634 



NA TURE 



[October 25, 1906 



LETTERS TO THE EDITOR. 



[The Editor does not 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 Nature. 

 No notice is taken of anonymous communications.] 



Absorption of the Radioactive Emanations by 

 Charcoal. 



Tiiii interesting properly of certain Icinds of charcoal, 

 notably that of the cocoa-nut, of rapidly absorbing gases, 

 except the inert gases belonging to the argon family, is 

 now well known since the recent experiments of Sir James 

 Dewar. 



In a recent investigation I had occasion to pass the 

 radium emanation through a tube filled with cocoa-nut 

 charcoal, and was surprised to find that the emanation was 

 completely absorbed by it. If a slow current of air, mixed 

 with the emanation of radium, thorium, or actinium, is 

 passed through a tube filled with charcoal, the issuing gas 

 is completely deprived of emanation. This occurs at 

 ordinary temperatures, and there is no necessity for initial 

 cooling of the charcoal. This property of charcoal of 

 absorbing the radium emanation can be shown by a very 

 simple and striking experiment. If a side tube containing 

 a fraction of a gram of charcoal is attached to a vessel 

 containing the emanation released from several milligrams 

 of radium bromide, in the course of time the emanation is 

 absorbed by the charcoal. At ordinary temperatures, several 

 hours or days, depending on the size of the vessel, are 

 required to effect a complete absorption as the emanation 

 diffuses slowly through the air. If some powdered 

 willemite is added with the charcoal, the gradual absorp- 

 tion of the emanation is shown by the increasing brilliancy 

 of the phosphorescence produced in the surrounding 

 willemite. 



It makes no difference whether the charcoal has been 

 initially heated to get rid of the absorbed air or whether 

 it has already absorbed its full quantity. .\t low pressures 

 of the gas, using charcoal which has been previously 

 heated, the removal of the emanation takes place rapidly. 

 This is probably due to the rapid absorption of the gas 

 which carries the emanation with it. The charcoal retains 

 the emanation at ordinary temperatures, for I have found 

 that the emanation retained in a charcoal tube open to 

 the air loses its activity at the normal rate observed in 

 scaled vessels. 



The greater part of the emanation is released by heating 

 the charcoal below a red heat. I have not vet settled 

 whether the release of the emanation is due to an alter- 

 ation in the absorptive power of the charcoal for (he 

 emanatfon at high temperatures, or whether the emanation 

 Is mechanically carried away by the rush of air which 

 takes place when the charcoal is heated. 



Since the emanations behave like inert gases of the 

 argon family, it Is somewhat surprising that charcoal 

 should so readily absorb them. It must be remembered, 

 however, that in ordinary experiments a verv minute 

 quantity of the emanation Is present, and It Is not unlikely 

 that even the gases argon and helium are absorbed by 

 charcoal to a small degree. 



This property of charcoal of retaining the emanation 

 promises to be of service In laboratories where radium is 

 kept in a state of solution. It is dangerous to keep radium 

 In the form of solution in sealed vessels, as the gradual 

 production of hydrogen and oxygen in the solution raises 

 the internal pressure, which would ultimately lead to the 

 bursting of the vessel. At the same time, the escape of 

 the emanation causes a radio-active contamination of the 

 laboratory which renders delicate experiments nn radio- 

 activity or ionlsatlon very difficult. 



This problem will be solved by the use of a small tube 

 containing cocoa-nut charcoal attached to the vessel, with 

 one end open to the air. The air inside the radium vessel 

 Is kept at atmospheric pressure, while the emanation is 

 completely stopped in the charcoal. The emanation inlxed 

 with a small quantity of gas can at any time be obtained 

 from the charcoal by heating. 



Experiments are In progress to test whether this properly 



NO. 1930, VOL. 74] 



of charcoal can be utilised to determine quantitatively the 

 amount of radium emanation existing in the air, and' also 

 the amount of emanation diffusing to the atmosphere from 

 'he soil. E. Rutherford. 



McGill University, Montreal, October 6. 



The Recent Radium Controversy. 



I w.\s absent from Montreal during the lime of the 

 interesting discussion on radium which appeared in the 

 Times, and it is only quite recently that 1 have had an 

 opportunity of reading the correspondence in full. In the 

 course of this discussion some weight has been attached 

 to a_ remark in the second edition of my book " Radio- 

 activity," viz. that radium is a compound of helium and 

 lead. It is far from my intention to reopen this discussion, 

 on which I think quite enough has already been said, but 

 m the last issue of Nature (September 27) which I have 

 just received, there appears a letter bv Lord Kelvin in 

 which this remark is still further emphasised. 



Lord Kelvin quite correctly quotes my words, but I feel 

 that the statement, apart from its context, is liable to leave 

 an erroneous impression of my views on the question, 

 especially in the minds of those who are not directly 

 acquainted with my writings. 



.\t the risk of being somewhat lengthy, I should like to 

 quote fully some statements made in my book which, I 

 think, clearly show my attitude on this question. 



I. p. 482: — "In order to explain the presence of 

 helium in radium on ordinary chemical lines, it has been 

 suggested that radium is not a true element, but a molecular 

 compound of helium with some substance known or un- 

 known. The helium composed gradually breaks down, 

 giving rise to the helium observed. It is at once obvious 

 that this postulated helium compound is of a character 

 entirely different from that of any other compound pre- 

 viously observed in chemistry. Weight for weight, it emits 

 during its change an amount of energy at least one million 

 times greater than any molecular compound known (see 

 section 249). In addition it must be supposed that the 

 rate of breaking up of the helium compound is independent 

 of great ranges of temperature — a result never before 

 observed in any molecular change. The helium compound 

 in its breaking up must give rise to the peculiar radiations 

 and also pass through the successive radio-active changes 

 observed in radium. 



" Thus in order to explain the production of helium and 

 radio-activity on this view, a unique kind of molecule must 

 be postulated — a molecule in fact which Is endowed with 

 every single property which on the disintegration theory is 

 ascribed to the atom of the radio-elements. On the other 

 hand, radium, as far as it has been examined, has fulfilled 

 every test required for an element. It has a well marked 

 and characteristic spectrum, and there is no reason to sup- 

 pose that it is not an element in the ordinarily accepted 

 sense of the term. 



" On the theory that the radio-elements are undergoing 

 atomic disintegration, the helium must be considered to be 

 a constituent of the radium atom, or in other words, the 

 radium atom Is built up of parts, one of which, at least. 

 Is the atom of helium. ..." 



P. 483 : — " Taking the view that the o particles are pro- 

 jected helium atoms, w^e must regard the atoms of the 

 radio-elements as compounds of some known or unknown 

 substance with helium. These compounds break up spon- 

 taneously, and at a very slow rate even in the case of 

 radium. The disintegration takes place in successive 

 stages, and at most of the stages a helium atom is 

 projected with great velocity. This disintegration is 

 accompanied by an enormous emission of energy. The 

 liberation of such a large amount of energy In the radio- 

 active changes at once explains the constancy of the rate 

 of change under the action of any of the physical and 

 chemical agencies at our command. On this view, 

 uranium, thorium, and radium are in reality compounds 

 of helium. The helium, however, is held in such strong 

 combination that the compound cannot be broken up by 

 chemical or physical forces, and, in consequence, these 

 bodies behave as chemical elements in the ordinarily 

 accepted chemical sense. 



