528 REPORT — 1908. 



■wliich produces them in some respects behaves in a manner contrary to that of 

 all proper chemicals. It does not lose its power of creatinjT heat even at the 

 extreme cold of liquid air, while at the greater degree of cold of liquid hydrogen 

 its activity is found by Professor Dewar to be actually greater. 



Unlike old-fashioned chemicals which, when they are formed, have all their 

 properties properly developed, radium and its salts take a month before they 

 have acquired their fall power (so Dewar tells us), and then, for anything we 

 know to the contrary, proceed to manufacture heat, emanations, three kinds of 

 rays, electricity, and gases for ever. For ever ; well, perhaps not for ever, but 

 for so long a time that the loss of weight in a year calculated, I suppose, rather 

 than observed, is next to nothing. Professor Rutherford believes that thorium or 

 uranium, which act in the same kind of way, but with far less vigour, would last 

 a million years before there was nothing left, or at least before they were worn 

 out ; while the radium, preferring a short life and a merry one, could not expect 

 to exist for more than a few thousand years. 



In this time one gramme of radium would evolve one thousand million heat 

 units, sufficient, if converted into work, to raise five hundred tons a mile high ; 

 whereas a gramme of hydrogen, our best fuel, burned in oxygen, only yields thirty- 

 four thousand heat-units, or one thirty-thousandth part of the output of radium. 

 I believe that this is no exaggeration of what we are told and of what is believed 

 to be experimentally proved with regard to radium ; but if the half of it is true 

 the term ' the mystery of radium ' is inadequate : the miracle of radium is the 

 only expression that can be employed. 



With all this mystery before us, which I must confess myself wholly unable 

 to follow, I feel sure that members of the Association who are interested in the 

 work of this Section will welcome the discussion, for which our secretaries have 

 been able to arrange, and hear from the lips of Professor Rutherford the con- 

 clusions to which his researches have at present brought liim. No one is more 

 fitted than Professor Rutherford to open such a discussion, for no one has attacked 

 the theoretical side with such originality and daring, or with such ingenuity of 

 experiment. 



As an example of the activity of mind and of research to which the activity 

 of radium has given rise, I may mention the fact that the last number of the 

 ' Proceedings of the Royal Society ' is wholly concerned with radium, there being 

 four papers, all of the first importance, dealing with entirely different phenomena. 



It is not my purpose to review these or the subject of radium generally; 1 

 am in no way htted to do so. But I cannot well let the present opportunity pass 

 of referring to another mjstery of which a conspicuous example is now leaving 

 us. I refer to the mystery of the comet and its tails. What is a comet ? of 

 what does its tail consist ? Gravitational astronomy has told us for many years 

 past that compared with the planets or their satellites a comet does not weigh 

 anything. It weighs pounds or perhaps hundreds, thousands, or millions of tons ; 

 but in comparison with inconspicuous satellites it weighs nothing. Yet some of 

 them as they approach the sun from remote regions begin to shoot out streamers 

 which pour awoy as though repelled by the sun, not being left as a trail behind 

 the comet, as is so often supposed. These streamers, ejected towards the sun, 

 bend round and pour away at speeds which are enormous compared with that of 

 the comet itself, thus producing the tail. Now these streams separate very often, 

 and give rise to comets with two or three tails. 



The comet's tail is still a mystery. Let me take the most recent explanation, 

 which was set forth only three months ago in the ' Astrophysical Journal ' in the 

 United States. Those admirable experimentalists Nichols and Hull have for 

 some years been investigating the back pressure exerted by the action of light 

 upon bodies on which it falls. In this they have followed the Russian physicist 

 Lebedew, but in minuteness and delicacy of measurement, and in their successful 

 elimination of disturbances, their results are unequalled. It is sufficient to say 

 that, difficult and minute as the experiment is, their success is such that the 

 discrepancy between the calculated force and that which they have found is under 

 1 per cent. Perhaps I may express some satisfaction that in this measurement 

 use was made of the quartz fibre. 



