DISCOVERY IN ATOMIC WEIGHTS

Professor Theodore W. Richards '86 and Mr. Max E. Lambert, who came to America from the Grand Ducal Technical school of Karlsruhe for the purpose of assisting Professor Richards, have recently been studying the atomic weight of lead obtained from radioactive minerals.

Many workers upon radioactivity have pointed out the fact that the most conclusive test concerning the recent theory the degeneration of radioactive elements is to be found in the determination of the atomic weights. Radium has a atomic weight of 226, but is known to decompose by emanations of helium. Each atom of helium has an atomic weight of 4, so that after five emanations, the substance remaining has an atomic, weight of 206. This substance is down as radium G. Lead is known to have an atomic weight of 207, and should therefore be practically indistinguishable from radium G. This is the theory which has been advanced. There was, however, some doubt as to whether or not this end product was really lead.

It was for the purpose of really determining whether lead from radioactive sources actually had an atomic weight as low as 206, that Professor Richards and Dr. Lambert undertook their experiment. The most notable thing in connection with the investigation, aside from the astonishing results secured, as the hearty co-operation of the chemists of other countries, prominent investigators of England, Germany, Norway, Russia, and Austria all contributing samples of lead from radioactive sources, for investigation.

The method of experimentation is complicated. The experiments were carried out with the greatest thoroughness, especial care being taken to have the materials as pure as possible. The results of the investigation were amazing. It was found that all of the radioactive speciments, obtained from uraninite, carnotite and thorianite, exhibited a lower atomic weight than ordinary lead, as determined under identical conditions, the deficiency in one case amounting to as much as 0.75 of a unit.

No simple linear quantitative relationship between the exact amount of radioactivity and the atomic weight was found. The ultra-violet spectrum of a typical specimen appeared to be exactly identical with that of ordinary lead. The necessary inference seems to be that lead from radioactive sources consists of a mixture of at least two substances, of which one is ordinary lead. The foreign substance must be very similar to ordinary lead and very difficult if not impossible to eliminate by chemical means; for many precautions were taken to purify the samples. This substance cannot be identified in the ultra-violet spectrum of the material, either because it has the same spectrum as lead, or because it has no spectrum in that part of the field, or because its spectrum is masked or absorbed by that of the lead.

This amazing outcome is contrary to the customary experience with several other elements, notably copper, silver, iron, sodium, and chlorine, each of which seems to give a constant atomic weight, no matter what the geographical source may have been. No attempt is made here to discuss the theoretical aspects of the facts presented, but attention is called to their qualitative agreement with the hypothesis brought forward by Dr. Fajans and by Dr. Soddy, that some of the places in the periodic table, corresponding to high atomic weight should perhaps include several elements, different in atomic weights, but very similar in other properties.