Emanation from the Soil to the Atmosphere. 633 



the emanation content of ground-gas drawn from 50 cm. 

 below the surface, the other with the escape of emanation 

 from the surface at an adjacent spot. 



The observations were made on a lawn beside the School 

 of Engineering, Trinity College, Dublin. The soil is cal- 

 careous, well-drained, and several metres deep. 



Method. 



1. Soil Gas. — The estimation of the amount of emanation 

 contained in the gases occupying the interstices of the soil is 

 carried out as follows. An iron pipe, of 15 mm. bore, is 

 closed and pointed at -the lower end. Holes are drilled in its 

 walls near this end. These slope downwards and inwards 

 to prevent clogging when the tube is being driven into the 

 ground. To reduce the capacity of the pipe as far as possible, 

 a loosely fitting, closed brass tube is inserted. The upper end 

 of the iron pipe is fitted with a glass leading tube, which is 

 usually kept closed. The pipe was in these experiments 

 driven into the soil so that the holes near the end were 50 cm. 

 from the surface. 



A litre boiling-flask was used to collect the soil gas. It is 

 exhausted of air and attached to the leading tube. The 

 connexions being opened, gas flows from the soil spaces into 

 the flask. The flow is observed by causing the gas to bubble 

 through a little water at the bottom of the flask. The flask, 

 when full, is allowed to stand at least 10 minutes to allow- 

 any thorium emanation present to decay to a negligible 

 amount. The flask is now attached to an exhausted electro- 

 scope (capacity 600 c.c.) and the gas allowed to flow into the 

 latter, water being allowed to flow into the flask to take the 

 place of the gas. The rate of discharge of the electroscope 

 is now read for a short time, and again in about three hours. 

 These rates, less the normal rate of discharge, ascertained 

 just before the experiment, allow one to estimate the amount 

 of emanation present, the electroscope having been calibrated 

 by the emanation from a known amount of radium. The 

 results are reduced to amounts of emanation per litre, and 

 are expressed in curies Xl0~ 12 . 



2. Exhalation. — To estimate the amounts of radium ema- 

 nation given off from the surface of the soil, the following 

 method was used. The collecting vessel is of tin-plate, 

 cylindrical, 25 cm. in diameter and 8 cm. deep. One end is 

 closed but for a central opening 3 cm. in diameter. The 

 other is open and its edge is pressed into the ground. Inside 

 this vessel is a circular plate of tin 24 cm. diameter, resting 

 on feet about 3 cm. long. This plate has a central opening 



