RADON AND MINERAL CONTENT OF SOILS 181 
avoid compacting the soil around the hole and thus reducing its 
porosity, which would tend, in clayey soils, to make it more difficult 
to obtain a sample of soil gas. The sampling tube (A) consists of a 
14-inch iron pipe, broken and coupled near the top to permit the 
insertion of nipples so that the tube can be extended when samples of 
soil gas from greater depths are desired. A wall packer (B) is welded 
on the outside of this tube about a foot from the bottom. The packer 
consists of a piece of tubing of the same diameter as the hole, around 
which an iron rod (C) of 14-inch square cross section was wrapped 
spirally and welded in place. This rod cuts into the walls of the hole 
and forms a tight seal against leakage from the atmosphere. The gas 
sample is pumped through drying tubes (D) by the piston pump (£) 
into the ionization chamber (F) of the electroscope. The ioniza- 
tion chamber is a cylinder whose bottom is a movable piston (G). To 
empty the ionization chamber of gas the outlet cock on the top is 
opened and the piston pushed up. The outlet cock is then closed and 
the inlet opened. The gas sample being pumped in thus exerts an 
outward pressure on the system and any leakage occurring is out- 
ward. This prevents the dilution of the gas sample with atmospheric 
air. The minute traces of radon in the soil gas, by virtue of radio- 
- active decay, produce an ionization of the gas in the ionization cham- 
ber and discharge the electrometer fiber which has been previously 
charged to a potential of about 200 volts. The rate of discharge is 
proportional to the amount of radon present in the gas. The sensi- 
tivity of the instrument is such that one part of radon can be de- 
tected in about 10” parts of soil gas. 
Several profiles were run across a known fault in the Balcones fault 
region. The measurements made in these profiles are shown in Figure 
2. It is difficult to express the results in terms of quantity of radon per 
cubic centimeter of soil or per gram of soil mineral, because of the 
great variation in the porosity of the soil and because of another factor 
which may be even more variable, the so-called emanating power of 
the soil grains. Since the radon gas is the product of the decay of 
radium atoms, the radon must be formed on the surface of the soil 
grains or within the grains themselves. As the radon is a gas formed 
from a solid, only that part formed at the surface of the grains is able 
to get into the soil gas. The emanating power is the ratio of the 
amount of radon which gets into the soil gas to the total formed in 
the soil. This obviously depends, then, not only upon the size of the 
soil grains, but upon their shape (ratio of volume to surface) and upon 
