the Constitution of Carbon Compounds. 75 



always to denote combination of the elements between which 

 it is placed in the proportions indicated by the symbols. This 

 disregard of molecular composition has undoubtedly served to 

 mislead. But the discussion of the amounts of energy to be 

 expended in the separation of atoms is an important part of 

 the present work ; and for this purpose Thomsen introduces 

 the practice of representing atomic proportions of elements by 

 means of small letters . Thus C is used by him as representing 

 carbon as we know it, and c to indicate a carbon atom; so that 

 if/. C denote the heat of combustion of ordinary (amorphous) 

 carbon,/, c is to be taken as denoting the heat of combustion 

 of carbon in the atomic condition. This appears to me to be 

 a most undesirable and unnecessary practice, and of set pur- 

 pose I have not adopted it, but have preferred instead to write 

 the equations so as to indicate the molecular composition of 

 the gases dealt with. It is far better to use C* as the symbol 

 of carbon of unknown molecular composition and thus remind 

 the reader of our ignorance; and Cx is the proper symbol of a 

 monatomic carbon molecule *. 



(2) Difference in the Heats of Combustion of Homologues. — 

 The first noteworthy conclusion arrived at by Thomsen is that 

 the heat of combustion increases from term to term in a series 

 of homologous compounds by an almost constant amount: on 

 an average by 157897 units. Of the forty-four differences 

 cited (Table I.), the minimum is 155120 and the maximum 

 159563, only five departing by more than one per cent, from 

 the mean f. 



* The failure to realize the importance of molecular composition in 

 relation to chemical change, which unfortunately is so clearly traceable 

 in our chemical literature, probably is in a great measure due to our want 

 of system in writing the formulas of elements. I have more than once 

 urged that an index should be used after the symbol of an element only 

 to denote the number of its atoms in the molecule. If this practice be 

 consistently followed and the index be used also in the case of monatomic 

 elements, the absence of an index after the symbol of an element would 

 at once serve to indicate that the molecular composition of the particular 

 element (as gas) is unknown. 



t It will be noted that all the compounds included in the Table are 

 members of the paraffinoid class; also that the comparison is made be- 

 tween closely related bodies, a large majority being true homologues, i. e. 

 bodies of the same type formed from one another by the mere introduc- 

 tion of CH 3 in place of H. If the heats of combustion of the benzenes 

 examined by Thomsen be compared, it will be observed that whereas the 

 difference between benzene and toluene 



C 7 H a -C 6 H G =955680-799350 = 156330, 



is very near the average value (157897), the difference between toluene 

 and pseudocumene or mesitylene, 



1281510—955680 = 2 . 162915, [continued, p. 78. 



G2 



