NEW LAW IN THERMOCHEMISTRY 3 



dition. These heats of formation are evidently not absolute, for 

 they involve the uncertain factors due to changes of physical 

 state ; they are, therefore, as I have already shown, complexes 

 rather than constants. Moreover, much of Thomsen's reason- 

 ing depends upon hypotheses which have been more or less 

 questioned, so that although his conclusions are highly interest- 

 ing, they have not won universal acceptance. This is espe- 

 cially true with respect to the thermal significance of single, 

 double and triple unions between atoms of carbon ; and on this 

 subject Thomsen's views have met with a good deal of criticism. 

 Nevertheless, he points out many remarkable and striking 

 thermochemical relations, though their full meaning is yet to be 

 elucidated. 



In one respect, Thomsen's data, as he has given them, are im- 

 perfectly suited to discussion. Although he starts each combus- 

 tion with gas and ends parti}- with gas, the water produced is 

 always in the liquid condition. For the best consideration, the 

 process should deal with gases throughout, both for substance and 

 for products ; then only can extraneous physical disturbances 

 be practically eliminated, and the chemical part of the phenom- 

 enon be studied with the fewest complications. Fortunately, the 

 obvious correction can be easily applied, at least approximately ; 

 and when that is done a new order of regularities appears. 



According to Thomsen ' the heat of formation of one gramme 

 molecule of liquid water from its elements at i8°, is 68357 

 calories. For gaseous water the value varies with temperature, 

 and may be represented at 100° by the quantity 58069. At 

 18° — the standard temperature of the experiments — this figure 

 reduces to 57 934. The difference in heat of formation between 

 gaseous and liquid water, then, is 10423 calories; and this 

 quantity should be subtracted from the heat of combustion of 

 any organic substance as many times as there are molecules 

 of water produced. Thus, for the heat of combustion of meth- 

 ane, CH^, Thomsen gives 211 930 calories. Two molecules of 

 water are formed; we subtract, therefore, twice 10423, and 

 the value for gaseous substances throughout becomes 191 084. 

 One gramme molecule of methane burning as gas, and with all 



» Therm. Unt., vol. 11, pp. 52-56. 



