Atomic Laws of Thermochemistry. 31 



of C0 2 by 121*1 ; but the heat of combustion of C0 2 is 

 nothing, therefore the heat of combustion of OC = CO would 

 be 121*1. Now this compound may be supposed to be burnt 

 by being first split into two molecules of CO, which will 

 double the gaseous volume, and then being burnt as 2C0, 

 which is known to give out 135*9 kcal. Accordingly 135*9 

 minus *58 for the doubling of the volume minus v 2 for break- 

 ing the double binding must be equal to 121*1, or, in symbols 

 121*1 = 135*3-r 2 , that is, 121*1 + u 2 = 135*3. But by section 3, 

 121*1 +v 2 is/.c, the heat of combustion of an isolated carbon 

 atom. Thus we have the two results, 



v 2 =U% 



/c=135*3. 



This curious piece of reasoning appears to have furnished 

 the values of two fundamental thermochemical constants by 

 the artifice of imagining an unknown compound OC = CO 

 and asserting that the heat of combustion of C0 2 is nothing. 

 These processes involve assumptions which may or may not 

 be legitimate or necessary ; bat it is certainly necessary that 

 we should know what they are, and realize what we are com- 

 mitting ourselves to. We can do so by stating in our symbols 

 the steps of the reasoning. The combustion of 2C0 to 

 produce 2C0 2 at constant volume is expressed by 



2(0) + 2/(C0 2 )-2/(CO) = 135-3; ... (6) 



and from equation (2), 



2(0)+/(C0 2 )-/(C:C)=m-l; 

 hence 



/(C0 2 )-2/(CO)+/(C:C)==14-2. ... (7) 



This corresponds to Thomson's assertion v 2 = 14*2 • and as his 

 v 2 is identical with/(C: C), we see that his equation v 2 = 14*2 

 is true only on condition that /(C0 2 ) = 2/(CO). This is a 

 legitimate assumption to make, if it is carefully tested by its 

 consequences. If it is true, then /(C: C) = 14*2, and equa- 

 tion (6) reduces to the form 2(0) +/(C0 2 ) =135*3, which 

 Thomsen calls the heat of combustion of the supposed isolated 

 carbon atom. But it is not so : it is only a portion of that 

 heat the whole of which in our notation is (C) + 2(0) +/(C0 2 ). 

 In Part I. we saw that (CI) = (Br) = (I) = 0, and supposed 

 this to be due to the fact that in the molecule of CI two atoms 

 are combined. If this is the true reason (0) is also zero, and 

 then 



(/C0 2 ) = 135*3 =2/(C0). .... (la) 



