406 Dr. C. R. A. Wright on the Relations between 



comparing these with the respective dissected formulae of the 

 substances in question correlations may a priori be anticipated. 



10. It is proposed thus to compare together the affinities per 

 metrogramme in various bodies all denoted by the general for- 

 mula C m H re O p , i. e. all capable of forming carbon dioxide and 

 water and nothing else by the continued action of oxygen, and 



, . , , „ 5> \2m + n + 16p 

 each one having a vapour-density ot o = -• 



It is, however, impossible in this case to compare the affinities 

 at such a uniform temperature that all the bodies concerned are 

 gaseous, carbon being non-volatile at all measurable tempera- 

 tures. Hence the affinity- values are calculated at some uniform 

 temperature T above or close to the boiling-points of all the 

 bodies concerned, the compound vapours being considered to be 

 under a constant pressure of 760 millims., and the constituents 

 carbon, hydrogen, and oxygen under such a pressure as to occupy 

 jointly the same volume as the compound vapour in each case 

 respectively, the carbon being considered as solid (and hence 

 occupying a space so small as practically to be negligible), and 

 the hydrogen and oxygen gaseous. 



100° C. is taken as the constant temperature T, as being prac- 

 tically convenient and sufficiently near to the boiling-points of 

 most of the substances examined to introduce but little error 

 into the calculations from incorrect estimates of latent heat of 

 vapours, &c. To avoid multiplicity of figures, a unit of heat is 

 taken equal to 1000 calories. 



11. To calculate the reduced values of 2S 1 F 1 , 2S 2 F 2 , &c. as 

 thus defined, the following data are employed: — 



A. By means of the calorimeter it is found that when one 

 gramme of a given body, C m H n P , is burnt with excess of 

 oxygen so as to form gaseous carbon dioxide and liquid water, 

 Hj heat-units are evolved, the materials and products being all 

 examined at 15° C. and 760 millims. The values of Hj are chiefly 

 taken from the experiments of Favre and Silbermann, the 

 results of Dulong, Andrews, and Thomsen being also used, and 

 the mean value being taken when different results are given by 

 different authorities. 



B. By means of the calorimeter it is found that, when one 

 gramme of solid carbon is burnt with excess of oxygen to form 

 carbon dioxide, h Y heat-units are evolved, the materials and pro- 

 ducts being all examined at 15° and 760 millims. ; the value of 



given vapour, i. e. is the weight indicated by the rational formula of the 

 body in question. The term is suggested as a substitute for "molecule " 

 in cases where no question arises as to theoretical views as to the constitu- 

 tion of matter: vide 'Chemical News/ July 18, 1873, p. 25. 



