2 CLARKE 



the final result being the sum of at least five independent factors. 

 These distinct quantities are not, as yet, separately measurable, 

 their relative importance is unknown ; and to many chemists, 

 for reasons like these, the problems of thermochemistry have 

 seemed to be hopelessly complex. Few general conclusions of 

 unimpeachable validity have been developed by thermochemical 

 research, and so, of late years, the entire subject has fallen 

 somewhat into disfavor. One constant, however, has attracted 

 almost universal attention — the neutralization constant of acids 

 by bases — and this, having an approximate value of 13700 

 calories, represents the union of hydrogen and hydroxyl ions 

 to form water. This constant has strengthened the theory of 

 electrolytic dissociation ; and I hope now to show that it has 

 still wider significance. 



In the fourth volume of his classical Thermochemische Untersuch- 

 ungen, Thomsen gives data relative to the heat of combustion of 

 1 18 organic compounds. These data have peculiar value, for the 

 reason that they are strictly comparable throughout. Every sub- 

 stance burned was taken in the state of gas, at constant pressure, 

 with the measurements finally reduced to a uniform temperature 

 of 18°. The products of combustion were also gaseous, except 

 in the case of the water, which was of course in liquid form. 

 Upon careful scrutiny, the results obtained exhibit systematic 

 regularities of homologous character ; and this fact, which is 

 elaborately discussed by Thomsen, tends to increase confidence 

 in the accuracy of his data. Other observers, doubtless, have 

 done their work equally well ; but no other body of thermo- 

 chemical measurements known to me is so homogeneous and so 

 self-consistent as this one. Furthermore, Thomsen gives abun- 

 dant details concerning every determination ; and, as a rule, 

 each quantity was measured several times. 



In reducing his observations, Thomsen uses the socalled " heat 

 of formation " of carbon dioxide from solid, amorphous car- 

 bon, and that of water in the liquid form. With these values, 

 by methods which are so well known as to need no description 

 here, he computes the heats of formation of the different com- 

 pounds studied ; but always on the supposition that the reactions 

 start from the several elements in their normal molecular con- 



