238 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



elude that in each gram of radium at least one-eighth of a milligram, 

 i. e., about 1 part in 8,000, must be in the ether. Considerations of 

 this nature induced me some time ago to make experiments on radium 

 to see if I could get any evidence of part of its mass being of an ab- 

 normal kind. The best test I could think of was to see if the pro- 

 portion between mass and weight was the same for radium as for 

 ordinary substances. If the part of the mass of radium which is in 

 the ether were without weight then a gram of radium would weigh 

 less than a gram of a substance which had not so large a proportion 

 of its mass in the ether. Now, the proportion between mass and 

 weight can be got very accurately by measuring the time of swing of 

 a pendulum; so I constructed a pendulum whose bob was made of 

 radium, set it swinging in a vacuum and determined its time of vibra- 

 tion, to see if this were the same as that of a pendulum of the same 

 length whose bob is made of brass or iron. Unfortunately radium 

 can not be obtained in large quantities, so that the radium pendulum 

 was very light, and did not therefore go on swinging as long as a 

 heavier pendulum would have done; this made very accurate deter- 

 minations of the time of swing impossible, but I was able to show that 

 to about 1 part in 3,000 the time of swing of a radium pendulum was 

 the same as that of a pendulum of the same size and shape made of 

 brass or iron. The minimum difference we should expect from theory 

 is 1 part in 8,000, so that this experiment shows that if there is any 

 abnormality in the ratio of the mass to weight for radium it does not 

 much exceed that calciilated from the amount of heat given out by 

 the radium during its transformation. With larger pendulums the 

 value of the ratio of mass to weight can be determined with far 

 greater accuracy than 1 part in 8,000 ; for example, Bessel three-quar- 

 ters of a century ago showed that this ratio was the same for ivory as 

 for brass to an accuracy of at least 1 part in 100,000 ; and with appa- 

 ratus specially designed to test this point an even greater accuracy 

 could be obtained. ^^Hien I made my experiments with the radium 

 pendulum the close connection between the amounts of uranium and 

 radium in radio-active minerals had not been discovered; this con- 

 nection makes it exceedingly probable that radium is derived from 

 uranium and that this metal may have weight for weight more elec- 

 tric potential energy, and therefore a larger proportion of its mass in 

 the ether, than radium itself. This points to the conclusion that the 

 proper substance to use for the pendulum exjDeriment is uranium 

 rather than radium, especially since uranium can easily be obtained 

 in sufficiently large quantities to enable us to construct the jDendulum 

 of the shape and size which would give the most accurate results, it 

 would not, I think, be impossible to determine the ratio of mass to 

 weight for uranium to an accuracy of 1 part in 250,000. 



