490 ^ Scientific Proceedings, Royal Dublin Society. 



Moreover, the cost is considerable, owing to the scale on which the salt must 

 be supplied. 



It was in connexion with the important problems arising in relation 

 to Irish water-power utilization, as recently put forward by Sir John Purser 

 Griffith before this Society, that I thought of the following procedure. At 

 the time I was in ignorance of the fact that the principle involved had ever 

 before been used or thought of. 



I propose to utilize, in determining river discharge, the extraordinary 

 accuracy with which radioactive measurements can be effected. A very 

 simple form of electroscope suffices to determine a quantity of radium to the 

 billionth part of a gram. The apparatus costs a very few shillings. If, 

 now, in place of introducing salt by the hundredweight into the river, we 

 flow into the river a few litres of a solution containing a trace of radium, and 

 taking samples down stream examine them by the electroscope, the discharge 

 of the river may be determined. 



In order to reduce the working conditions to figures, I take the case of 

 the Poulaphuca and Golden Falls on the Upper Liffey. The flow I take as 

 20,000 cubic feet per minute. 



The constant of the electroscope I take as O'Si x 10"^^. That is to say, 

 this quantity of radium, or rather the emanation in equilibrium with it, will 

 cause the gold leaf of the electroscope to show a gain in rate of fall of 1 scale 

 division per hour. I have made many such electroscopes. We require to 

 know how much radium is to be put into the river, so that we can estimate 

 the discharge to 1 per cent. ; and I assume that the feed of radioactive water 

 into the river is maintained for twenty minutes. 



Suppose we withdraw samples of 10 litres volume for testing by the 

 electroscope. To evaluate the radium to 1 per cent., it must increase the 

 rate of fall of the leaf of the electroscope by 100 scale divisions per hour. 

 That is, the 10 litres must contain 54 x 10"'' gram radium ; i.e., there 

 must be 5'4 x 10"'^ gram radium per litre of water passing down the river. 

 Now. the flow of the river as assumed is about 600,000 litres per 

 minute. The quantity of radium fed in per minute must therefore be 

 6 X 10' X 5'4 X 10"" = 32 X 10"' gram, q, p; and for twenty minutes' flow 

 we require 64 x 10"* gram radium : that is, 0'064 milligram must be put 

 in. 



Now, the most economical form in which the radium can be supplied is 

 pitchblende, the mineral oxide of uranium. In this ore 200 milligrams of 

 radium are associated with 1,000 kilos of the mineral, from which we find 

 that O'Ol milligram of radium are contained in 50 grams of the ore. 300 

 grams are required to supply 0'06 milligram. 



