POLYMORPHIC TRANSFORMATIONS OF SOLIDS. 



61 



always positive; hence Ar is negative, and the transition line is de- 

 pressed. 



The procedure is easily modified so as to give the pressure shift of 

 the equilibrium line. Let now the pressures only be altered. The 

 pressure on (2) is increased by Ai>y, and that on (1) by Api. (See 

 Fig. 3.) Under these conditions pure (1) at p + Api, t is to be in 



Figure 3. Showing the displacement of pressure produced by impurities. 



equilibrium .with pure (2) at p + Api, r. If now we choose Ap-2 — 

 Api, equal to the osmotic pressure of the impurity dissolved in (1), 

 then we have impure (1) at p + Ap2, r in equilibrium with pure (1) 

 at p + Api, T, and hence in equilibrium with pure (2) at p + Ap2, r. 

 That is, Api gives the pressure shift of the transition line. 

 We have 



or 



also 



Whence 



AZi = AZ.2 



ro Ap2 = ("1 Api 

 Ap2 — Api = Ap 



(Ap = osmotic pressure) 



Ap2 



l\ 



Ap 



This gives the pressure shift (= Ap2) of the line due to impurity. It 



might haxe been obtained from the value given above for the tempera- 



.. . r/r 



ture shift 1)\" putting Ar = -— Ap, but the method of deduction there 



given did not make it evident that this substitution would be allowable 



(It . . . 



when -T- = oo , as it is in those cases to whicli we wish to applv the 

 dp 



