544 



SCIENCE 



[N. S. Vol. L. No. 1302 



these splendid opportunities might well be 

 made available for the holders of such fellow- 

 ships as I have suggested. 



These fellowships should not be restricted to 

 the purely scientific branches. What better 

 place than Washington for historical research ? 

 Where else than in the Department of Labor 

 with such a division as the Children's Bureau 

 could certain social problems be studied to 

 better advantage? Yet it is probable that the 

 majority of such fellowships would be in con- 

 nection with the scientific activities of the 

 government. For that reason it is from our 

 national scientific societies that the initial im- 

 pulse for a movement leading to the establish- 

 ment of such an institution must come. 



G. F. Ferris 



SPECIAL ARTICLES 



THE TEEM " INVERSION " 



The diversity among the phenomena which 

 are referred to by the term " inversion " is 

 so great that at present the word has lost any 

 precise meaning which it may have had in the 

 I)ast. The organic chemist uses it when dis- 

 cussing the behavior of the allotropic forms 

 of a substance and also when alluding to that 

 well-known single-phase phenomenon, the in- 

 version of cane sugar; while the inorganic 

 chemist generously uses it to denote, in ad- 

 dition to the meanings already referred to, 

 such a phenomenon as an incongruent melting. 

 The writer is not an organic chemist and 

 therefore does not vsdsh to criticize the ter- 

 minology used by the organic chemists, but 

 feels in duty bound to protest against the 

 present use of the term " inversion " by most 

 inorganic chemists. 



When an inorganic chemist hears the term 

 "inversion" used, he invariably associates it 

 with phenomena like the change of rhombic 

 to monoclinic sulfur'^ or phenomena like the 

 thermal dehydration of sodium sulfate deca- 

 hydrate.^ The first use of the term is, I 

 think, a very satisfactory one, but for the 

 second type of phenomena a difi^erent term 

 should be employed, since otherwise there can 



iFindlay, "Phase Rule," page 34 (1908). 

 2 Hid., p. 138. 



not but result an overlooking of the impor- 

 tant temperature interval which character- 

 izes this type of " inversion " in systems of 

 three or more components. The term " tran- 

 sition," now used synonymously with " in- 

 version," could well be confined to phenomena 

 of the second class mentioned above. 



The distinctive feature about an inversion 

 such as that of rhombic to monoclinic sulfur 

 is the fact that the inversion temperature is 

 a fixed point^ at constant pressure regardless 

 of the complexity of the system, provided no 

 solid solutions are formed, whereas in the 

 case of the transition of sodium sulfate deca- 

 hydrate to the anhydrous salt the transition 

 temperature is dependent, in addition, upon 

 the composition of the whole system. Thus 

 both hydrated and anhydrous salt can coexist 

 in the presence of suitable liquids over a wide 

 range of temperatures. The change of tran- 

 sition temperature by the addition of a third 

 component is not an isolated phenomenon but 

 rather an example of one of the types met 

 with most frequently in phase rule studies 

 of complex systems. 



On the theoretical side Bancroft^ has 

 pointed out as a corollary of the theorem of 

 Van Alkemade^ that in ternary systems at 

 constant pressure involving no solid solution 

 the temperatures at which two solid phases 

 can coexist in the presence of a suitable 

 liquid will rise as the composition of the 

 liquid approaches the side line, i. e., will be 

 a maximiun in the binary system. In other 

 words, in a system such as sodium sulfate: 

 water : sulfuric acid, the temperatures at which 

 the solid phases, sodivmi sulfate and sodium 

 sulfate decahydrate, can coexist will rise as 

 the liquid becomes less acid and be a max- 

 imum when the solution contains no free 

 acid. 



With this corollary in mind, the effect of 



3 The variable inversion temperature of cristo- 

 balite (SiO.) found by C. N. Fenner, Am. J. Sci., 

 36, 331 (1913), is a phenomenon of unstable equi- 

 librium and is hence excluded from this discussion, 

 discussion. 



4 "The Phase Rule," W. D. Bancroft, p. 149 

 (1897). 

 , .^Z. pliysilc. Chem., 12, 371 (1893). 



