To the Editor: I n reply t o the proposals of James E. McDonald (J. CHEM.EDUC., 35, 205 (1958)) that the words "suhlimation" and "deposition" he used to denote solid-tovapor and vapor-to-solid phase transitions, respectively, I would like t o make the proposal that these phase transitions be termed SG- and GS-transitions, respectively. I n other terms, by use of the initial letters S, F, G t o denote the solid, fluid, and gaseous states (initials also appropriate in French and German) it is possible to represent in a perfectly clear way the six possible common types of transition bet~veenthe three states of matter. The continuance of the use of the word "sublimation" is open to objection on several grounds; it is ambiguous, since it may mean (to physicists and some chemists) the SG-transition, or (to meteorologists) the GS-transition, or (to alchemists and chemists) the SGScycle of transitions, as James E. McDonald (loc. n't.) clearly points out; it surely means t o many scientists the formation of a sublimate, by analogy with such terms as freezing (the formation of frost), or melt-
ing (the formation of a melt), vaporization (the formation of a vapor), or condensation (the formation of a condensate) and in this sense is synonymous with deposition (the formation of a deposit). James E. McDonald also would have us retain the use of "freezing" t o signify the FS-transition, but it is really only the formation of frost; it would he just as correct to call all GS-transitions "riming" (the formation of rime) or "hoar freezing" (the formation of hoar frost) or t o call all SF-transitions "thawing." The word "evaporation" is clearly ambiguous, since it means the formation of a vapor and so applies equally t o FGand SG-transitions. The proposal to use initial letters is easily extended to possible or actual phase transitions involving diierent phases of the same kind; a solid-solid transition is an S&-transition; a meritectic transition (binary incongruent melting) and ternary semicongruent melting are &(SzF)-transitions; binary incongruent vaporization (Smith, Norman O., J. CHEM.EDDC., 35, 128 (1958)) is an S1(SzG)-transition; ternary incongruent melting is an S1(SzS3F)-transition. The proposal is also suited t o deal with triple-point phase transitions (FINDLAY,A,, A. N. CAMPBELL, AND N. 0. SMITH, "The Phase Rule and its Applications," Dover Publications Inc., New York, 1951, p. 34); for example, the four triple points in the sulfur system involve the phases S,S,,,F, S.S,G, S,FG, S,FG, where the subscripts o and m denote the orthorhombic and monoclinic forms of sulfur. G. N. COPLEY
JOURNAL OF CHEMICAL EDUCATION
