176 Mr. Carl Barus on the Fusion 



changes which lie at the margin, as it were, of the more 

 profound and chemically significant volume-changes (poly- 

 meric passage from homogeneity to organized rock-structure), 

 even though the latter may be conceived as producible by 

 pressure alone, under conditions of nearly constant (high) 

 temperature. I may add, in passing, that the magnitude of 

 the chemical changes of volume makes it advisable to carry 

 the work on the effect of pressure on the chemical equilibrium 

 of solids and of liquids, and on the solution behaviour solid- 

 liquid, into greater detail than I have thus far attempted*. 



Apparatus. 



4. Temperature Measurement. — In work of the present kind 

 an apparatus for the accurate measurement of high tempera- 

 tures is the fundamental consideration. I may, however, 

 dismiss this subject here, since I discussed it in the intro- 

 ductory paper "f. The temperature-measurements of this 

 paper were made with a thermocouple of platinum and pla- 

 tinum with 20 per cent, of iridium, which had been frequently 

 compared with my re-entrant porcelain air-thermometer 

 throughout an interval of 1100° C., and tested for freedom 

 from anomalies beyond this interval. Inasmuch as the 

 electric method consists in expressing thermoelectromotive 

 force by aid of a zero method, in terms of a given Latimer- 

 Clark standard cell, the temperature-apparatus is of the same 

 order of constancy as to time as the standard cell. 



To find in how far my earlier data were trustworthy after 

 the lapse of upwards four years, I made fresh check measure- 

 ments of the boiling-points of mercury and of zinc. The 

 latter only need be instanced here. My original mean datum 

 of the boiling-point of zinc, expressed as electromotive force 

 for the couples under consideration, was (1887) £ 20 = 11,074 

 microvolts, the cold junction being at 20° C. The new expe- 

 riments (1891) gave me data as follows : — 



Thermocouple No. 35, e 20 = 11,168 (microvolts), 



No. 36, „ 11,127 



No. 39, „ 11,116 



No. 40, ,, 11,136 



No. 36, „ 11,127 

 No. 39, „ 11,116 



„ ^V. J-W, ,, J.J.yJ.^yj „ 



Mean . . . 11,137 „ 



* American Journal, xxxvii. p. 339 (1889); ibid, xxxviii. p. 408 

 (1889) ; ibid. xl. p. 219 (1890) ; ibid. xli. p. 110 (1891) ; ibid. xlii. p. 46 

 fit seq. (1891). Also Phil. Mag. [5] xxxi. p. 9 (1891). 



t See this Magazine, July 1892, p. 1. 



