750 
On the contrary the value substituted for R may be in error to 
a considerable extent. A change of a full unit however in this 
value (one eighth of the amount) produces a change in w of about 
14°/, only. So we may perhaps conclude that the value of w will not 
differ much from 1 and that the values found for the vapour pressure 
of mercury agree in a rather satisfactory way with the theory of 
Terrope, if we give the elementary domain ( the value hey, 
Nevertheless, in ‘my opinion, we may not attach much value to 
this result. Besides the difficulties which we pointed out already 
there is still another serious objection. 
Formula (15) connects the vapour pressure with the entropy- 
difference between gaseous and fluid mercury or, when we take 
into account the relation (19), with the difference between gaseous 
and solid mereury. Now we must doubt seriously whether this 
difference can be rightly evaluated if the undeterminate constants in 
S and S" are fixed in the above mentioned rather arbitrary way. 
On the ground of Bourzmany’s formula we may account for the 
entropy S", viz. for the change which the entropy of solid mercury 
undergoes when heated from 0° to 7°: to this effect we have to 
compare the probabilities of different states of the solid mercury. 
This is done eg. by Desye in his theory of specific heats. In this 
comparison we are concerned only with quantities referring to the 
solid state, e.g. the modulus of elasticity. In the deduction of (6), 
on the other hand, only the gaseous state has been considered. The 
question arises whether it will be possible, by a combination of 
these results, to determine the difference S—S", which according 
1) The ‘objection might be raised that in the above calculation Herrz’s formula 
for the vapour pressure has been applied for a temperature at which this pressure 
has never been measured. In reality however the value of w given by (16), (17) 
and (18) is independent of the choice of the temperature. 
Indeed, the differential coefficient of the right hand side of (16) with respect to T'is 
1 ds! ? (7 / ) uf 
— ———— og p — 
par ear lot aT 
dn 
a well known thermodynamic theorem this quantity must be zero. In virtue of 
(18) it becomes P 
LVS) AaB 5) 
BAT TL 
and this expression really is zero because Hertz has chosen the coefficient (3 in 
accordance with the theorem in question. 
gow ‘ 
CG is the specific heat of the fluid under its vapour pressure) According to 
