210 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



ing differences in the atomic weights, according to the source of the 

 material. No such differences whatever were found. More recently 

 Professor Baxter has compared the atomic weights of iron and nickel 

 in meteorites (from an unknown, perhaps inconceivably distant 

 source) and the same terrestrial metals. In these cases also the re- 

 sults were negative. Thus copper, silver, sodium, iron and nickel all 

 appeared to be perfectly definite in nature, and their atoms, each 

 after its own kind, all alike. 



The general question remained, nevertheless, one of profound in- 

 terest to the theoretical chemist, because it involved the very nature 

 of the elements themselves; and in its relation to the possible dis- 

 covery of a difference between uranium lead and ordinary lead, it 

 became a very crucial question. 



Early in 1913, when the hypothesis of radioactive disintegration 

 had assumed definite shape, Doctor Fajan'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. The Carnegie Institution of 

 Washington gave generous pecuniary assistance toward providing 

 the necessary apparatus in this and subsequent investigations. 



The most important precautions to be taken in such work are 

 worthy of brief notice, because the value of the results inevitably 

 depends 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 unheeded impurities, otherwise its weight 

 will mean little. This is an end not easily attained, for liquids 

 often attack their containing vessels and absorb gases, crystals 

 include and occlude solvents, precipitates carry down polluting im- 

 purities, dried substances cling to water, and solids, even at high 

 temperatures, often fail to discharge their imprisoned contamina- 

 tions. Especial care was taken that each specimen was as pure as 

 it could be made, for impurity in one would vitiate the whole com- 

 parison. 



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 re- 

 maining in solution, or minute losses by evaporation at high tem- 

 peratures. 



In brief, " the whole truth and nothing but the truth," is the aim. 

 The chemical side of the question is far more intricate and uncer- 

 tain than the physical operation of weighing. The real difficulties 



