August 26, 1909J 



NA TURE 



?57 



its age may be. On this view tlie different atoms of a 

 radio-active substance are not, in all respects, identical. 



The energy developed by radio-active substances is ex- 

 ceedingly large, one gram of radium developing nearly 

 as much energy as would be produced by burning a ton 

 of coal. This energy is mainly in the a particles, the 

 positively charged helium atoms which are emitted when 

 the change in the atom takes place ; if this energy were 

 produced by electrical forces it would indicate that the 

 helium atom had moved through a potential difference of 

 about two million volts on its way out of the atom of 

 radium. The source of this energy is a problem of the 

 deepest interest ; if it arises from the repulsion of similarly 

 electrified systems exerting forces varying inversely as the 

 square of the distance, then to get the requisite amount 

 of energy the systems, if their charges were comparable 

 with the charge on the a particle, could not when they 

 start be further apart than the radius of a corpuscle, 

 10-'^ cm. If we suppose that the particles do not acquire 

 this energy at the explosion, but that before they are shot 

 out of the radium atom they move in circles inside this 

 atom with the speed with which they emerge, the forces 

 required to prevent particles moving with this velocity 

 from flying off at a tangent are so great that finite charges 

 of electricity could only produce them at distances com- 

 parable with the radius of a corpuscle. 



One method by which the requisite amount of energy 

 could be obtained is suggested by the view to which I 

 have already alluded — that in the atom we have electrified 

 systems of very different types, one small, the other large ; 

 the radius of one type is comparable with lo-" cm., that 

 of the other is about 100,000 times greater. The electro- 

 static potential energy in the smaller bodies is enormously 

 greater than that in the larger ones ; if one of these small 

 bodies were to explode and expand to the size of the 

 larger ones, we should have a liberation of energy large 

 enough to endow an a. particle with the energy it possesses. 

 Is it possible that the positive units of electricity were, to 

 begin with, quite as small as the negative, but while in 

 the course of ages most of these have passed from the 

 smaller stage to the larger, there are some small ones 

 still lingering in radio-active substances, and it is the 

 explosion of these which liberates the energy set free 

 during radio-active transformation? 



The properties of radium have consequences of enormous 

 importance to the geologist as well as to the physicist or 

 chemist. In fact, the discovery of these properties has 

 entirely altered the aspect of one of the most interesting 

 geological problems, that of the age of the earth. Before 

 the discovery of radium it was supposed that the supplies 

 of heat furnished by chemical changes going on in the 

 earth were quite insignificant, and that there was nothing 

 to replace the heat which flows from the hot interior of 

 the earth to the colder crust. Now when the earth first 

 solidified it only possessed a certain amount of capital in 

 the form of heat, and if it is continually spending this 

 capital and not gaining any fresh heat it is evident that 

 the process cannot have been going on for more than a 

 certain number of years ; otherwise the earth would be 

 colder than it is. Lord Kelvin in this way estimated the 

 age of the earth to be less than 100 million years. Though 

 the quantity of radium in the earth is an exceedingly 

 small fraction of the mass of the earth, only amounting, 

 according to the determinations of Profs. Strutt and Joly, 

 to .^bout five grams in a cube the side of which is 100 

 miles, yet the amount of heat given out by this small 

 quantity of radium is so great that it is more than enough 

 to replace the heat which flows from the inside to the 

 outside of the earth. This, as Rutherford has pointed 

 out, entirely vitiates the previous method of determining 

 the age of the earth. The fact is that the radium gives 

 out so much heat that we do not quite know what to do 

 with it, for if there was as much radium throughout the 

 interior of the earth as there is in its crust, the tempera- 

 ture of the earth would increase much more rapidly than 

 it does as we descend below the earth's surface. Prof. 

 Strutt has shown that if radium behaves in the interior 

 of the earth as it does at the surface, rocks similar to 

 those in the earth's crust cannot extend to a depth of 

 more than forty-five miles helow the surface. 



It is remarkable that Prof. Milne from the study of 



NO. 2078, VOL. 81] 



earthquake phenomena had previously come to the con- 

 clusion that rocks similar to those at the earth's surface 

 only descend a short distance below the surface; he 

 estimates this distance at about thirty miles, and con- 

 cludes that at a depth greater than this the earth is fairly 

 homogeneous. 



Though the discovery of radio-activity has taken away 

 one method of calculating the age of the earth it has- 

 supplied another. 



The gas helium is given out by radio-active bodies, and 

 since, except in beryls, it is not found in minerals which 

 do not contain radio-active elements, it is probable that 

 all the helium in these minerals has come from these 

 elements. In the case of a mineral containing uranium, 

 the parent of radium in radio-active equilibrium, with 

 radiiim and its products, helium will be produced at a 

 definite rate. Helium, however, unlike the radio-active 

 elements, is permanent, and accumulates in the mineral ; 

 hence if we measure the amount of helium in a sample 

 of rock and the amount produced by the sample in one- 

 year we can find the length of time the helium has been 

 accumulating, and hence the age of the rock. This- 

 method, which is due to Prof. Strutt, may lead to deter- 

 minations, not merely of the average age of the crust 

 of the earth, but of the ages of particular rocks and the 

 date at which the various strata were deposited ; he has, 

 for example, shown in this way that a specimen of the 

 mineral thorianite must be more than 240 million years 

 old. 



The physiological and medical properties of the rays 

 emitted by radium is a field of research in which enough' 

 has already been done to justify the hope that it may lead 

 to considerable alleviation of human suffering. It seems 

 quite definitely established that for some diseases, notably 

 rodent ulcer, treatment with these rays has produced re- 

 markable cures ; it is imperative, lest we should be pass- 

 ing_ over a means of saving life and health, that the 

 subject should be investigated in a much more systematic 

 and extensive manner than there has yet been either time 

 or material for. Radium is, however, so costly that few 

 hospitals could afford to undertake pioneering work of 

 this kind; fortunately, however, through the generosity 

 of Sir Ernest Cassel and Lord Iveagh a Radium Institute, 

 under the patronage of his Majesty the King, has been 

 founded in London for the study of the medical properties 

 of radium, and for the treatment of patients suffering 

 from diseases for which radium is beneficial. 



The new discoveries made in physics in the last few 

 years, and the ideas and potentialities suggested by them, 

 have had an effect upon the workers in that subject akin 

 to that produced in literature by the Renaissance. 

 Enthusiasm has been quickened, and there is a hopeful, 

 youthful, perhaps exuberant, spirit abroad which leads 

 men to make with confidence experiments which would 

 have been thought fantastic twenty years ago. It has 

 quite dispelled the pessimistic feeling, not uncommon at 

 that time, that all the interesting things had been dis- 

 covered, and all that was left was to alter a decimal or 

 two^ in some physical constant. There never was any 

 justification for this feeling, there never were any signs 

 of an approach to finality in science. The sum of know- 

 ledge is at present, at any rate, a diverging, not a con- 

 verging, series. As we conquer peak after peak we see 

 in front of us regions full of interest and beauty, but we 

 do not see our goal, we do not see the horizon ; in the 

 distance tower still higher peaks, which will yield to 

 those who ascend them still wider prospects, and deepen 

 the feeling, the truth of which is emphasised by every 

 advance in science, that " Great are the Works of the 

 Lord." 



SECTION A. 



MATHEMATICS AND PHYSICS. 



Opening Address by Prof. E. Rutherford, M.A., D.Sc, 

 F.R.S., President of the Section. 

 It is a great privilege and pleasure to address the 

 members of this Section on the occasion of the visit of 

 the British Association to a country with which I have 

 had such a long and pleasant connection. I feel myself 

 in the presence of old friends, for the greater part of what 



