[moran] radium standard SOLUTIONS 29 



to the remaining 750 c.c. to make up one litre, the strength of which 

 was now three-fourths the strength of the previous Utre, or 9 • 15 X 10"^^ 

 gram of radium per c.c. Radium contents declared right with the 

 International standard to an accuracy of at least one-third per cent. 

 Density of solvent HCl 1-08. Combined errors scarcely one per 

 cent." 



Experiments were carried out by the writer both with the strong 

 and the weak solutions of the Rutherford-Boltwood standard. These 

 served to compare further the Rutherford-Boltwood standard solu- 

 tion with the Washington, and thus also indirectly with the Inter- 

 national. In addition they served to show if the strong and the weak 

 solutions had remained unchanged throughout the eleven years which 

 have elapsed since their preparation. These solutions having been 

 prepared in the ratio of 100:1, if any deposits of radium had occurred 

 during this interval the amounts deposited would probably be in a 

 different ratio, and there would be a discrepancy in the results. 



Section 2. Method. 



The emanation method was employed in making the above 

 comparisons. The required volumes of radium solution were drawn 

 off with a clean pipette and accurately weighed. Distilled water and 

 a little HCl were then added, and the solution was sealed up air-tight 

 in a 500 c.c. flask, the solution occupying about half the volume of the 

 flask. The flask was then put aside for the emanation to collect, and 

 boiled off at intervals of about a week. 



The apparatus consisted of a sensitive, airtight, gold-leaf electro- 

 scope, carefully silvered on the inside, and well earthed. It was 

 exhausted by a water-pump, a sufificient exhaust usually being obtained 

 in a couple of minutes. The air was deprived of moisture by phos- 

 phorus pentoxide contained in a U-tube. The gold-leaves were pro- 

 tected from a sudden inrush of air by using a capillary tube. The 

 emanation was admitted by means of a three-way tap, and air allowed 

 to enter afterwards until the pressure inside the electroscope was 

 atmospheric. 



The solutions were v/ell-boiled in order to drive off all the emana- 

 tion, which was collected with the air over water in a bell- jar at room 

 temperature. Care was taken throughout to minimise possible 

 errors, by having the solutions of nearly equal strength, by boiling each 

 the same length of time, and by observing the maximum value of the 

 ionisation current. In the earlier part of the work, readings were 

 taken every few minutes, and the growth of the ionisation current 

 traced. The practical maximum was reached in about five hours. 



