94: Day and Allen — Isomorphism and Thermal 



he offers some very interesting speculations on the conditions 

 of equilibrium for substances above and below the melting 

 temperature under different pressures. The behavior of crys- 

 talline minerals, which melt at temperatures considerably higher 

 than he was able to command, offers peculiarly advantageous 

 opportunities for verifying the truth of his inferences and of 

 contributing further to the knowledge of this most important 

 change of state of matter. 



Temperature Measurements. — It is only a short time since 

 it became possible to measure even moderately high tempera- 

 tures with certainty and to express them in terms of a well- 

 established scale. Temperature is a peculiar function in that 

 it is not additive. Two bodies, each at a temperature of 50°, 

 can not be united to obtain a temperature of 100°, nor can any 

 number of bodies, at a temperature of 50° or below, give us 

 information about the temperature 51° or above. Further- 

 more, temperature is not independently measurable : we can 

 only measure phenomena like the expansion of gases or the 

 conductivity of platinum wire or the energy of thermal radia- 

 tion, which we have good reason to suppose will vary with the 

 temperature uniformly or according to a known law. 



The measure of temperature now generally accepted as 

 standard is the expansion of hydrogen gas between the melting 

 point of ice and the normal boiling point of water, divided 

 into 100 equal increments or degrees. Temperatures above 

 this point* have been determined by continuing the expansion 

 of hydrogen or nitrogen in the same units, as far as it has 

 been found possible to provide satisfactory containing vessels 

 for the expanding gas. Such determinations are then rendered 

 permanent and available for general use by establishing fixed 

 points, such as the melting temperatures' of easily obtainable 

 pure metals, at convenient intervals. Beyond 1150° no trust- 

 worthy gas measurements have been made and we have there- 

 fore no standard scale. For higher temperatures it is usual to 

 select some convenient phenomenon which is measurable up to 

 the temperature desired, to compare it with the gas scale as 

 far as the latter extends and then to continue on the assump- 

 tion that the law of its apparent progression below 1150° will 

 continue to hold above that point. In this way we obtain 

 degrees which, if not identical with the degrees of the gas 

 scale, approximate very closely to them and can receive a 

 small correction if necessary, whenever the gas scale shall be 

 extended or another scale substituted. 



* To 600°, Chappuis et Harker, Travaux et memoires du bureau inter- 

 national des poids et mesures, xii, 190?. To 1150°, Holborn and Day, Ann. 

 der Physik, ii, 505, 1900 ; English translation, this Journal [4], x, 171, 

 1900. 



