JANU.VKT 3, 1919] 



SCIENCE 



chemist, because it involved the very nature 

 of the elements themselves; and in its relation 

 to the possible discovery of a difference be- 

 tween uranium lead and ordinary lead, it be- 

 came a very crucial question. 



Early in 1913, when the hjix)thesis of radio- 

 active disintegration had assumed definite 

 shape. Dr. Fajans's assistant, Max Lembert, 

 journeyed to Cambridge, bringing a large 

 quantity of lead from Bohemian radioactive 

 sources, in order that its atomic weight might 

 be determined by Harvard methods, with the 

 precision attainable there. The Carnegie In- 

 stitution of Washington gave generous pecu- 

 niary assistance toward providing the nec- 

 essary apparatus, in this and subsequent in- 

 vestigations. 



The most important precautions to be taken 

 in such work are worthy of brief notice, be- 

 cause the value of the results inevitably de- 

 pends upon them. The operation consists in 

 weighing specimens of a salt of the element 

 in question, and then precipitating one of the 

 constituents in each specimen, determining 

 the weight of the precipitate, and thus the 

 composition of the salt. In the first place, 

 each portion of substance to be weighed must 

 be free from the suspicion of containing un- 

 heeded impurities, otherwise its weight wiU 

 mean little. This is an end not easily at- 

 tained, for liquids often attack their con- 

 taining vessels and absorb gases, crystals in- 

 clude and occlude solvents, precipitates carry 

 down polluting impurities, dried substances 

 cling to water, and solids, even at high tem- 

 peratures, often fail to discharge their im- 

 prisoned contaminations. Especial care was 

 taken that each specimen was as pure as it 

 could be made, for impurity in one would 

 vitiate the whole comparison. 



In the next place, after an analysis has 

 once begun, every trace of each substance to 

 be weighed must be collected and find its 

 way in due course to the scale pan. The 

 trouble here lies in the difficulty in estimating, 

 or even detecting, minute traces of substances 

 remaining in solution, or minute losses by 

 evai>oration at high temperatures. 



In brief, " the whole truth and nothing but 



the truth " is the aim. The chemical side of 

 the question is far more intricate and un- 

 certain than the physical oi)oration of weigh- 

 ing. The real difficulties precede the intro- 

 duction of the substance into the balance case. 

 Everj' substance must be assumed to be im- 

 pure, every reaction must be assumed to be in- 

 complete, everjy measurement must be as- 

 sumed to contain error, until proof to the con- 

 trary can be obtained. Only by means of the 

 utmost care, applied with ever-watchful judg- 

 ment, may the imexpected snares which always 

 lurk in complicated processes be detected and 

 rendered powerless for evil. 



After all these digressions, made in order 

 that the problems concerned should be clearly 

 recognized, let us turn to the main object of 

 our quest. In the present case, each form of 

 lead was first weighed as pure chloride, and 

 the chlorine in this salt after solution was 

 precipitated as silver chloride, the weight of 

 which was determined. Precautions too nu- 

 merous to mention were observed. Thus the 

 weight of chlorine in the salt was foimd, and 

 by difference the weight of the lead. From 

 the ratio of weights, the atomic weight of lead 

 was easily calculated. 



The outcome of the first Harvard trials, 

 published in July, 1914, brought convincing 

 evidence that the atomic weight of the speci- 

 men of uranium-lead from Bohemia is really 

 less than that of ordinary lead, the value found 

 being 206.6, instead of 207.2 — a difference of 

 0.3 i)er cent., far beyond the probable error of 

 experiment. Almost simultaneously prelimi- 

 nary figures were made public by Drs. Honig- 

 schmid and St. Horovitz and Maurice Curie, 

 pointing toward the same verdict. 



This result, interesting and convincing as 

 it was, was only a beginning. Other experi- 

 menters abroad have since confirmed it, espec- 

 ially Professor Honigschmid, who had studied 

 at Harvard and understood the necessary re- 

 finements of analysis; and many new deter- 

 minations have been made at the Wolcott 

 Gibbs Memorial Laboratory, with the assist- 

 ance of Dr. Charles Wadsworth, 3d, and Dr. 

 i^orris F. Hall, upon various samples of lead 



