696 Prof. J. Joly on the Radioactivity of 



dealt with, becausethese are primarily involved in the wider 

 geological applications of radioactivity. 



The method pursued is different from what has hitherto 

 been adopted in such investigations. Individual rock 

 specimens are not dealt with. A large number of typical 

 rock samples are reduced to powder. A similar weight of 

 each is taken and the added powders thoroughly mixed. 

 The determination in the furnace is made upon one or more 

 samples of this composite powder. If there is anything to 

 suggest abnormality in the result, a second determination is 

 made upon a composite powder compounded of about one 

 half the original samples. If a high result was due to the 

 presence of one very radioactive rock, this fact must be 

 detected in this second experiment. If it is desired to 

 eliminate such an excessively radioactive material it may be 

 rapidly isolated by a few successive experiments upon one 

 half, one quarter, &c. oi: the original samples separately 

 compounded. This method has many advantages. At this 

 stage of the inquiry it yields all the information required. 

 The labour does not increase with the number of rock 

 specimens ; on the contrary, the more rocks enter into the 

 composite material the less need to consider individual 

 abnormalities, or, we might say, the more the necessity for 

 their inclusion. As the number of available rock specimens 

 increases I hope to place the means arrived at in this manner 

 upon a more secure basis. 



There is a certain small latitude of uncertainty in the 

 results. The " constant " of furnace and electroscope working 

 in conjunction, as determined by dealing with known amounts 

 of radium, may vary sornewmat. In a series of experiments 

 carried out to ascertain the constant proper to the present 

 series of experiments, variations of its value from 07 to 0*8 

 were obtained. The mean of these values has been taken. 

 Comparison with results obtained when this method was first 

 being developed (he. cit.) will show that this value is well 

 supported by other observations. In experiments upon the 

 sedimentary rocks, carried out in my laboratory, a somewhat 

 lower constant, i. e. 0'6, was arrived at*. The difference is 

 probably due to the electroscopes used in the two cases. It 

 is satisfactory that the results on composites of the sedi- 

 mentary rocks, given further on, wherein the higher constant 

 is used, almost exactly substantiate means arrived at on the 

 detailed results calculated on the lower constant. This not 

 only brings the latter measurements into line with those on 



* Fletcher, Phil, Ma?. Feb. 1912. 



