234 PROCEEDINGS OF THE AMERICAN ACADEMY. 



was then placed in one of the two compartments of a glass vessel which 

 was connected to a mercury pump and exhausted. Heat was then 

 applied to the other compartment of the vessel which was full of 

 mercury, and the mercury slowly distilled over until it covered the 

 capillary, as high a vacuum as possible being maintained all the while 

 by constant operation of the pump. This distillation acts as an addi- 

 tional purification of the mercury, that coining over being dry and 

 presumably free from air. When the capillary was covered with 

 mercury, air was admitted though the pump and mercury forced into 

 the capillary through the open cup, any small possible bubble of air 

 rising to the top of the other cup. In this way nearly all the air can 

 be removed, the slight quantity remaining having probably clung to 

 the inner walls of the capillary throughout the exhaustion. The 

 quantity of air left was usually large enough to introduce an appre- 

 ciable correction. This correction was determined by measuring the 

 resistance of the mercury at low pressures compared with a calibrated 

 Bourdon gauge of the Societe Genevoise, and extrapolating back for 

 the zero from 50 kgm. The tube must be refilled if the correction is 

 large, because it will not remain constant, as obviously the effect of 

 the occluded air on the resistance depends on its position as well as 

 on its quantity. If the correction is small, however, it remains con- 

 stant apparently indefinitely. In the tubes used, the correction ranged 

 from 0.6 mm. to 0.1 mm. of the bridge wire, that is, a mean correction 

 of about ^oVo °f the total resistance. That the permanent change of 

 the zero mentioned above was due to set in the glass and not to a 

 curious behavior of the contained air, is proved by the fact that no 

 set was found after filling in the manner above a tube once seasoned, 

 but the correction for air assumed at once its final value. 



In addition to showing the necessity of seasoning the glass and 

 removing all air from the mercury, the preliminary comparisons 

 shoved that the mercury must be purified with some care. Later, 

 a quantitive determination of the effect of two common impurities 

 will be given. It was found that the mercury could be got sufficiently 

 pure for present purposes by distilling commercial mercury, clean- 

 ing with acid, washing and drying, and finally distilling into the U 

 capillary as described above. 



When all these precautions are taken, the mercury gauge seems 

 to be reproducible at pleasure. The results of a comparison of two 

 such gauges is shown in Table I. The two mercury resistances com- 

 pared were each contained in capillaries of the same kind of glass, 

 Jena No. 3880 a. One capillary (R 9), however, was twice the linear 

 dimensions of the other (R 10) because it seemed desirable to eliminate 



