January i6, 19 13] 



NATURE 



DO/ 



the highest success in his calling the science-master 

 must, of course, be thoroughly versed in his subject, 

 alike theoretically and practically. He should, if pos- 

 sible, be a man who has himself done some original 

 research, or at least is intimately familiar with 

 methods of experimentation and investigation, and 

 able to guide his pupils along the lines of independent 

 research. I am strongly of opinion that his efficiency 

 will be much augmented if he has had a good 

 literary as well as a scientific training. When he 

 enters on his teaching career he will soon find the 

 great advantage of a cultivated style, both in dis- 

 coursing and in writing. Unfortunately some able 

 men of science who have neglected the literary side 

 of their education cannot arrange their thoughts in 

 proper sequence or express them with clearness and 

 terseness. I would urge the science-master to keep 

 his hold on literature, ancient as well as modern. 

 Many a time when weary with his labours, and dis- 

 couraged, perhaps, by the difficulties whi.rewith they 

 are beset, he will find in that delightful field ample 

 consolation and refreshment. 



But, above all, the science-master must be 

 thoroughly in love with his subject and possess the 

 power of infusing some of his affection for it into 

 his pupils. His evident and genial enthusiasm should 

 be infectious and become an inspiration that appeals 

 to his boys in everything he does, whether as he 

 lectures and demonstrates to them in the class-room 

 or as he shows them how to work in the laboratory. 

 There are probably few other callings in the educa- 

 tional domain where the personal touch, the stimulat- 

 ing influence that springs from earnest devotion to a 

 subject, has so many opportunities of manifesting 

 itself and tells more promptly and powerfully^ on the 

 pupils. The teacher who is gifted with such an in- 

 spiring power may do more in the way of developing 

 a love of science with the meagre outfit of a parish 

 school than a man without this influence can do with 

 all the resources of a modern laboratory. 



RADIATIONS OLD AND NEW.''- 

 A AT' HEN, therefore. X-rays are projected into any 

 • • material we must think of them as a stream of 

 separate entities, each one of which has complete 

 independence of its neighbours and pursues a life of 

 its own. It changes to a yS ray and back again ; as a 

 (3 ray it is liable to loss of energy and much deflection, 

 so that those rays which do not pass through the 

 body but are held therein end as electrons moving 

 about in the bod)' with the velocities of thermal agita- 

 tion ; that is to say, with those velocities which free 

 electrons in the body must possess on account of the 

 share which they take in carrying the heat of the 

 body. 



Now we may ask ourselves what will be the result 

 if transformations continue to take place at these 

 lower energies ; for the moment let us assume that 

 they do. Let us consider some substance like a block 

 of metal. Within it we know that there are innumer- 

 able electrons travelling to and fro with various speeds. 

 In their motion is stored up energy ; the communica- 

 tion of heat to the body makes them dance more 

 quickly. When the quicker motion is begun in one 

 part of the body, diffusion hands on the motion to 

 the rest ; that is to say, heat has been conducted 

 through the body. If we try to pass an electric cur- 

 rent through the body, it is the movement of the 

 electrons that constitutes the current. This is the 

 accepted theory at the present time. It is even pos- 



1 Evening discourse delivered on September 6, igi2, before the British 

 Association at Dundee by Prof. W. H. Bragg F. R.S. Continued from 

 P- 532- 



NO. 2 2 55, VOL. 90] 



sible — but this is not accepted by all — that the energy 

 of the moving electrons in the body constitutes the 

 main store of heat therein. The electrons do not all 

 move at the same speed, of course ; but there is a 

 certain well-known distribution of their energies about 

 a mean value. At any time a certain percentage of 

 the electrons are moving with speeds Ijdng within 

 definite limits, although the individuals possessing 

 such speeds are continually changing. If we now 

 take into account the transformations of which I have 

 been speaking, we find that there must be X-ray 

 quanta — this name will do for them as well as any 

 others — in such numbers as to be in equilibrium with 

 the electrons of every variety of speed. In the case 

 of the X-rays and electrons which we have been 

 handling in our experiment, we find that the greater 

 the energy the larger the number of X-ray quanta 

 required to be in equilibrium with the corresponding 

 electrons, for quanta of large energy are transformed 

 into electrons much more rarely than quanta of small 

 energy, whereas electrons of large energy are trans- 

 formed as often, and perhaps more often, than those 

 of small energy. Thus the distribution of energy 

 amongst the quanta is not the same as the distribu- 

 tion amongst the electrons ; in the former there is a 

 much larger number — relatively — of the quanta of 

 larger energy. 



The electrons which we are considering have very 

 little power of penetration or of breaking away from 

 the substances in which they are. At high tempera- 

 tures, when they move more quickly, there is a con- 

 siderable emission, an effect which has been much 

 studied recently. But at ordinary temperatures the 

 emission of electrons is small. Recently R. W. Wood 

 has suggested that there must be an "aura" of elec- 

 trons surrounding a conductor and extending a minute 

 distance away, since only in this way can we account 

 for the fact that electricity passes freely from one con- 

 ductor to another when they are separated by a space 

 of the order of a wave-length of light. But if the 

 electrons have such difficulty in breaking away from a 

 substance, this is not true of the X-ray quanta. If 

 they behave like those we have been investigating of 

 recent years, they have far greater powers of 

 penetration than the electrons, and every body 

 must be emitting them in streams. Moreover, 

 if bodies be placed near each other, there will be 

 an interchange which will hand energy from one 

 to the other until there is an equilibrium. If a hot 

 body is placed near a cold one, the former contains 

 some electrons and corresponding quanta of great 

 energy, and as these stream over to the cold body, 

 they go through transformations which permit of loss 

 of energy, since for a time they put the energy into 

 electron carriers which can exchange, and do exchange, 

 energies with others — through the mediation of the 

 atoms, it may be. X-ray quanta have not that power 

 of themselves. Thus in time the two bodies are 

 brought to the same temperature. 



In this way we have a conception of radiation which 

 on the surface differs from that which is ordinarily 

 held. But does it do so really? May it be that we 

 have merely found a different method of regarding 

 the processes of radiation? If so, that would be a 

 verv good thing, for it is one of the best aids to 

 inquiry to have more than one hypothesis which will 

 link together a number of experimental facts. Nor 

 need we be afraid if the hypotheses differ considerably. 

 On the contrary, that means that we have the greater 

 number of interesting things to discover between the 

 two points of view and their final point of convergence. 



Now we know that when light falls upon material 

 substances there is an emission of electrons of slow 

 speed ; in other words, light radiation resembles 



