THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES. J 



Akt. XX. — The Transition Temperature of Sodic Sulphate, 

 a New Fixed Point in Thermometry; by Theodore 

 William Richards. 



I. On the Constancy of the Point in Question. 

 With the assistance of Jesse Briggs Churchill. 



According to the Phase Rale of Willard Gibbs, in order to 

 obtain a nonvariant point in any system two more conditions 

 must be definitely fixed than the number of components in the 

 system. In other words, if the system contains only one com- 

 ponent, three conditions must be fixed; if two components, four 

 conditions, and so on. Since such a nonvariant* point is 

 invariable in temperature, as well as in every other respect, 

 there evidently exist a great number of definite temperatures 

 obtainable by the combinations of different substances. 



Of these multitudinous nonvariant points, the world uses 

 only two, the simplest possible, as the standards of thermome- 

 try. The fixed conditions are : — a definite pressure, and two 

 phases of one component, water. While undoubtedly 0° and 

 100° Centigrade will always remain the standards of reference, 

 it would be extremely convenient to have at least one definitely 

 determined point between these. Many thermometers do not 

 cover this whole range, and all are noticeably upset as to their 

 internal condition by such wide variations of temperature. 



Landolt's very carefully made determinations of the melting 



* Trevor's term "nonvariant" is a peculiarly happy selection. His similar 

 terms " mono variant," etc , would be equally suitable if they wore not hybrids of 

 Greek and Latin. Would it not be well to use instead of these the homogenous 

 words "univariant," "bivariant," etc.? These latter terms are not likely to 

 cause trouble because of their similarity to "univalent," etc , for both the num- 

 bers of syllables and the accents are different in the two series. 



Am. Jour. Sci. — Fourth Series, Vol. VI, No. 33. — September, 1898. 

 14 



