xii, a, 3 Wright and Heise: Philippine Waters 151 



The radioactivity of any water expressed in grams of radium 

 may be then readily calculated from the formula 



m 1 =am r7 (2) 



Z 

 in which 



a is the constant derived from (1) ; 

 Z is the leak caused by m grams of radium (determined with 



the standard solution) ; and 

 Z ' is the leak caused by the emanation from the water in 

 question. 



Since it was not practicable, especially in the field, to wait the 

 three hours necessary for the leak due to emanation to reach 

 maximum value, readings were taken, as previously mentioned, 

 fifteen minutes after filling the ionization chamber, and the 

 maximum leak was calculated by means of a radium-emanation 

 decay curve. 



For use in the above equation the readings for the leak (Z 

 and Z') were reduced by means of a calibration curve from 

 divisions of the telescope scale to volts, and the natural leak of 

 the instrument was in each case subtracted from the leak ob- 

 served with emanation in the ionization chamber. 



Limits of accuracy and probable error. — Although the object 

 of this work was primarily to get reliable comparative data 

 rather than exceedingly accurate absolute values, it is probable 

 that the error was not great. The radium solution used for 

 standardization may be considered accurate within 5 per cent. 

 Duplicate determinations made with the standard solution 

 checked within 1.5 per cent. Duplicate determinations made 

 in the field on the same water checked within the limits of ob- 

 servational error. We have reason to believe that, except for 

 the isolated cases noted in which proper samples could not be 

 secured because of the nature of the source, the maximum error 

 in field determinations was not greater than 7 per cent. There- 

 fore the probable error was much smaller and hence was well 

 within the limits of accuracy to be expected for this class of work. 



The radium-emanation content of Philippine waters. — No at- 

 tempt was made in the field to determine anything but radium- 

 emanation content. That we were actually dealing with radium 

 emanation was shown by the fact that when the gases from a 

 number of waters studied were allowed to remain in the ioniza- 

 tion chamber for long periods of time the typical decay curve 

 of the radium emanation was obtained. 



The radioactivity of the Philippine waters examined is shown 

 in Table I. 



