Constants of Igneous Bock. Ill 



agreeing very closely with subsequent specially careful 

 measurements, viz.: — 



Thermocouple No. 36, e 20 = 11,131 (microvolts), 

 No. 39, „ 11,134 



The difference of values, new and old, is 60 microvolts, only 

 about \ per cent, as regards electromotive force, and corre- 

 sponding to about 4° at 1000°. In view of the excessive use 

 and abuse to which the couples had been put in the lapse of 

 time this result is gratifying. 



Endeavouring to ascertain where this discrepancy was to 

 be sought, I also made new comparisons between the Clark 

 cell and a normal Daniell of my own (Bull. 54, U. S. Geolog. 

 Survey, p. 100), in which the cells are separate, and only 

 joined during the time of use. Supposing the electromotive 

 force of the former to have been e= 1*435 throughout, the 

 following succession of values obtained for the standard 

 Daniell : — 



March 1886, 20° C, e=M38, 

 August 1887, 28° C., 1-139, 

 September 1891, 27° C., 1-147. 



If, therefore, instead of regarding the Clark cell as constant 

 I had attributed* this virtue to the standard Daniell, the 

 small thermoelectric discrepancy equivalent to 4° at 1000° 

 would be altogether wiped out. Now I made the Clark cells 

 in question as far back as 1883, when less was known about 

 details of construction than is now available. I conclude, 

 therefore, that the discrepancy is very probably in the standard 

 cells, and that the thermocouples have remained absolutely 

 constant, a result which is borne out by my boiling-points of 

 cadmium. 



Apart from this, the new standardization with boiling zinc 

 fixes the scale relatively to the accepted value for this datum. 



5. General disposition of Apparatus. — This is given in 

 Plate Y. on a scale of 1 : 4, where figs. 1 and 3 are sectional 

 elevations showing the parts chiefly with reference to their 

 vertical height, and fig. 2 is a sectional plan, in which the 

 parts are given with reference to the horizontal. 



The molten rock, Z Z, contained in a long cylindrical pla- 

 tinum tube, largely surrounded by a tube of fire-clay, F F, is 

 fixed vertically in a tall cylindrical furnace, D D, L L. The 

 heat is furnished by six burners, B, B, . . . , fed by gas and an 



* Our laboratory affords insufficient facilities for the direct measure- 

 ment of electromotive force. 



