RADIOACTIVE LEAD RICHARDS. 211 



precede the introduction of the substance into the balance case. 

 Every substance must be assumed to be impure, every reaction must 

 be assumed to be incomplete, every measurement must be assumed 

 to contain error until proof to the contrary can be obtained. Only 

 by means of the utmost care, applied with ever- watchful judgment, 

 may the unexpected snares which always lurk in complicated proc- 

 esses be detected and rendered powerless for evil. 



After all these digressions, made in order that the problems con- 

 cerned 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 numerous to mention were observed. Thus, the 

 weight of chlorine in the salt was found, and by difference the weight 

 of the lead. From the ratio of weights, the atomic weight of lead 

 was easily calculated. Since the question involved especially a com- 

 parison of the two kinds of lead, particular care was taken that 

 each sample should be treated in precisely the same way. Even if 

 a constant error had existed in the method, it could not have affected 

 the comparison, since each result would have been influenced in 

 identical degree. 



The outcome of our earliest trials, published in July, 1914, brought 

 convincing evidence that the atomic weight of the specimen 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 per 

 cent, far beyond the probable error of experiment. Almost simul- 

 taneously preliminary figures were made public by Doctors Honig- 

 schmid and St. Horovitz and Maurice Curie, pointing toward the 

 same verdict. 



This result, interesting and convincing as it was, was only a begin- 

 ning. Other experimenters abroad have since confirmed it, especially 

 Dr. Otto Honigschmid; and many new determinations have been 

 made at the Wolcott Gibbs Memorial Laboratory, with the assist- 

 ance of Dr. Charles Wadsworth, 3d, and Dr. Norris F. Hall, upon 

 various samples of lead from radioactive sources in widely separated 

 parts of the world. Messrs. E. R. Bubb and S. Radcliff, of the Ra- 

 dium Hill Co., of New South Wales, kindly sent a large quantity 

 of lead from their radium mines, and a particularly valuable speci- 

 men prepared from selected crystals of pure mineral was put at our 

 disposal by Dr. Ellen Gleditsch — not to mention other important 

 contributions from others, including Professor Boltwood and Sir 

 William Ramsay. Each of these samples gave a different atomic 

 weight for the lead obtained from them, and the conclusion was 

 highly probable that they contained varying admixtures of ordinary 

 lead in the uranium-radium lead. This was verified by the knowledge 



