DYNAMICAL PRINCIPLES TO PHYSICAL PHENOMENA. 
491 
169 x 10 2 p 
87 r er — p 
p/(<r — p) for water at atmospheric pressure is about gwo, so that 8p, the alteration in 
pressure, which equals 1W 8p, is equal to 
- 169 x 10 2 
87t x 800 
which is roughly about '87, so that the maximum change in the vapour-pressure 
which can be produced by electrification is about ttoo'o °f the vapour-pressure of 
water at 15° C. In sulphuric acid the ratio would be very much greater. 
We can calculate in a similar way the alteration in the vapour-pressure produced 
by any alteration in the state of the liquid. All we have to do is to calculate the 
change in the value of L due to this alteration. If this change be y, then we may 
prove, just as before, that 
R0S P = .(25) 
' <7 — p 
It should be noticed that 8p and dy]d£ are of the same sign, so that the presence of 
any kind of energy which causes L to increase as evaporation goes on will facilitate 
the evaporation. Thus, in the case of surface-tension, the potential energy due to 
the surface-tension diminishes as evaporation goes 011 : this corresponds to an increase 
in L, so that the surface-tension will facilitate the evaporation ; again, in the electrical 
case, the potential energy due to the electrification increases as evaporation goes on : 
this corresponds to a decrease in L, so that the electrification will tend to stop the 
evajDoration. These are only special cases of a general principle, of which we shall 
find frequent illustrations in the subsequent work. 
Dissociation. 
§ 8 . Another problem to which the method can be applied is that of a gas partly 
dissociated into two components. Let us suppose that we have a quantity of gas 
contained in a vessel whose volume is V, and that part of it is in its normal condition, 
which we shall call A, while the molecules of the rest of the gas have been split 
up: we shall call this state of the gas B. Let 17 , he the masses of the gases in 
the states A and B respectively. Then the value of L for the gas in the state A is, 
by the investigation on p. 487, 
f (a ,0 + 11,0 log 
3 E 2 
