RICHARDS AND MERIGOLD. — ATOMIC WEIGHT OF URANIUM. 387 



There is another phase of this subject that deserves to be considered, 

 namely, the possible effect of radio-active matter, even if present, upon 

 the atomic weight value. The purest specimen of radium or "polonium" 

 3'et obtained has consisted of a mixture containing ^^robably little more 

 than fifty per cent of the active element, as nearly as could be estimated. 

 This highly impure material, however, possesses 8,000 times the radio- 

 activity of uranium. The radio-active power of the pure material is 

 undoubtedly very much greater than that of the impure mixture. Con- 

 sequently the quantity of ratio-active substance necessary to give to 

 uranium the comparatively slight degree of activity that it possesses must 

 be exceedingly minute. Giesel has recently shown * that a quantity of 

 radium so small that it cannot be detected by sulphuric acid is sufficient 

 to affect a photographic plate. Crookes also says on this point, " Con- 

 sidering my most active UrX does not contain sufficient of the real 

 material to show in the spectrograph, yet is powerful enough to give a 

 good impression on a photographic plate in five minutes, what must be 

 its dilution in compounds which require an hour, a day, or a week to 

 give an action ? " f Even in the ordinary active uranium compounds it 

 is most unlikely that the active element — if indeed it is an element — 

 could possibly be present in quantity sufficient to exert any influence 

 whatever upon the atomic weight of uranium. 



Pure carbon was obtained by ignition of sugar. Large, clear crystals 

 of the best " rock candy " of commerce were ground up in a porcelain 

 mortar and ignited at low heat in a platinum dish as long as organic 

 gases were given off. The resulting charcoal was then powdered in an 

 agate mortar and ignited in a hard glass combustion tube ; first in a 

 stream of pure, dry nitrogen, and finally in a stream of bromine vapor. 

 In this way the carbon was freed from any impurities which might, if 

 present, be acted upon during the sublimation and contaminate the 

 sublimate. Owing to the presence of undecomposed carbohydrates, or 

 possibly of water, most of the bromine was converted into hydrobromic 

 acid. Heatin? in bromine was continued until acid fumes ceased to be 

 given off. Finally, the carbon was again heated in a current of dry 

 nitrogen. Five grams of carbon, thus prepared, left no visible or weigh- 

 able residue after combustion in oxygen. 



The method of bromine purification was essentially identical with that 

 used in many other atomic weight investigations in this laboratory, and has 



* Berichte der deutschen chemischen Gesellschaft, 33, 3569 (1900). 

 t Proceed. Lond. Royal Soc, 66, 422 (1900). 



