March 27, 1919] 



NATURE 



77 



dried substances cling to water, and solids, even at 

 high temperatures, 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 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 difticultv in esti- 

 mating, or even detecting, minute traces of substances 

 remaining in solution, or minute losses by evaporation 

 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 uncertain than the physical 

 operation of weighing. The real difficulties precede 

 the introduction of the substance into the balance- 

 case. Every substance must be assumed to be impure, 

 every reaction to be incomplete, every measurement 

 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 processes be 

 detected and rendered powerless for evil. 



After all these digressions, made in order that the 

 problems concerned should be clearly recognised, 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 solu- 

 tion 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. 



The outcome of the first Harvard 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 ordinan,' lead, the value 

 found being 2o6-6 instead of 2072 — a difference of 

 03 per cent., far beyond the probable error of experi- 

 ment. Almost simultaneously preliminary figures 

 were made public by Drs. Honigschmid and St. 

 Horovitz and by Maurice Curie, pointing towards the 

 same verdict. 



This result, interesting and convincing as it was. 

 was only a beginning. Other experimenters abroad 

 have since confirmed it, especially Prof. Honigschmid, 

 who had studied at Harvard and understood the neces- 

 sary refinements of analysis; and many new deter- 

 minations have been made at the Wolcott Gibbs 

 Memorial Laboratory, with the assistance of Dr. 

 Charles Wadsworth and Dr. Norris F. Hall, 

 upon various samples of lead from radio-active 

 sources in widelv separated parts of the world. 

 Messrs. E. R. Bubb and S. Radcliff, of the Radium 

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

 quantity of lead from their radium mines, and a par- 

 ticularly valuable specimen prepared from selected 

 crystals of pure mineral was put at our disposal by 

 Prof. Gleditsch — not to mention other important con- 

 tributions from others, including Prof. 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 know- 

 ledge that in at least some cases the uranium ore 

 actually had been contaminated with lead ore. The 

 purest Norwegian specimen thus acquired especial 

 importance and significance, because it was only very 

 slightly, if at all, vitiated in this way. As a matter [ 

 of fact, it gave 206-08 for the atomic weight in ques- | 

 ' NO. 2578. VOL. 103] 



tion— the lowest of all. Here are typical results, 

 showing the outcome ; many more of similar tenor 

 were obtained : — 



AAomxc Weights. 

 Common lead ... {VorS^Y'^r^^ 



1 206 32 1 

 Australian radio-active lead containing ) 2o6'36 f ' V 

 probably 25 per cent, ordinary lead ... ) 206 33 ( ^'^'^^ 



( 20636 ) 



Purest uranic-lead. 



** f 206 



J 206 08 » ^^ o 



Honigschmid, from similar pure material, had 

 found figures (20605) agreeing almost exactly with 

 the last value. One cannot help believing that this 

 last specimen of lead is a definite substance, probably in 

 a state almost pure, because of the unmixed quality 

 of the carefully selected mineral from which it was 

 obtained. 



A further question now arises : Is it a permanent 

 substance — really an end-product of the disintegration ? 

 Soddy's hypothesis assumes that it is. The only im- 

 portant fact militating against this view is the ob- 

 servation that uranium-lead is always radio-active, 

 and hence might be suspected of being unstable. In 

 various impure specimens, however, the radio-activity 

 is not proportional to the change in the atomic weight ; 

 hence the radio-activity is probably, at least in part, 

 to be referred, not to the lead itself, but rather to 

 contamination with minute, unweighable amounts of 

 intensely radio-active impurities — other more transi- 

 tory products of disintegration.* If weighable, such 

 impurities would almost certainly increase, not 

 diminish, the atomic weight ; hence their presence 

 could not account for the low value. 



Let us compare the actual result for the atomic 

 weight of this kind of lead with the theory of Soddy 

 and Fajans. If this theory is sound, the simple sub- 

 traction of eight times the atomic weight of helium 

 from that of uranium, or five times the atomic weight 

 of helium from that of radium, should give the atomic 

 weight of the lead resulting from the disintegration as 

 follows : — 



Hypothetical Calculation of Atomic Weight of 

 Uranium-lead. 



... =238-18 

 ... = 3200 



Atomic weight of uranium 

 8 X atomic weight of helium 



Residue (lead?) 



Atomic weight of radium 

 5 X atomic weight of helium 



Residue (lead ?) 



Average hypothetical value for lead.. 

 Observed value for uranium -lead* . 



20618 = 20618 

 =225 -96 



: 20 'OO 

 205 -96 =205 -96 



= 206-07 

 = 20608 



Difference 001 



The agreement is remarkably good. Each of the 

 individual calculated values shows less than 005 per 

 cent, deviation from the average, and the average 

 itself shows essential identity with fact — a striking 

 confirmation of the theory. ' This is, perhaps, the 

 most successful attempt on record to compute an 

 atomic weight from hypothetical assumptions. 

 Usually we are wholly at a loss as to the theory 

 underlying the precise relationships, and must deter- 

 mine our values by careful experiment alone. 



2 For this reason the term "radio-active lead," although it docribes the 

 fact, is from a theoretical point of view perhaps not the best designation of 

 either uranium- or thorium-lead ; hut the term is convenient, because it dis 

 tinguishes between these two forms and common lead. 



' This is the Harvard result. If HCSnigschmid's value is Riven eqiial 

 weieht, the average observed value would be 206-07, exactly identical with 

 the hypothetical value. 



