December 12, 1919] 



SCIENCE 



545 



adding a component to the systems under dis- 

 cussion may be deduced. In the first case 

 suppose we had both rhombic and monoclinic 

 sulfur coexisting, and suppose, keeping the 

 temperature and pressure constant, we added 

 another component. The total quantity of 

 crystalline sulfur would probably be changed 

 but the crystalline sulfur remaining (if any) 

 would consist of crystals of both the rhombic 

 and monoclinic forms. In the second case 

 suppose we had a saturated solution of sodium 

 sulfate and sodium sulfate decahydrate and, 

 again keeping the temperature and pressure 

 fixed, we added a third component. The 

 sodium sulfate decahydrate would promptly 

 disappear and only a saturated solution of 

 the anhydrous salt, together with anhydrous 

 salt, would be left. This will probably appear 

 clearer after a discussion of the diagram 

 given in Fig. 1. 



Pig. 1. 



Let B in Fig. 1 represent the composition, 

 on one of the side lines of a three-component 

 triangular diagram, of a hydrated salt, and A 

 the composition of the anhydrous form. CE 

 represents the boundary line between the 

 fields of A and B and the temperature falls 

 from C to E as shown by the arrow, l^ow 



suppose we take a saturated solution of B in 

 contact with crystals of B, with total com- 

 position represented by X and with the com- 

 position of the solution represented by Y. 

 The isotherms (saturated solutions of A and 

 B) on the concentration diagram are shown 

 by the lines running from F to F^ to Y^- If 

 now to X we add the third component the 

 total composition will vary along XX^. While 

 the total composition varies from X to X„ the 

 hydrated salt will persist, and while the total 

 composition varies from X, to X^ both salts 

 and the solution Fj^ will be obtained, but 

 when the composition passes Xg only the an- 

 hydrous salt and a solution will be obtained.^ 



Enough has been given, I think, to demon- 

 strate the essential differences in the char- 

 acter of the phenomena under discussion and 

 I would suggest' that the term inversion be 

 restricted to phenomena like the change of 

 rhombic to monoclinic sulfur, and the term 

 transition to phenomena similar in type to 

 the dehydration of sodium sulfate decahy- 

 drate, and that in cases in which the com- 

 position of all the phases concerned may be 

 represented by simple chemical formulas, the 

 word dissociation be employed. 



In conclusion I should like to draw atten- 

 tion to a distinction made by Findley^ be- 

 tween transitions in the solid state and melt- 

 ing point phenomena. He states : 



The transition point, however, differs in so far 

 from a point of fusion, that while it is possible to 

 supercool a liquid, no definite case is known where 

 the solid has been heated above the triple point 

 without passing into the liquid state. Transfor- 

 mation, therefore, is suspended only on one side 

 of the melting point. In the case of two solid 

 phases, however, the transition point can be over- 

 stepped in both directions, so that each phase can 

 be obtained in the metastable condition. 



This has been interpreted by many to mean 

 that a crystalline substance can not exist even 



6 See Roozeboom, "Die Heterogenen Gleichge- 

 wichte, " III., part 1, page 190. 



'Writers like Findlay and W. C. MoC. Lewis 

 use the words "transition," "inversion," and 

 "transformation" synonymously, while Bancroft 

 in denoting such phenomena uses the word "in- 

 version ' ' almost exclusively. 



8 "The Phase Eule," p. 37. 



