418 A. L. Day and J. K. Clement — Gas Thermometer. 



through accident or oversight, it reaches the bulb almost 

 immediately, and once there, it is a matter of two weeks 

 boiling with nitric acid to get rid of it again. 



Even with this valve, it sometimes happened that when gas 

 was bubbled through the mercury in filling, even at the bot- 

 tom of the tube some 80 cm distant from the valve opening, 

 tiny globules of mercury were shot upward with such speed 

 and accuracy of aim as to pass up beside the little valve and 

 into the capillary tube, after which their ultimate destination 

 is inevitably the bulb. The altogether insignificant size of the 

 opening and the distance required to be traversed by such a 

 globule did not convey to us a suspicion that a globule might 

 hit and pass it, but it actually happened on two different 

 occasions, with the consequence of an exasperating delay. 



In the present arrangement of the gas thermometer, this 

 accident is also provided against by introducing a gold capillary 

 instead of platinum, between the fixed point and the bulb. 

 Such microscopic globules of mercury are taken up by the gold 

 without reaching the bulb and therefore remain harmless. 



Barometer. — It was deemed advisable from the start not to 

 attempt to combine the barometer with the manometer as has 

 usually been done by the French observers. It is a convenient 

 method and is rather necessary if a single observer is to make 

 all the readings, but the combination brings three or four 

 essentially different errors into one reading in a way that does 

 not admit of an intelligent evaluation of their individual mag- 

 nitudes. 



Two barometers were used throughout this investigation, 

 both of Fuess manufacture and of the same type (Wild-Fuess 

 Normal Barometer, 14 mm tube). The corrected readings of the 

 two instruments were in perfect accord and were correct in 

 their absolute value within *05 mm .* 



Thermoelectric Apparatus. — The thermoelectric measure- 

 ments were made with apparatus and by methods which have 

 already been described in varying degrees of fullness in pre- 

 vious publications from this laboratory. f 



Briefly, it may be noted in passing that all the thermoelectric 

 measurements without exception were made with platinum- 

 platin-rhodium thermoelements of Herseus manufacture on a 

 potentiometer of Wolff standard construction by direct com- 



* One of these instruments was compared with the normal barometer at 

 the U. S. Weather Bureau at Washington, the other at the Bureau of 

 Standards. 



f Day and Allen, The Isomorphism and Thermal Properties of the Feld- 

 spars, Publications of the Carnegie Institution of Washington, No. 31. 1905 , 

 Allen and White, On Wollastonite and Pseudo-Wollastonite, Polymorphic 

 Forms of Calcium Metasililicate, this Journal (4), xxi, p. 89, 1906 ; Walter P. 

 White, Potentiometer Installation, especially for High Temperature and 

 Thermoelectric Work, Phys. Eev., xxv, p. 334, 1907. 



