Affinity in the Combustion of Organic Compounds, 67 



ploding mixtures at high initial pressures. They found that 

 in a mixture OH 4 -t-0 2 + 2H 2 the distribution of oxygen in 

 the products of combustion was, under the best conditions, 

 97 # 1 per cent, combined with methane, 2*9 per cent, with 

 hydrogen — a ratio of 33*4. The molecular weight of methane 

 is 16, with which four atoms of oxygen of total weight 64 

 combine, the product of these being 1024. Hydrogen of 

 molecular weight 2 combines with oxygen of weight 16, 

 giving a product of 32. If the affinity of these gases for 

 oxygen is proportional to the product of their combining 

 masses, the affinity of methane for oxygen relative to that 

 of hydrogen should be 1024/32 = 32. The coincidence 

 between, this and Bone's observed value, the only direct 

 determination available, is remarkably close. The case of 

 carbon monoxide is complicated by oxygen already in com- 

 bination, and by the uncertain role of steam in its explosion 

 with hydrogen present. 



3. In an explosive mixture before ignition molecules 

 collide without liberation of heat ; but when activated in the 

 wave front they combine with increased kinetic energy of 

 which the heat set free is a measure. This old conception 

 of the source of heat in combustion has been criticised as 

 inadequate, but it has the merit of giving a clear picture of 

 the commencement of the process, and the only way in which 

 it can be extended is by further consideration of the velocities 

 of combination. It takes no account of changes of heats of 

 formation. 



Let two masses, m 1? ?n 2 , of combustible gas and oxygen 

 respectively, combine. Their kinetic energies at collision 

 are ^m 1 v 1 2 = JAj and ±m 2 v 2 2 = Jh 2 , where A lt h 2 are the heat 

 equivalents of the translational energies of each and v\, v 2 

 the components of velocity due to attraction. Since the force 

 of combination is applied equally to both, li 1 = h 2 = h say. 

 The total heat H contained in the products of combustion 

 is 2/i, or, if there is suppression of heat at collision, 2kh, 

 where k is less than unity. 



The product 



m 1 m 2 



V\v 2 J \ViV 2 J 



It has been shown * that H is proportional to m 2 , the mass 

 of oxygen burnt, in the ratio H/??i 2 = 3-31. The heat of 

 ■combustion H is also proportional to m 1? the mass of the 

 combustible molecule, and on an average for organic com- 

 pounds H/?/i 1 = ll-6. It follows that H 2 /^i??i 2 and (k/v^) 2 



* " The Relation of Oxygen to the Heat of Combustion of Organic 

 Compounds," Phil. Mag. vol. xxxiii. p : 196, Feb. 1917. 



F 2 



