120 DBS. L. MONO, W. RAMSAY, AND J. SHIELDS, ON THE 



the platinum plate was replaced by palladium, so that the heat evolved on the 

 occlusion of the hydrogen was also included. 



Independent of direct measurement in the calorimeter, the heat of a reaction can 

 be calculated, as was first shown by HORSTMANN (' Berichte,' 1869, vol. 2, p. 137), 

 by an application of the second law of thermodynamics to the dissociation pressures 

 of the substance at different temperatures. From measurements of the dissociation 

 pressures of palladium hydrogen by TROOST and HAUTEFEUILLE ('Ann. Chim. Phys.,' 

 (5) vol. 2, p. 279), MOUTIER (' Comptes Rend.,' vol. 79, p. 1242) calculated that the 

 heat of occlusion of hydrogen by palladium at 20 C. was 4T5 K. per gram, and 

 moro recently DEWAR (' Proc. Chem. Soc.,' 1897, No. 183, p. 197) obtained, from 

 similar measurements by PtOOZEBOOM (HoiTSEMA, ' Zeitschr. Physikal. Chem.,' 1895, 

 vol. 17, p. 1), the value (45'61 + 0'2378 T.) K. At C. (273 abs.), therefore, the 

 heat evolved per gram of hydrogen occluded becomes 46'26 K., which agrees very 

 closely with the value, 4G'44, found by us. The magnitude which is directly 

 measured in the calorimeter, however, represents the true heat of the reaction plus 

 the heat corresponding to the work done by the atmosphere, viz., 27 K.,* and hence 

 the true heat of occlusion of hydrogen by palladium black is only 46'4 27 = 437 K. 

 per gram of hydrogen. 



VIII. Investigation of the Ratio Palladium : Hydrogen. 



From the analysis of palladium black (v. p. 108) it follows that the black as 

 prepared contained 97'63 per cent, palladium, the remainder consisting of 1'65 per 

 cent, oxygen and 072 per cent, water. Hence, in calculating the ratio of 

 the number of atoms of palladium to the number of atoms of hydrogen in palladium 

 fully charged with hydrogen, we ought to reduce the weighed amount of palladium 

 black to palladium metal per se. Strictly speaking, this ought also to be done in 

 expressing the number of volumes of hydrogen occluded by unit volume of palladium 

 existing in the substance we call palladium black. 



The following table contains the atomic ratio of palladium to hydrogen for fully 

 charged palladium black, sponge, and foil (the atomic weight of palladium being 

 taken as 107), and also the corrected number of volumes of hydrogen occluded by 

 unit volume of palladium : 



V. p. 123. 



