﻿484 Mr. J. H. J. Poole on the Average 



temperature of about 1000° C. is requisite. The melt is 

 then treated with water and filtered. The filtrate is discarded 

 as it has been found to contain only a negligible quantity of 

 thorium. The residue left on the filter-paper is dissolved in 

 distilled water containing 50 c.c. of pure HC1, and the solu- 

 tion made up to a convenient bulk with added distilled water. 

 This solution is set aside for some days to allow the thorium 

 disintegration products time to grow, as some of them are 

 lost in the process of getting the rock into solution. The 

 thorium content is determined by boiling off die thorium 

 emanation in a constant stream of air, which air then passes 

 through an ordinary gold-leaf electroscope. By determining 

 the rate of leak of the electroscope when the solution is 

 boiling and when it is not, we can estimate the amount of 

 thorium in the solution. The solution before being tested is 

 boiled in a separate vessel to drive off the radium emanation 

 if there is any present. 



The electroscope is standardized with a thorianite solution 

 of known strength, as Dr. Joly has previously described. 

 One c.c. of this solution was either added to a known amount 

 of distilled water or to a rock solution whose relative activity 

 had already been determined. In either case, by observing 

 the new rate of leak we can easily calculate what quantity 

 of thorium corresponds to a gain of rate of leak of one scale- 

 division per hour. 



The latter method was adopted to see whether rock solu- 

 tions had the power of concealing, as it were, the emanation 

 that they contained. If this were the case, one would expect 

 a higher constant for the electroscope to be obtained by the 

 second method. However, by both methods a practically 

 identical value was obtained. The whole method is of course 

 only a comparative one, the effect of any rock solution being 

 compared with that of the standard. For this reason great 

 care has to be taken to keep the conditions under which the 

 standard solution and the rock solution are boiled as similar 

 as possible. In all cases some powdered steatite has to be 

 added to the solution to make it boil freely and to prevent 

 bumping. Many solutions were found to be very sensitive 

 to any change in the form of ebullition, and in these cases 

 it was often necessary to add some fresh steatite, as the 

 steatite after a certain time lost its power of producing free 

 ebullition. 



The constant of the electroscope depends largely on the 

 rate of the draught of air through the boiling flask, and 

 thence through the condenser and electroscope. There is 

 one rate of flow which gives the lowest constant for the 



