Boltwood — Radio-activity of Natural Waters. 383 



the solution of the mineral to fall on its side and fill with 

 water. The contents of the flask were now boiled for about 

 20 minutes, a few snips of platinum foil having been placed 

 in the flask to prevent bumping. The gases which accumu- 

 lated in the tube C were transferred to the gas burette E, 

 allowed to stand for 15 minutes and then introduced into the 

 electroscope. The water in the flask B, the tube C and the 

 gas burette E contained a little sodium hydroxide to absorb 

 any chlorine which might be produced in the reaction. The 

 time which transpired from the boiling off of the gas to its 

 introduction into the electroscope (about 30 minutes) was suffi- 

 cient to reduce the activity of the thorium and actinium ema- 

 nations also present to a negligible value. 



The rate of leak as determined at the end of three hours 

 was taken as the basis of calculation, and was considered as 

 equal to the activity of the radium emanation associated with 

 0*0100 gram of uranium. This leak was equal to 1*76 division 

 per minute. A fall of the gold-leaf equal to 0*001 division 

 per minute was therefore equivalent to 5*68 X 10 -6 grams 

 uranium. 



Calculation of Initial Activity. 



In most cases it is either impractical or altogether impossible 

 to carry out the measurement of the activity of the water at 

 the source of the spring. A certain length of time, either a 

 few hours or several days, must elapse between the time of 

 collection of the sample and the time that the test is conducted. 

 Since it is highly probable that the activity of most, if not all, 

 radio-active waters is due chiefly to dissolved radium emanation, 

 this activity will become regularly less on standing. The decay 

 of the radium emanation will follow the simple exponential 

 law expressed by the equation : 



I - I e- at , 

 in which I represents the activity after the interval t, I the 

 initial activity, e the base of the Napierian system of loga- 

 rithms and t the elapsed time expressed in hours. The value 

 of the constant a is 0*00724 as determined by Curie, 0*00778 

 as determined by Rutherford and Soddy, and 0*00741 as deter- 

 mined by Bumstead and Wheeler.* 



The initial activity of the water can therefore be calculated 

 from the observed value of I. A convenient form of the 

 equation is then 



I = Ie at 



Radium Salts in Solution. 

 The presence or absence of radium salts in solution can be 

 demonstrated by boiling about 10 liters of the water, a little 

 * This Journal xvii, 97 (1904). 



