222 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1932. 



In j there is a cone of blotting-paper, which is moistened with 

 water. Pouring in Uquid air, it forms a vessel tight for liquid air, 

 because it consists of ice. The tube which it surrounds is so cooled 

 that the emanation freezes in its interior and is deposited upon the 

 walls. We may now open the tap h, for the emanation can no longer 

 be contaminated, and the hydrogen is pumped off. But even at the 

 temperature of liquid air the emanation possesses some little vapor 

 pressure, and, consequently, a certain quantity accompanies the 

 hydrogen. To take this quantity into account, its radio-activity is 

 next determined, and by comparing it with that of the gas before its 

 introduction into the apparatus the loss is ascertained, and conse- 

 quently the quantity remaining in the apparatus. 



Havuig removed the hydrogen as far as possible, v/e raise the reser- 

 voir/ in order to protect the tap Ji against attack by the emanation, 

 and heat the tube j. The emanation is volatilized. Raising the 

 reservou* d, it is compressed mto the capillary tube m, where its 

 volume is measured. But this tube must be chosen of such a 

 diameter that its volume can be determined at a pressure little 

 removed from that of the atmosphere. If we measure toder too 

 low a pressure, the correction for the capillarity, which may rise to 

 two centimeters of mercury, becomes too great. However, it is diffi- 

 cult to estimate the magnitude of the correction, for we have been 

 unable to find constant results for the capillarity; it seems to be 

 influenced by the state of electrification of the mercury, due to the 

 presence of the emanation. But that is of little consequence when 

 the measurement is made at a pressure near 760 milluneters, for we 

 may almost disregard a pressure of 5 millimeters, which does not 

 exceed 0.7 per cent of the total pressure. 



To give you an idea of the exactness of this measurement, allow 

 me to state the result of a recent experuuont b}- which we ascertained 

 that a sample of helium had a volume of 0.042 cubic millimeter; the 

 length of the tube measured was 20 millimeters, which permits the 

 attainment of an approximation of 1/200. A platinum wire sealed 

 through the top of the capillary tube makes it possible to test the 

 purity of this gas by passing thi'ough it an electric discharge. The 

 other electrode consists of the mercury of the reservou", and we may 

 prevent the lines of mercury from being seen by solidifying it wdth 

 a paper cone filled v/ith liquid air, surrounding the tube. 



We have compressed the emanation so as to liquefy it; the tube 

 containing it is of thick glass and can resist a considerable pressure. 

 We have cut it off and mounted it m a compression apparatus like 

 that of Amagat. The emanation liquefies at the ordinary tempera- 

 ture at a pressure of about 10 meters of mercury. The volume of 

 gas, which was 0.1 cubic millimeter, was reduced to 0.00025 cubic 

 milluneter; it occupied about 0.24 millimeter of the length of the 



