1256 



NA JURE 



[August 26, 1909 



rwould seem meaningless still to regard them as forming 

 -one unit. Thus the energy in the bundles of light, after 

 - they have suffered partial reflection, will not be the same 



• as in the bundles when they were emitted. The study of 



• the dimensions of these bundles, for example, the angle 

 they subtend at the luminous source, is an interesting sub- 

 ject for investigation ; e.xperiments on interference between 

 rays of light emerging in different directions from the 

 luminous source would probably throw light on this point. 



I now pass to a very brief consideration of one of the 

 most important and interesting advances ever made in 

 physics, and in which Canada, as the place of the labours 

 .of Profs. Rutherford and Soddy, has taken a conspicuous 

 part. I mean the discovery and investigation of radio- 

 activity. Radio-activity was brought to light by the 

 Rbntgen rays. One of the many remarkable properties 

 •of these rays is to e.xcite phosphorescence in certain sub- 

 stances, including the salts of uranium, when they fall 

 upon them. Since Rbntgen rays produce phosphorescence, 

 it occurred to Becquerel to try whether phosphorescence 

 would produce Rbntgen rays. He took some uranium 

 salts which had been made to phosphoresce by exposure, 

 not to Rbntgen rays, but to sunlight, tested them, and 

 found that they gave out raj's possessing properties 

 similar to Rbntgen rays. Further investigation showed, 

 however, that to get these rays it was not necessary to 

 make the uranium phosphoresce, that the salts were just 

 as active if they had' been kept in the dark. It thus 

 appeared that the pi-operty was due to the metal and 

 not to the phosphorescence, and that uranium and its 

 compounds possessed the power of giving out rays which, 

 'like Rbntgen rays,- affect a photographic plate, make 

 certain minerals phosphoresce, and make gases through 

 which they pass conductors of electricity. 



Niepce de Saint-Victor had observed some years before 

 this discovery that paper soaked in a solution of uranium 

 nitrate affected a photographic plate, but the observation 

 ■excited but little interest. The ground had not then been 



• prepared, by the discovery of the Rbntgen rays, for its 

 -reception, and it withered and was soon forgotten. 



Shortly after Becquerel 's discovery of uranium, Schmidt 

 found that thorium possessed similar properties. Then 

 M. and Mme. Curie, after a most difficult and laborious 

 investigation, discovered two new substances, radium and 

 polonium, possessing this property to an enormouslv 

 greater extent than either thorium or uranium, and this 

 was followed by the discovery of actinium by Debierne. 

 Now the researches of Rutherford and others have led 

 to the discovery of so many new radio-active substances 



"that any attempts at christening seem to have been 

 abandoned, and they are denoted, like policemen, by the 

 letters of the alphabet. 



Mr. Campbell has recently found that potassium, 

 though far inferior in this respect to any of the substances 

 I have named, emits an appreciable amount of radiation, 

 the amount depending only on the quantity of potassium, 

 and being the same whatever the source from which the 

 potassium is obtained or whatever the elements with which 

 it may be in combination. 



The radiation emitted by these substances is of three 

 types, known as a, 0, and y rays. The o rays have 

 been shown by Rutherford to be positively electrified 

 atoms of helium, moving with speeds which reach up to 

 about one-tenth of the velocity of light. The j3 rays are 

 negatively electrified corpuscles, moving in some cases with 

 very nearly the velocity of light itself, while the 7 rays 

 are unelectrlfied, and are analogous to the Rbntgen ravs. 



The radio-activity of uranium was shown by Crookes 

 to arise from something mixed with the uranium, 

 which differed sufficiently in properties from the uranium 

 itself to enable it to be separated by chemical analysis. 

 He took some uranium, and by chemical treatment 

 separated it into two portions, one of which was radio- 

 active and the other not. 



Next Becquerel found that if these two portions were 



'kept for several months, the part which was not radio- 

 active to begin with regained radio-activity, while the 

 part which was radio-active to begin with had lost its 

 radio-activity. These effects and many others receive a 



-complete explanation by the theorv of radio-active change 

 which we owe to Rutherford and Soddy. 



NO. 2078, VOL. 81] 



.'\ccording to this theory, the radio-active elements are 

 not permanent, but are gradually breaking up into 

 elements of lower atomic weight ; uranium, for example, 

 is slowly breaking up, one of the products being radium, 

 while radium breaks up into a radio-active gas called 

 radium emanation, the emanation into another radio-active 

 substance, and so on, and that the radiations are a kind 

 of swan's song emitted by the atoms when they pass 

 from one form to another ; that for example, it is when 

 a radium atom breaks up and an atom of the emanation 

 appears that the rays which constitute the radio-activity 

 are produced. 



Thus, on this view, the atoms of the radio-active 

 elements are not immortal ; they perish after a life the 

 average value of which ranges from thousands of millions 

 of years in the case of uranium to a second or so in the 

 case of the gaseous emanation from actinium. 



When the atoms pass from one state to another they 

 give out large stores of energy ; thus their descendants 

 do not inherit the whole of their wealth of stored-up 

 energy; the estate becomes less and less wealthy withj 

 each generation ; we find, in fact, that the politician, 

 when he imposes death duties, is but imitating a process 

 which has been going on for ages in the case of these 

 radio-active substances. 



Many points of interest arise when we consider the rate 

 at which the atoms of radio-active substance disappear. 

 Rutherford has shown that whatever be the age of these 

 atoms, the percentage of atoms which disappear in one 

 second is always the same ; another way of putting it is 

 that the expectation of life of an atom is independent of 

 its age — that an atom of radium one thousand years old 

 is just as likely to live for another thousand years as one 

 just sprung into existence. 



Now this would be the case if the death of the atom 

 were due to something from outside which struck old 

 and young indiscriminatelv ; in a battle, for example, th'* 

 chance of being shot is the same for old and young ; so 

 that we are inclined at first to look to something coming 

 from outside as the cause why an atom of radium, for 

 example, suddenly changes into an atom of the emana- 

 tion. But here we are met with the difficulty that no 

 changes in the external conditions that we have as yet 

 been able to produce have had any effect on the life of 

 the atom ; so far as we know at present, the life of a 

 radium atom is the same at the temperature of a furnace 

 as at that of liquid air — it is not altered by surrounding 

 the radium by thick screens of lead or other dense 

 materials to ward oft radiation from outside, and, what 

 to my mind is especially significant, it is the same when 

 the radium is in the most concentrated form, when its 

 atoms are exposed to the vigorous bombardment from the 

 rays given off by the neighbouring atoms, as when it is 

 in the most dilute solution, when the rays are absorbed 

 by the water which separates one atom from another. 

 This last result seems to me to make it somewhat improb- 

 able that we shall be able to split up the atoms of the 

 non-radio-active elements by exposing them to the radiation 

 from radium ; if this radiation is unable to affect the un- 

 stable radio-active atoms, it is somewhat unlikely that it 

 will be able to affect the much more stable non-radio- 

 active elements. 



The evidence we have at present is against a disturb- 

 ance coming from outside breaking up the radio-active 

 atoms, and we must therefore look to some process of 

 decay in the atom itself ; but if this is the case, how are 

 we to reconcile it with the fact that the expectation of 

 life of an atom does not diminish as the atom gets older !- 

 We can do this if we suppose that the atoms when they 

 are first produced have not all the same strength of con- 

 stitution, that some are more robust than others, perhaps 

 because they contain more intrinsic energy to begin with, 

 and will therefore have a longer life. Now if when the 

 atoms are first produced there are some which will live 

 for one year, some for ten, some for a thousand, and 

 so on ; and if lives of all durations, from nothing to 

 infinity, are present in such proportion that the number 

 of atoms which will live longer than a certain number 

 of years decreases in a constant proportion for each 

 additional year of life, we can easily prove that the ex- 

 pectation of life of an atom will be the same whatever 



