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



instrument, for if the draught is too fast the emanation will 

 only be left in the electroscope for a very short time, and 

 hence it will only produce a small effect; but if, on the other 

 hand, the draught is too slow, most of the emanation will 

 have died out before the air reaches the electroscope at all, 

 and so in this case also the effect will be small. Some of 

 the results given were obtained when the instrument had a 

 higher constant than in the subsequent experiments. The 

 higher constant had a value of 3'2xl0 -5 , while the sub- 

 sequent lower one was 2*1 x 10 -5 . The change in constant 

 was due to the draught through the instrument being altered. 

 The results obtained with the constants are shown separately 

 in the table of results. It will be seen that whenever a 

 solution was retested with the lower constant, the new result 

 obtained agreed fairly well with the old. This is a satisfactory 

 feature of the experiments. 



Dr. Joly has described, in his previous paper, how the 

 composite rock mixtures were originally made. Amounts 

 of the rock mixtures varying from five to fifteen grams were 

 used in the solutions. The smaller amounts were generally 

 only used in the repeat experiments owing to a certain 

 scarcity of material. One advantage, however, of varying 

 the amount of rock in the solution is that it would show if 

 any contamination of the solutions had occurred, as we would 

 expect a higher value for the thorium content of the weaker 

 solution if we assume that in each solution there was the 

 same absolute amount of contamination approximately. 

 Anyway it would seem justifiable to assume that there could 

 not be any lar^e amount of contamination if the two solutions 

 gave nearly the same result, as it appears highly improbable 

 that each solution should be contaminated in exact propor- 

 tion to the amount of rock it contained. This argument of 

 course does not apply to any contamination which the original 

 powder as a whole may have received. Ordinary incan- 

 descent gas-mantles are a very dangerous source of contami- 

 nation, and accordingly their presence in the laboratory 

 should be avoided. 



The composite rock mixtures used may be roughly divided 

 into three classes, i. e. acid, intermediate, and basic. Some 

 results obtained for some miscellaneous rocks are also given. 

 For the sake of comparison the radium content of these 

 rocks, as determined by Dr. Joly by the electric furnace 

 method, are also given. It will be seen from the results 

 that the thorium content decreases as we pass from acid to 

 basic rocks. In this respect the thorium content resembles 

 the radium content. However, there is no exact numerical 



