214 RADIO-ACTIVE EMANATIONS [CH. 
By means of this relation the emanating power of compounds 
which are not of equal weight can be compared. 
It was found that thorium compounds varied enormously in 
emanating power, although the percentage proportion of thorium 
present in the compound was not very different. For example, 
the emanating power of thorium hydroxide was generally 3 to 4 
times greater than that of ordinary thoria, obtained from the manu- 
facturer. Thorium nitrate, in the solid state, had only 1/200 of the 
emanating power of ordinary thoria, while preparations of the 
carbonate were found to vary widely among themselves in emanat- 
ing power, which depended upon slight variations in the method 
of preparation. 
141. Effect of conditions on emanating power. The 
emanating power of different compounds of thorium and radium is 
much affected by the alteration of chemical and physical conditions. 
In this respect the emanating power, which is a measure of the 
rate of escape of the emanation into the surrounding gas, must not 
be confused with the rate of decay of the activity of the emanations 
themselves, which has already been shown to be unaffected by 
external conditions. 
Dorn (loc. cit.) first observed that the emanating power of 
thorium and radium compounds was much affected by moisture. 
In a fuller investigation of this pomt by Rutherford and Soddy, it 
was found that the emanating power of thoria is from two to three 
times greater in a moist than in a dry gas. Continued desiccation 
of the thoria in a glass tube, containing phosphorus pentoxide, did 
not reduce the emanating power much below that observed in 
ordinary dry air. In the same way radium chloride in the solid 
state gives off very little emanation when in a dry gas, but the 
amount is much increased in a moist gas. 
The rate of escape of emanation is much increased by solution 
of the compound. For example, thorium nitrate, which has an 
emanating power of only 1/200 that of thoria in the solid state, 
has in solution an emanating power of 3 to 4 times that of thoria. 
P. Curie and Debierne observed that the emanating power of 
radium was also much increased by solution. 
Temperature has a very marked effect on the emanating power. 
