March 17, 1921] 



NATURE 



79 



ing vessels many years ago; and, thirdly, by the 

 Italian Sanzo, who in 1919 gave a figure of a 

 specimen 28 mm. in length from the Straits of 

 Messina. Richardson referred his — or rather 

 Hooker's — specimen to the trunk-fishes, and 

 termed it Ostracion hoops ; the other authors, 

 however, realised that they were dealing with the 

 young of sun-fishes, but were unable to make 



any closer determination of the species. Judging 

 from the new material provided by the Dana 

 expedition, I can now with full certainty state that 

 all the specimens in question are larvae of the 

 oblong sun-fish (Ranzania). The tiny stages of the 

 short sun-fishes (Mola), however, do not appear 

 to have been figured or mentioned in literature up 

 to now. 



Electrons.^ 



By Sir William Bragg, K.B.E., F.R.S. 



IN recent years the results of experimental re- 

 search on the properties of electrons have 

 accumulated with startling rapidity. As know- 

 ledge grows, the importance of the part played 

 by the electron in the mechanics of the world be- 

 comes even clearer. There are all the right signs 

 that progress is being made along a road that 

 really leads somewhere ; we are continually find- 

 ing that, through some electron action, pheno- 

 mena are linked together between which, we had 

 hitherto seen no connection. Precision is given 

 to our views : we find ourselves able to express, 

 quantitatively and with confidence, laws and rela- 

 tions which have been matters of vague surmise. 

 Every experiment that is finished suggests others 

 that are promising. The whole world of experi- 

 mental physics is full of new life, and of the con- 

 sciousness that after a period of hesitation the 

 tide of discovery is sweeping on again. While 

 knowledge grows by experiment, theory is also 

 busy. The attempts to co-ordinate the new dis- 

 coveries are of singular interest because of their 

 daring, their width, and their strength : because 

 they are so often fruitful in prediction : and, not 

 least perhaps, because they seem so often to be 

 irreconcilable with each other. 



It helps to a right appreciation of the position 

 as regards the electron if we observe its strong 

 resemblance to the older state of things when first 

 the atomic theory of matter was clearly defined. 

 Just as chemistry has grown and prospered on its 

 recognition of the unit of matter, so electrical 

 science has already begun a new life, and, to all 

 seeming, a most vigorous one, based on the under- 

 standing of Nature's unit of electricity. There 

 are many different atoms of matter — nearly a 

 hundred are distinguishable by their different chem- 

 ical reactions ; but the number of different kinds 

 of electrical atoms is very much more limited. 

 We have for some years been clear as to the exist- 

 ence of the electron. Nature's unit of negative 

 electricity. More recently the work of Rutherford 

 and Aston indicates that the nucleus of the hydro- 

 gen atom is to be regarded as the positive 

 counterpart. 



If the chemist has found so much profit in his 

 recognition of the fact that Nature has just so 

 many ways, and no more, of doing up parcels of 

 matter, the electrician will surely gain in the same 



1 The Twelfth Kelvin Lecture delivered before the Inst'tution of Elec- 

 trical Engineers on January 13. 



NO. 2681, VOL. 107] 



way when he grasps the fact that not merely is 

 electricity measurable in quantity, but that there 

 is already a unit of Nature's choice, possibly no 

 more than one unit. We may say with justice that 

 already the most wonderful advances in modern 

 physics are the reward for our appreciation of this 

 truth, and we may hope with equal justice that we 

 are yet far from reaping the full benefit. 



The first suggestion of the atomic character of 

 electric charge came, it is well known, from 

 observation of the laws of electrolysis. Since the 

 movement of atoms or atom clusters or ions across 

 the electrolytic cell was accompanied by a simul- 

 taneous transfer of electricity, in which each ion, 

 of whatever nature, bore always the same charge 

 or at least a simple multiple of it, there was a 

 clear indication that this division of electricity into 

 parcels of constant magnitude implied the exist- 

 ence of some natural unit charge. No progress, 

 however, was or could be made so long as 

 the charge could be observed only as an attach- 

 ment to an ion : it was not even clear that it could 

 ever have a separate existence. In the long series 

 of researches which finally led to the isolation of 

 the electron and the determination of its proper- 

 ties, there were certain that marked definite stages 

 in the forward movement. Crookes examined the 

 electric discharge in bulbs exhausted to a high 

 degree by the new air pumps which he had suc- 

 ceeded in making ; and he observed the so-called 

 cathode rays streaming away from the negative 

 electrode. He showed that they possessed the 

 properties to be expected from a stream of par- 

 ticles projected across the bulb and carrying nega- 

 tive electricity with them ; for on one hand 

 they could heat up bodies on which they fell, and 

 on the other they were deflected in crossing a 

 magnetic field. Crookes spoke of a fourth state 

 of matter and defended his view against the op- 

 posing hypothesis, held largely on the Continent, 

 that the stream consisted of electromagnetic 

 waves in some form or other. Hertz showed that 

 the rays could pass through thin sheets of matter 

 such as aluminium leaf, and Lenard took advan- 

 tage of this to coax them outside the bulb and 

 display their effects in the air outside. 



In the later years of last century came the 

 great experiments of Wiechert, Thomson, and 

 many other well-known observers, who weighed 

 the electron and measured its charge, and showed 

 that there was only the one electron, though it was 



