1 6 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



TEMPERATURE MEASUREMENTS. 



It is only a short time since it became possible to measure even 

 moderately high temperatures 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 ioo, nor can any num- 

 ber of bodies at a temperature of 50 or below give us information 

 about the temperature 51 or above. Furthermore, 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 radiation, 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 ves- 

 sels for the expanding gas. Such determinations are then rendered per- 

 manent and available for general use by establishing fixed points, such 

 as the melting temperatures of easily obtainable pure metals, at con- 

 venient intervals. Beyond 1150 no trustworthy gas measurements 

 have been made, and we have, therefore, 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 

 assumption that the law of its apparent progression below 1 150 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. 



The application of measurable high pressures at the higher tem- 

 peratures has never been successfully accomplished, and until some- 

 thing can be done in this direction, our knowledge of the rock-forming 

 minerals, and in fact all the generalizations relating to equilibrium 



* To 6oo, Chappuis et Harker, Travaux et memoires du bureau international 

 des poids et mesures, 12, 1902. To 1 150 , Holborn and Day, Ann. der Physik, 2, 

 505, 1900. English translation, Am. Journ. Sci., (4), 10, p. 171, 1900. 



