1909-10.] Isothermal Change of State. 469 
which show that, at a pressure a little below the triple-point pressure, 
water evaporates at a lower temperature than ice, demonstrate the existence 
of the two courses between a and /. No ambiguity comes in at these points. 
Thus, in passing from B to / and onwards, the course is to e if solidification 
nuclei are present; it is to d if solidification nuclei are absent, while 
lr 
liquefaction nuclei are present. But, if we postulate Thomson’s waved 
form of isothermal between a and b, c and d, e and /, difficulty appears at 
the points c and d. The parts a c and / d are portions respectively of the 
waved isothermals joining a and b, f and e. From d, or beyond it, two 
waved isothermals must proceed respectively to the points e and c ; from c, 
or beyond it, two waved isothermals must proceed respectively to the points 
d and b ; and, in either case, the two isothermals correspond to the absence 
