1174 
tomic gas‘), from thermochemicat data; they have found with the 
atmosphere as pressure unity: 
C= — 1,308 
with an uncertainty which they estimate at most at + 0,15. They 
calculate from (16): 
C= — 1,17, 
which may be considered as a very satisfactory agreement. 
On the other hand QO. Srern*) has also derived the entropy of 
monatomic gaseous iodine from thermochemical data, and found a 
value which very greatly deviates from that following from (16). 
He infers from this that either the heat theorem is not valid for 
the reaction 2I,oia@ 2 Isola, that therefore the difference of entropy 
remains finite for 7’=0, or that the vapour tension of monatomic 
iodine cannot be accurately calculated with the aid of (16). He 
seems to think the former rather probable. It seems to me that they 
come to the same thing. It may namely be very well the case 
that the heat theorem only holds for substances that really exist, 
and this eannot be said of monatomic solid iodine. Then the calcu- 
lation of the vapour tension as we have carried it out in this 9, 
has no longer any meaning: monatomic gaseous iodine cannot exist 
at low temperatures either. The formulae of this and the following 
§§ for the entropy constant can lay claim to validity only for such 
substances as also occur in the same molecular form at low tempe- 
ratures, as the gases of the He group, the metal vapours, further 
also gases as H,,O,,CO, 1, ete.; but not gases as I, Br or such like 
ones. Of course the possibility continues to exist that an unexpeetedly 
great error occurs in the data used by STERN. 
§ 4. On the vapour pressure of a diatomic solid substance and 
the entropy constant of the gas. 
In a corresponding way the vapour pressure of a diatomic sub- 
stance and the entropy of the vapour can be caleulated, when it is 
assumed that here too the atom motions censist of sine vibrations, 
while moreover the two atoms of one molecule are always at a 
definite distance from each other*) as they also are in the gas for 
1) A. Eucken, Sitz. Ber. Berl. Akad., 1 Febr. 1912. 
2) O. Stern, Ann. d. Phys. 44, 497 (1914). 
8) When the possibility of existence of the solid substance falls below the region 
within which classical mechanics may still be applied to the rotation of the mole- 
cules, the calculation has of course no direct meaning, and it will be preferred 
to follow another method; see § 5. 
A possible mutual vibration of the aloms with a zero point energy ghy would 
