ITS J. C. Sanderson — Probable Influence of tfa 



method by which either all or a definite fraction of the 

 thorium emanation in equilibrium with a known amount 

 of a thorium salt may be obtained with certainty. It may 

 reasonably be assumed, however, that if the thorium salt could 

 be obtained in the form of a sufficiently thin coating on the 

 surface of some neutral material, all of the emanation pro- 

 duced within it would escape. For this reason the -thorium 

 solution was thoroughly mixed with a considerable quantity of 

 neutral sea sand so that each grain was covered with a very 

 thin film of radioactive material. 



Another point which must be taken into consideration is the 

 requirement that all the disintegration products of thorium, 

 preceding the emanation, shall be present in equilibrium 

 amounts. In the present experiment this was accomplished by 

 taking a solution of a thorium mineral containing a known 

 proportion of thorium. 



The mineral chosen was thorite, containing 52 per cent of 

 ThO., in the form of a silicate. When this mineral was dis- 

 solved in hydrochloric acid, diluted and mixed with a granular 

 neutral material, a thin gelatinous film of free silicic acid, con- 

 taining the ThCl 4 , was formed on each grain, and it was thought 

 that such a film would possess a very great emanating power, 

 an assumption which was amply justified by comparison with 

 thorium in other forms. By adopting as a standard the most 

 emanating form of thorium which could be found, it was 

 assured that the results obtained for the thorium content of 

 various materials were very nearly minima. The standard was 

 prepared by dissolving 0*1 gram of thorite in a little hydro- 

 chloric acid. It was then diluted and intimately mixed with 884 

 grams of white beach sand, which had been previously tested 

 and found free from any measurable radioactivity. This mix- 

 ture was thoroughly dried and then moistened with 12 cubic 

 centimeters of water. The standard thus prepared was placed 

 in a vertical tin cylinder, about 35 centimeters high, closed at 

 its lower end, except for a brass connecting tube, and provided 

 with a cover having a small inlet hole in its center. About a 

 centimeter from the bottom was a platform of wire gauze on 

 which was placed a disk of cotton. The air was sucked in 

 at the top, through the sand, out at the bottom and thence, 

 through a rubber connecting tube, to the ionization chamber. 



The theoretical considerations governing this case are much 

 the same as before. "When the underground air was drawn 

 directly into the brass tube, it retained its equilibrium concen- 

 tration up to the instant of entering the tube. In the case of the 

 cylinder, a horizontal layer of air enters the sand practically 

 devoid of thorium emanation and passes through it in a time 



k 



— - , where k. is the volume of air in the interstices of the 

 q 



