24 CLARKE 



What now, does the correction— 12 mean? If we write the 

 equations of combustion for the several compounds and evaluate 

 r, we shall find in every case that the absolute heat of formation, 

 in henotherms, is 2« + 3. Thus for benzene, if n=i2, we 

 have 2« = 24, and 2^ + 3 = 27. This constant increase un-, 

 doubtedly is connected with the structure of the aromatic ring, 

 and may be interpreted in either of two ways. Either the three 

 double bonds of the Kekule ring have higher thermal values 

 than usual, being three henotherms instead of two henotherms 

 each, or else there are three cross linkings, as in the centric 

 formula, thermally equal to one henotherm apiece. The first 

 explanation is negatived by the evidence regarding double and 

 triple bonds which is furnished by the aliphatic compounds ; the 

 centric explanation, therefore, seems to be the more probable. 

 The latter, however, is difficult to apply in the case of thiophene, 

 and so, for the time being, the question must be left open. 



In piperidine, C^H^jN, we have a seemingly exceptional com- 

 pound. The heat of combustion is given by Thomsen as 

 883790 calories. For gaseous substances throughout this be- 

 comes 777464, and ^/i= 3 105856. The divisor, with the cor- 

 rection of — 12 applied is 211, and hence the quotient is 14720. 

 If, however, we neglect the correction, and accept 223 as the 

 normal divisor, we have a quotient of 13927, which is in har- 

 mony with the other regular values. That is, piperidine, re- 

 garded as pyridine hexhydride, has no double bonds nor cross 

 linkings, and it responds to the general formula exactly like the 

 amines. If we include its normal quotient with the others of 

 the aromatic group, the mean of all the eleven compounds be- 

 comes 13795. This mean will be adopted in the final sum- 

 mation. ' • 



Three compounds still remain to be studied, and these form 

 a group by themselves. They are carbon monoxide, CO, car- 

 bonyl chloride, COCI2, and carbonyl sulphide, COS. Since 

 they contain oxygen, they are only in part dissociated when 

 burned, and their thermal interpretation is, therefore, difficult. 

 Their heats of combustion, as given by Thomsen, are as follows : 



