calorific value of 

 any hydrocarbon of known composition. Thus the difference 

 between the heats of combustion of successive members of 

 the paraffins, defines, acetylenes, aromatic hydrocarbons^ 

 alcohols, aldehydes, ketones, and esters is also nearly constant 

 and is equal to 158 - 57 calories, for the radicle CH 2 . A + B 

 in the above formula =158*34, A' + B' = 158*94, the mean 

 being 158*59. Since these effects are additive, when com- 

 bustion is complete with one oxygen atom the heat set free, 

 irrespective of bonds, is 52*4 calories per gram molecule of 

 combustible gas, when with two atoms 105*9, with three 

 158*59. 



2. Taking the heats of combustion H of hydrocarbons and 

 other compounds, without attempting to discuss their energy 

 of internal strain, and dividing them by the number of 

 oxygen atoms necessary for complete combustion, values of 

 H/w are obtained, which under the conditions are remarkably 

 constant. These are given in the Tables below, in which 

 the compounds of Tables 35 to 44 in Thomsen's ' Thermo- 

 chemistry ' are taken for examples. It is difficult from such 

 * Communicated by the Author. 



