Certain Minnesota Soils. 395 



whose equilibrium quantity of emanation would give one 

 division per minute. Equating, substituting for Y/ and P, and 



, . , . MN p % (p-p t ) 



solving lor ??, we get : n = vrf- 7— — v , — — 7 



Then w , the mass of radium per cubic centimeter of soil 



which would account for its equilibrium amount of free ema- 



n 

 nation, is given by: n r= _^ where X is the constant for 



radium emanation and t the time during which the soil was 

 left standing. 



The Thorium Content and Thorium Standard. 



The method by which the thorium content of a soil can be 

 determined was fully described in the writer's previous article. 

 On account of the short life of thorium emanation, a continu- 

 ous current of air was drawn through the soil and through the 

 ionization vessel. It was necessary to consider the building 

 up of the emanation as a layer of air passed through the soil, 

 its partial disintegration in transit to the ionization vessel, and 

 its average concentration while passing through that vessel; 

 the equation took the form : 



Where N is the leak of the electroscope in scale divisions 

 per minute, n is the fraction of a gram of thorium per cubic 

 centimeter of the soil which would account for its free emana- 

 tion, q is the air current in cubic centimeters per second, X is 

 the constant for thorium emanation, P is the porosity (from 

 radium measurements), &, is the volume of the connecting 

 tubes and filters between the cylinder and ionization vessel, & 2 

 is the volume of the ionization vessel, k 3 is the volume of air 

 in the soil and K is the constant of the apparatus. The con- 

 stant K was determined, as before, by making a run on neutral 

 quartz sand impregnated with a known quantity of ThCl 4 in 

 solution. On treating thorite with hydrochloric acid, free 

 silicic acid is produced, which, on drying, forms a thin film on 

 each particle of sand. This film contains the thorium and has 

 very great emanating power. It seemed probable that there 

 would be a critical thickness for this film beyond which some 

 of the emanation would be occluded and the emanating power 

 would diminish. To test this point, about three liters of the 

 neutral sand were mixed with enough of the standard solution to 

 give it a thorium content about equal to that of the more active 



