152 Scientific Proceedings, Royal Dublin Society. 



loosely, and closed at each end, is inserted into the iron tube, thereby, in 

 effect, reducing its bore and its gas-content. 



In order to withdraw the ground-gas, an ordinary boiling-flask is exhausted 

 of air, carried to the iron tube, and attached to it by the rubber connecting- 

 tube. On opening the connexions, air flows from the ground into the 

 flask. A little water is kept in tlie flask, and the entering air is caused to 

 bubble through this water; in this manner the rate of entry of air is under 

 observation, and it is known when the flask is quite filled. 



When the flask is full, it is brought into the laboratory, let stand for about 

 15 or 20 minutes, in order to allow of the decay of the emanation of thorium, 

 and then attached to the exhausted electroscope. Tlie latter is filled from the 

 flask, water being let flow into the flask according as air leaves it. In our 

 experiments the flask held about one litre, and the electroscope about 600 ccs, 

 Tiius, a portion only of the air withdrawn from tlie soil was used for 

 measurement. 



The electroscope used is calibrated by the emanation from a known amount 

 of radium in the usual manner. Before the admission of the giouud-gas the 

 rate of discharge of tlie electroscojpe is observed, and again for about thirty 

 minutes after the admission of the gas, and finally three iiotirs after admission. 

 From the knowledge of the calibration constant of the instrument the amount 

 of emanation being dealt with is easily arrived at. The quantity of emanation 

 in a litre of similar gas is then calculated on the known volume of the electro- 

 scope. This method of determiniug the emanation content of the ground-gases 

 is free from instrumental factors, being simply and directly comparative. 



In the foregoing manner the following observations were carried out. 

 Three suction-pipes were in use. Their depths were finally brought to 25, 

 100, and 150 cms. The pipes were within 4 or 5 metres of one another, 

 in the grass on the east side of the School of Engineering, and in ground which 

 had not been disturbed for many years. The soil is calcareous, and presents 

 no abnormal features. It extends to a depth of several metres, and is well 

 drained. 



In the third column of the Table we give the radium equivalent of the 

 emanation per litre of ground-gas in billionths of a gramme, and in the fourth 

 column the ratio of this to the normal emanation content of the atmosphere, 

 taking the latter as 8 X 10'" per litre. 



