RICHARDS. — CHANGING HEAT CAPACITY. 299 



of the remaining data cannot be utilized at present because of a lack in 

 each case of one or two of the necessary figures. 



More complicated cells, in which also the osmotic tendencies are nearly 

 balanced, show the same tendency. For example, in the cell Hg, HgCl, 

 KCL KN0 3 , KBr, HgBr, Hg, which has been studied by Bugarszky,* 

 there is a loss of heat capacity of about 26 mayers per gram equivalent 

 during the reaction, while the observed heat energy exceeds the electro- 

 motive energy by 4.3 kilo-joules. When the iodides are substituted for 

 the bromides, the loss of heat capacity is 13 mayers, and the electro- 

 motive energy is less than the heat energy by 8.2 kilojoules. On the 

 other hand, when mercurous oxide and potassic hydroxide are combined 

 with the calomel electrode, a cooling reaction yields an electromotive 

 energy algebraically 23.5 kilojoules greater than the heat energy, while 

 the heat capacity gains 44 mayers. These figures are of the same order 

 of magnitude as before, and in the expected direction. They are not 

 included in the table because in them the osmotic energy may amount to 

 an appreciable quantity, which cannot be wholly determined until the 

 solubilities of the nearly insoluble salts are known. 



Among all the cells which have been studied, including nearly if not 

 quite all of those for which even moderately accurate data exist, only the 

 cells containing aluminum as one of the metals disagree with the generali- 

 zation. The solutions of salts of aluminum have usually a very large 

 heat capacity, but yet the heat evolved is greater than the electromotive 

 energy. Great discrepancies exist in the determination of this last quan- 

 tity, however ; for Wright and Thompson give as the potential of the 

 cadmium-aluminum chloride cell only 0.05 volt f» while .Neumann's t 

 values indicate 0.87 volt. It is not impossible that even the latter may 

 be too low ; for aluminum may be like chromium in its anomalous elec- 

 trochemical behavior. § In the light of the discrepancy it seems permis- 

 sible to reject the cells containingaluminum until more certain knowledge is 

 obtained. From the accepted data two important connected inferences may 

 be drawn with considerable security. .In the first place, it appears that un- 

 changing heat capacity is an essential condition in determining the equality 

 of the free electrical and the total heat energy changes, in reactions from 



* Bugarszky, Zeitschr. anorg. Chem., 14, 145 (1897). 



t Wright and Thompson, Phil. Mag. [4], 19, 117 (1885). 



t Neumann, Zeitschr. phys. Chem., 14, 225 (1894). 



§ The use of aluminum as a current rectifier is well known (Graetz, Wied. Ann., 

 62, 189 ; Pollack, Zeitschr. phys. Chem., 24, 546.). The same question is concerned 

 in this phenomenon. 



