322 



NA TURE 



[August 2, 1906 



their wurk is placed upon me as your President, and it is 

 for my effort to discharge that duty that I ask your 

 generous consideration. 



As one might expect, the progress of the knowledge of 

 nature (for it is to that rather than to the historical, 

 moral and mental sciences that English-speaking people 

 refer when they use the word " science ") has consisted, 

 in the last twenty-five years, in the amplification and fuller 

 verification of principles and theories already accepted, and 

 in the discovery of hitherto unknown things which either 

 have fallen into place in the existing scheme of each 

 science or have necessitated new views, some not very 

 disturbing to existing general conceptions, others of a more 

 startling and, at first sight, disconcerting character. 

 Nevertheless I think I am justified in saying that, exciting 

 and of entrancing interest as have been some of the dis- 

 coveries of the past few years, there has been nothing to 

 lead us to conclude that we have been on the wrong path 

 — nothing which is really revolutionary ; that is to say, 

 nothing which cannot be accepted by an intelligible modifi- 

 cation of previous conceptions. There is, in fact, continuity 

 and healthy evolution in the realm of science. Whilst 

 some onlookers have declared to the public that science 

 is at an end, its possibilities exhausted, and but little of 

 the hopes it raised realised, others have asserted, on the 

 contrary, that the new discoveries — such as those relating 

 to the X-rays and to radium — are so inconsistent with 

 previous knowledge as to shake the foundations of science, 

 and to justify a belief in any and every absurdity of an 

 unrestrained fancy. These two reciprocally destructive 

 accusations are due to a class of persons who must be 

 described as the enemies of science. Whether their atti- 

 tude is due to ignorance or traditions of self-interest, such 

 persons exist ; and it is one of the objects of this .(Vssoci- 

 ation to combat their assertions and to demonstrate, by 

 the discoveries announced at its meetings and the conse- 

 <iuent orderly building up of the great fabric of " natural 

 knowledge, "that Science has not come to the end of her 

 work — has, indeed, only as yet given mankind a foretaste 

 of what she has in store for it— that her methods and her 

 accomplished results are sound and trustworthy, serving 

 with perfect adaptability for the increase of true' discovery 

 and the expansion and development of those general con- 

 ceptions of the processes of nature at which she aims. 



Ne-w Chemical Elements.— There can be no doubt that 

 the past quarter of a century will stand out for ever in 

 human history as that in which new chemical elements, 

 not of an ordinary type, but possessed of truly astounding 

 properties, were made known with extraordinary rapidity 

 and sureness of demonstration. Interesting as the other's 

 are, it is the discovery of radio-activitv and of the element 

 radmm which so far exceeds all others in importance that 

 we may well account it a supreme privilege that it has 

 fallen to our lot to live in the days of this discovery. No 

 single discovery ever made by the' searchers of nature even 

 approaches that of radio-activity in respect of the novelty 

 of the properties of matter suddenly revealed by it. A new 

 conception of the structure of matter is necessitated and 

 demonstrated by it, and yet, so far from being destructive 

 and disconcerting, the new conception fits in with, grows 

 out of, and justifies the older schemes which our previous 

 knowledge has formulated. 



Before saying more of radio-activitv, which is apt to 

 eclipse in interest every other topic of discourse, I must 

 recall to you the discovery of the five inert gaseous' elements 

 by Rayleigh and Ramsay, which belongs to the period on 

 which we are looking back. It was found that nitrogen 

 obtained from the atmosphere invariably differed in weight 

 from nitrogen obtained from one of it's chemical combin- 

 ations ; and thus the conclusion was arrived at by Rayleigh 

 that a distinct gas is present in the atmosphere, to the 

 extent of i per cent., which had hitherto passed for 

 nitrogen. This gas was separated, and to it the name 

 argon (the lazy one) was given, on account of its incapacity 

 to combine with any other element. Subsequently this 

 argon was found by Ramsay to be itself impure, and from 

 it he obtained three other gaseous elements equally inert : 

 namely neon, krypton, and xenon. These were all dis- 

 tinguished from one another by the spectrum, the sign- 

 manual of an element given by the light emitted in each 



NO. 1 918, VOL. 74] 



case by the gas when in an incandescent condition. A fifth 

 inert gaseous element was discovered by Ramsay as a 

 constituent of certain minerals which was proved by its 

 spectrum to be identical with an element discovered twenty- 

 five years ago by Sir Norman Lockyer in the atmosphere 

 of the sun, where it exists in enormous quantities. Lockyer 

 had given the name helium to this new solar element, and 

 Ramsay thus found it locked up in certain rare minerals 

 in the crust of the earth. 



But by helium we are led back to radium, for it was 

 found only two years ago by Ramsay and Soddy that 

 helium is actually formed by a gaseous emanation from 

 radium. Astounding as the statement seems, yet that is 

 one of the many imprecedented facts which recent study 

 has brought to light. The alchemist's dream is, if not 

 realised, at any rate justified. One element is actually 

 under our eyes converted into another ; the element radium 

 decays into a gas which changes into another element, 

 namely helium. 



Radium, this wonder of wonders, was discovered owing 

 to the study of the remarkable phosphorescence, as it is 

 called — the glowing without heat — of glass vacuum-tubes 

 through which electric currents are made to pass. Crookes, 

 Lenard, and Rontgen each played an important part in 

 this study, showing that peculiar rays or linear streams 

 of at least three distinct kinds are set up in such tubes — 

 rays which are themselves invisible, but have the property 

 of making glass or other bodies which they strike glow 

 with phosphorescent light. The celebrated Rontgen rays 

 make ordinary glass give out a bright green light ; but they 

 pass through it, and cause phosphorescence outside in 

 various substances, such as barium platino-cyanide, calcium 

 tungstate, and many other such salts ; they also act on a 

 photographic plate and discharge an electrified body such 

 ■as an electroscope. But the most remarkable feature 

 about them is their power of penetrating substances opaque 

 to ordinary light. They will pass through thin metal 

 plates or black paper or wood, but are stopped by more 

 or less dense material. Hence it has been possible to 

 obtain " shadow pictures " or skiagraphs by allowing the 

 invisible Rontgen rays to pass through a limb or even a 

 whole animal, the denser bone stopping the rays, whilst 

 the skin, flesh, and blood let them through. They are 

 allowed to fall (still invisible) on to a photographic plate, 

 when a picture like an ordinary permanent photograph is 

 obtained by their chemical action, or they may be made to 

 e.xert their phosphorescence-producing power on a glass 

 plate covered with a thin coating of a phosphorescent salt 

 such as barium platino-cyanide, when a temporary picture 

 in light and shade is seen. 



The rays discovered by Rontgen were known as the 

 X-rays, because their exact nature was unknown. Other 

 rays studied in the electrified vacuum-tubes are known as 

 kathode rays or radiant corpuscles, and others, again, as 

 the Lenard rays. 



It occurred to M. Henri Becquerel, as he himself tells 

 us, to inquire whether other phosphorescent bodies besides 

 the glowing vacuum-tubes of the electrician's laboratory 

 can emit penetrating rays like the X-rays. I say " other 

 phosphorescent bodies," for this power of glowing without 

 heat — of giving out, so to speak, cold light — is known to 

 be possessed by many mineral substances. It has become 

 familiar to the public in the form of " phosphorescent 

 paint," which contains sulphide of calcium, a substance 

 which shines in the dark after exposure to sunlight — that 

 is to say, is phosphorescent. Other sulphides and the 

 minerals fluor-spar, apatite, some gems, and, in fact, a 

 whole list of substances have, under different conditions of 

 treatment, this power of phosphorescence or shining in the 

 dark without combustion or chemical change. All, how- 

 ever, require some special treatment, such as exposure to 

 sunlight or heat or pressure, to elicit the phosphorescence, 

 which is of short duration only. Many of the compounds 

 of a somewhat uncommon metallic element, called uranium, 

 used for giving a fine green colour to glass, are phos- 

 phorescent substances, and it was, fortunately, one of them 

 which Henri Becquerel chose for experiment. Henri 

 Becquerel is professor in the Jardin des Plantes of Paris ; 

 his laboratory is a delightful old-fashioned building, which 

 had for me a special interest and sanctity when, a few 



