1891 - 92 .] Dr W. Peddie on Transformation of Energy. 257 
But \dl = Od(f) , 
so dO = dp ^^ — 
A per 
which gives the change of the boiling- point dependent on the 
change dp of pressure. The result applies also to the change in the 
melting-point, p being the density of the solid. 
III. Eeat and Electric Energy . — Let I and i represent respec- 
tively electromotive force and quantity of electricity. Then d\di 
and dQd^ both represent energy supplied to the system, so that 
dldi - d6drf) = 0^ whence 
Odcj> _ ^dl 
~dT~ ~^Te 
represents the amount of heat which must be withdrawn from the 
system (say a voltaic cell), during the passage of unit quantity of 
electricity, in order that the temperature may remain constant. 
This is Helmholtz’s well-known expression. 
If I represents the (internal) electromotive force in a thermo- 
electric circuit we get 
dl Odej) _ 7T 
dO B di 0 
where tt is the heat absorbed at the junction when unit quantity of 
electricity passes across it at temperature 0. 
IV. Electrical Energy and Work . — Consider a sphere of capacity 
C charged to potential V. The electrical energy is JCV^. Hence 
dVdfyV) -f dpdv = 0, and 
, ..jdKWf) 
dp = dY \ ^ = 
dv dv 
since we have to take the value of d{CY)jdv when V is constant, 
How = f 7 tC^, whence 
dp = YdVj^. 
If Q be the total charge of electricity 
dp = 
QrfQ 
4tC^‘ 
VOL. XIX. 
22 / 2 / 93 . 
R 
