736 . Prof. E. Rutherford on the 



was very unsuitable for several reasons. A trace of sul- 

 phuric acid remaining in the solution after the removal of 

 the barium might possibly precipitate the radium as sulphate 

 — a form in which it would be very unlikely to release all 

 its emanation by aspiration of the solution. 



After three months' observations, this solution was put 

 aside with the intention of testing its radium content at 

 intervals; but the pressure of other work and the recognition 

 of the danger of contaminating the solution in a laboratory 

 in which a large quantity of radium was in use, led to a 

 postponement of further tests for a period of over two and a 

 half years. On the appearance of Boltwood's paper I imme- 

 diately examined this solution to see whether there had been 

 a growth of radium in this long interval. A preliminary 

 test showed that there had been a considerable increase in 

 the content of radium, but in making a more accurate deter- 

 mination, the solution was unfortunately contaminated with 

 radium, probably by the use of some tap grease for a stop- 

 cock. This accident brought home to me the danger of 

 making experiments of this character in a laboratory con- 

 taminated with radium, so that most of the experimental 

 work recorded in this paper was carried out in the Chemical 

 Laboratory, in which no radioactive matter had been intro- 

 duced. 



At the same time that the actinium solution had been pre- 

 pared, a quantity of a solid actinium preparation weighing 

 *32 gram of activity about 250 times uranium, had been set 

 aside in a closed glass tube. The radium content of a gram 

 of the same sample had been determined in 1904 by com- 

 parison with a standard radium solution prepared at that 

 time. Corrected in terms of the recently prepared radium 

 standards of Rutherford and Boltwood, the amount of radium 

 per gram of actinium in 1904 was 1*16 X 10 — 8 gram. 

 Assuming, as was probable, that the content of radium was 

 equally distributed throughout the whole mass of the actinium, 

 the amount of radium in the '32 gram was 3*7 X 10~ 9 gram. 

 After an interval of 2' 7 years this actinium was removed and 

 dissolved in hydrochloric acid, and the amount of radium 

 present was found by the emanation method, described later, 

 to be 1*05 x 10 -8 gram. The growth of radium in the in- 

 terval of 2*70 years between the two tests was thus 6*9 x 10 -9 

 gram, or assuming the rate of growth constant 2*9 x 10~ 9 

 gram per year. While not much weight can be attached to 

 this result by itself, on account of the imperfect aspiration 

 method employed in the initial determination of the radium 

 constant, yet the rate of growth observed will be seen to be 



