ss 
* (Nid 
ss ee 
454 
- We have thus found five independent values of the heat of formation 
of silver oxide, varying from 59 K to 74 K. These values all include 
the external work. In order to obtain the mere change of internal 
energy, 3 K must be subtracted from each of the above numbers. We 
have then, 
(a) 56 
(b) 67 
(c) 65 
(d) 63 
(e) 71 
The average of these values is 64 K, which happens to be precisely 
the value which we obtained in the preceding section. ‘This very satis- 
factory agreement proves conclusively that the silver oxide which exists 
between 302° and 445° is, from a thermo-dynamic point of view, iden- 
tical with that which exists at ordinary temperature. 
DECOMPOSITION PRESSURE OF SILVER OXIDE AT 25°. 
We are now able to calculate from equation 5 the equilibrium pressure 
between silver oxide, silver, and oxygen at 25°. As data we will take 
20.5 atmospheres as the pressure at 302°; 64 K as the heat of formation 
of silver oxide at 25°; and for C the value 3, as before. The third term 
of the equation containing V may be here neglected. We thus obtain 
Po, = 4.9°10 — atmospheres. 
When we consider the question of how much this value is affected by 
possible errors in the data used, we find that errors in pz. and in C have 
much less influence than an error in U,,. An error of 1 K in the latter 
would cause an error of nearly 20 per cent in p,;. In the following 
paper I shall show that from this value of the decomposition pressure of 
silver oxide at 25° it is possible to calculate the electrolytic potential of 
oxygen and that the value thus obtained differs very considerably from 
the one at present accepted. 
SUMMARY. 
The equilibrium pressures in the system, silver oxide, silver and oxygen, 
at the temperatures 302°, 325°, and 445° are shown to be respectively 
20.5, 32 and 207 atmospheres. 
It is proved that no silver suboxide was present in any of the experi- 
ments and that probably it is incapable of existence at these temper- 
atures. 
From the change of equilibrium pressure with the temperature, the 
heat of formation of silver oxide is found to be 64 K (Ostwald calories). 
The mean of five determinations of this quantity based on calorimetric 
data is likewise 64 K. 
The decomposition pressure of silver oxide at 25° is calculated to be 
about 4.0¢10 —* atmospheres. 
