Mav 5, 1904] 



NA TURE 



LETTERS TO THE EDITOR. 

 [The Editor does twt hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Natl'RE. 

 No notice is taken of anonymous communications.] 



The Disaster to Submarine Ai. 



At the inquest on the victims of the disaster to sub- 

 marine Ai, Commander Bacon is reported to have expressed 

 the opinion that as the result of the collision every soul 

 on board was instantly stunned, since the failure to set in 

 action the mechanism for bringing the boat to the surface 

 could not otherwise be accounted for. It is surprising that 

 this opinion should have been received and adopted without 

 comment by both the coroner and the lay Press, seeing that 

 such a result is contrary to all experience of collisions at 

 sea. The occupant of the conning tower, which was the 

 part struck, was no doubt stunned, probably killed, by the 

 blow, but it is difficult to believe that the same fate should 

 have befallen every other person on board, however remote 

 from the point of concussion. 



The fact that the naval authorities can suggest no other 

 reason for the failure to rise to the surface after the collision 

 is not in itself a sufficient justification for the acceptance 

 of an opinion which, from the physiological point of view, 

 is, to say the least, highly improbable, and certainly 

 requires confirmation by experiment. 



University, Edinburgh, May i. E. A. Schafer. 



The Life-history of Radium. 



Evidence of a convincing nature is rapidly accumulating 

 to the effect that helium may be produced as a result of the 

 disintegration of the radium atom. On the other hand, it 

 has been suggested by Rutherford and others that radium 

 is analogous to the first products of the disintegration of 

 uranium and thorium — to the substances known as 

 uranium X and thorium X — rather than to those elements 

 themselves. Such an idea points to a search for the parent 

 atom, by the dissolution of which radium is formed. 



In Prof. Rutherford's recent book on radio-activity, 

 reasons are given for suspecting that in uranium itself we 

 shall find the origin of radium. The atomic weight of 

 uranium is greater than that of radium. Radium is dis- 

 covered in minerals rich in uranium, and the amount of 

 radium in good pitchblende is about that to be expected on 

 the view that a balance exists between the rate of develop- 

 ment of the radium by the uranium present and the rate 

 at which it decays by the ordinary process of radio-activity. 



My wife and I have been investigating lately the slight 

 amounts of radium emanation that are almost invariably 

 found in samples of salts and oxides of uranium sold as 

 chemically pure. By the kindness of Mr. H. J. H. Fenton 

 we have been able to examine several specimens of uranium 

 compounds, known to have been preserved in the Cambridge 

 University Chemical Laboratory for periods of from 

 seventeen to twenty-five years. In all cases greater amounts 

 of radium emanation have been obtained from these old 

 specimens than from more recently prepared samples of 

 the corresponding compounds. 



It is, of course, possible that a limited number of such 

 results may be accidental, and, in order that indirect evidence 

 of this kind should possess any weight, enough specimens 

 must be examined to enable us to deal with the subject 

 statistically. I should be very grateful if anyone possess- 

 ing uranium compounds of known pedigree, prepared thirty 

 years ago or upwards, would either test them quantitatively 

 for radium emanation, or send a few grammes of them to 

 me for examination. 



If, in most cases, an excess of radium is discovered in 

 the older samples, it would be presumptive evidence in 

 favour of the view that radium is formed by the disintegra- 

 tion of uranium, but the possibility of some general change 

 in the methods of preparation of uranium salts renders even 

 such a confirmation of doubtful validity. 



The only convincing evidence would be supplied by tracing 

 the gradual growth of radium in a mass of a compound of 

 uranium. .At first sight, it would seem that the time re- 



NO. 1801, VOL. 70] 



quired for such growth would put the possibility^ of such 

 a confirmation beyond the reach of one human life. But 

 a short calculation shows that the attempt is not so hopeless 

 as might be imagined. 



The average life of a radium atom is taken by Ruther- 

 ford, on a minimum estimate, as about fifteen hundred 

 years. The process of decay occurs in a geometrical pro- 

 gression, and thus in one year about half a milligramme 

 per gramme of radium should disintegrate. On a maximum 

 estimate for the life, the fraction disintegrated per year is 

 i/ioo milligramme. Taking this maximum estimate as 

 the least favourable for our purpose, we see that in one 

 year the one hundred thousandth part will break up. 



If in pitchblende, radium is in radio-active equilibrium 

 with its source of supply, the same fraction must be re- 

 placed in the year by the disintegration of uraniuni. In 

 presence of a large excess of uranium, the production of 

 radium would go on at a constant rate. Thus in one year 

 about the one hundred thousandth part of the proportion 

 of radium in pitchblende would be developed in an equiva- 

 lent mass of uranium. 



We find that, using a good electroscope, it is easy to detect 

 with certainty the radio-activity from the radium emanation 

 evolved on heating a milligramme of good pitchblende. 

 In order to produce from uranium an amount of radium 

 large enough to detect by its radio-activity in a reasonable 

 time — let us say one year — it is merely necessary to work 

 with a sufticient quantity of uranium to give, in that tirne, 

 a mass of radium of which the emanation has an activity 

 equal to that evolved from a milligramme of pitch- 

 blende. The requisite quantity of uranium is clearly about 

 0001x100000=100 grammes. This, as we said, is a 

 maximum estimate ; it is probable that less would suffice. 



In this manner, by putting on one side a few hundred 

 grammes of some compound of uranium, carefully freed 

 from radium and tested for emanation, it should be possible 

 to detect the growth of radium in a time measured in 

 months, or, on the other hand, to show that it is necessary 

 to look elsewhere for the parent atom of radium. 



At the present time we have such an investigation in 

 progress, and trust that eventually we may obtain definite 

 results. But, in the hope that others may undertake a 

 similar task, I venture to place the principles of the method 

 before your readers. On such a fundamental point, several 

 indeperident experiments are greatly to be desired. 



W. C. D. Whetham. 



Upwater Lodge, Cambridge, April 30. 



Graphic Methods in an Educational Course on 

 Mechanics. 



Though no one, I venture to think, will gainsay Mr. 

 W. Larden's main contention that " analytical methods 

 give a grasp of the principles of statics, while graphical 

 methods disguise them," yet it should not be forgotten that 

 the analytical treatment has its own set of snares and 

 pitfalls. 



Mechanics is a physical science, and like other sciences 

 should be approached from the experimental side. If the 

 initial stages are treated experimentally, the principles 

 underlving the subject will come prominently into view. 

 One need only mention the principle of moments, which 

 every boy has' surely grasped, in a general sort of way, 

 long before he has opened a text-book on statics. He has 

 only to carry out a few simple experiments on levers to 

 find out the law for himself in its exact form. Let the 

 beginner hang up two spring balances from nails and then 

 attach a weight by a couple of strings to the hooks of the 

 balances, and he will soon discover for himself whether or 

 not the pulls in the strings are proportional to their 

 lengths. 



The graphical treatment lays stress on the empirical and 

 tentative side, which in the symbolical is completely lost 

 sight of. But the superlative advantage of graphical work 

 is its essentially practical character. All cases of a problem 

 can be solved with equal facility. Ladders are not as a 

 rule inclined to the ground at an angle of 60°, coefficients 

 of friction are never quadratic surds, and weights of 

 v/Tpoundals belong to some other world which is not the 

 one in which we live. Again, the question is on a screw 

 jack, and a bov taking 7r = 22 7 has worked out an answ^er 



