Manchester Memoirs, Vol. xlviii. (1904), No. 8. 27. 



and Travers, into the rare elements of the atmosphere,^ 

 the methods of deaHng with infinitesimal quantities of 

 gases with ease and certainty have been developed into 

 an exact science. In addition, the extreme delicacy and 

 distinctive character of the spectroscopic reaction of 

 helium contributed to make the result possible. The- 

 smallest visible bubble at the top of a fine thermometer 

 capillary, say one-hundred-thousandth of a cubic centi- 

 metre, or one-five-hundred-millionth of a gram, could be 

 with certainty detected by the methods employed. The 

 quantity actually obtained from the radium was certainly 

 not many times greater than this. Even the production 

 of this minute quantity of helium proves that the life of 

 the radium cannot be indefinitely great, and this result 

 had previously been arrived at by Professor Rutherford 

 from the consideration of the probable number of a particles 

 expelled per gram of radium per second, and the total 

 number of atoms in a gram of radium. This point will 

 be again referred to, but for the present some further and 

 more recent results obtained by Sir William Ramsay and 

 myself may be mentioned. 



We have attempted by methods similar to those 

 employed in dealing with minute amounts of helium,, 

 to obtain a measure of the absolute volume occupied by 

 a given quantity of the radium emanation. This may be 

 expected to be even less than the quantity of helium 

 produced for two reasons. First, the emanation is much 

 denser, and from measurements of the coefficient of diffia- 

 sion undertaken by Professor Rutherford and Miss Brooks 

 it is probable that the density lies in the neighbourhood of 

 80 (H = i). Secondly, the quantity does not continuously 

 accumulate, but in the course of a few weeks attains an 

 equilibrium value as already described. The equilibrium 



