Radioactivity of Meteorites. 239 



Estimation of Madiwn 



The estimation of radium is usuallj carried out in one of 

 two ways : 



1. By the gamma ray method. 



2. By the emanation method. 



The gamma ray method is entirely nnsuited for measuring 

 minute quantities of radium. The emanation method, how- 

 ever, can be used to measure extremely small amounts of radi- 

 um with a fair degree of accuracy. When radium decomposes, 

 the first disintegration product is radium enianation. If the 

 radium-containing substance is kept in a sealed tube, the ema- 

 nation reaches its equilibrium amount in about thirty days. 

 The amount of emanation N present at any time t is : 

 :N- ^ N' (1 _ e-^^^) 



where IN^ is the maximum amount of em.anation, and X a con- 

 stant. 



The emanation can be separated completely from the solution 

 or fused material by bubbling air through the liquid. The air 

 may be introduced into a suitable electroscope, and the ioniza- 

 tion due to the included emanation measured.* 



Standardization of the Electroscope 



The electroscope has been in use for a number of years, and 

 has a very slow natural leak. It consists of a brass cylinder 

 13cm higli^ and 9^"" in diameter. The gold leaf is supported by 

 an L shaped brass strip which is insulated by an amber pillar. 

 The leaf is charged by a battery of small dry cells giving 400 

 to 500 volts. The movement of the gold leaf is observed with 

 a low power microscope, w^ith a scale in the eyepiece. 



The electroscope w^as standardized by means of standard 

 radium solutions furnished by Dr. H. JST. McCoy. Three dif- 

 ferent solutions were used : 



^o. 1—8-78 X 10~" gm Ka element 



No. 2—3-78 X 10-'^^" '' 



No. 3—3-78 X 10-'^ " " 

 The standards were made by diluting a standard radium 

 solution, and adding a few cc. of dil. HCl. 



The electroscope w^as first evacuated, and dry air from out- 

 doors admitted. The natural leak was then determined by 

 timing the rate of discharge over five divisions of the scale. 

 The electroscope was then evacuated, and the standard which 

 had been sealed for over a period of thirty days was inserted 

 in the air line. The ends of the sealed tubes which had been 



* Cf . Herman Schlundt, 26th meeting, Am. Electrochem. Soc. , 1914. H. M. 

 Plum, Jour. Am. Chem. Soc, vol. xxxvii, p. 1811, 1915. 



