January 31, 19 18] 



NATURE 



437 



defined boundary on the side of the shorter wave- 

 lengths, the position of such boundary depending on 

 the voltage on the tube. 



(6) One or more characteristic radiations (of the 

 ... J, K, L, M, . . . series), each approximately homo- 

 geneous and characteristic of the metal of the anti- 

 cathode. The higher the atomic weight the more pene- 

 trating the radiation in rhe same series. 



The proportions of (a) and (b) depend entirely on the 

 conditions. With very soft tubes a large proportion of 

 the radiation may be wholly characteristic. 



With reference to the spectrum of general rays, it 

 has recently been shown that the maximuin frequency 

 of X-ray which a tube can yield can be readily calcu- 

 lated by a simple extension of Planck's quantum 

 theory. The relation in question (due to Einstein) is 

 Ve = hv, where V is the voltage on the tube, c the 

 elementary charge on each cathode ray, v the frequency 

 of the hardest X-ray produced, h is Planck's constant. 

 e and h are known with considerable exactness, so 

 that we have the means of calculating' very readily the 

 voltage necessary to generate a particular X-ray. In- 

 serting Millikan's latest values of these constants, we 

 have 



Wave-length in A.U.= ^'400 

 *^ voltage 



The accuracy of this simple relation has been con- 

 firmed experimentally over a wide range of voltages 

 in America. It will be noticed that the result is in- 

 dependent of the material of the anticathode. 



With reference to the characteristic radiations, each 

 consists of a number of spectral lines. For these, Ein- 

 stein's simple law does not hold, a greater voltage 

 being required. Webster noticed that the various si>ec- 

 tral lines of a series all spring into being together as 

 the voltage is increased through the critical value. 



Through the medium of the X-rays we have unveiled 

 a few of the secrets of the structure of the atom. The 

 biggest development has resulted from the discovery of 

 the wave-like character of the X-rays. It was Laue 

 and his pupils in 1913 who first demonstrated the 

 diffraction of X-rays by crystals, but it was in this 

 country that the first real insight into the problem 

 came. The Braggs showed how the crystal reflection 

 of X-rays could be utilised to separate out different 

 waves in a fashion exactly analogous to the production 

 of interference colours by thin plates. The X-ray spec- 

 trometer revealed both the atomic spacings of a large 

 number of crystals and the absolute wave-lengths of a 

 variety of monochromatic X-rays. 



The work of Moseley stands out pre-eminently here. 

 Moseley photographed many characteristic X-ray spec- 

 tra, and measured the wave-lengths of the principal 

 lines. He was able at once to obtain the very remark- 

 able and simple relation now associated with his name, 

 namely, that the frequency of a characteristic X-ray 

 from any element is proportional to the square of the 

 atomic number of the element. This atomic numb •■ 

 must be distinguished from the atomic weight. It de- 

 notes merely the order in which the elements come 

 when arranged according to their atomic weights. 

 Thus the atomic number of hydrogen is i, of helium 2,- 

 of lithium -?, and so on. The atomic numbers follow 

 the order of atomic weights except in three instances : 

 argon and potassium, cobalt and nickel, iodine and 

 tellurium are interchanged. 



The X-ray spectra are revealed as an extreme type of 

 light-ray spectra, and are even more characteristic of 

 the parent atom. Later work has shown that X-ray 

 spectra contain many lines and are much more com- 

 plicated than was first believed. 



Moselev's work ha.^ been extended bv others, notablv 

 bv Siegbahn and Friman. We now know the atomic 

 numbers of all the known elements, beginning with 



Na 2518, VOL. 100] 



hydrogen and ending with uranium — with an atomic 

 number of 92. Each of the atomic numbers is repre- 

 sented by an element, with the exception of numbers 

 43» ^i. 75. ^5. and 87, which stand for five elements 

 waiting to be discovered. It by no means follows, 

 however, that there are only five missing elements; 

 five is a lower limit, for we now know that several 

 elements may have the same atomic number. Such 

 isotopes, as Soddy has called them, cannot be distin- 

 guished one from another by ordinary chemical or 

 physical tests. They are grouped together under the 

 one atomic number in the periodic classification of the 

 elements, but, nevertheless, they may, and do, possess 

 atomic weights differing by several units. It is ap- 

 parent that the atomic number is something more than 

 a mere integer; it undoubtedly represents some funda- 

 mental attribute of the atom, and as the work of 

 Rutherford and others has shown, the atomic number 

 equals the excess number of positive charges in the 

 nucleus of the atom. 



The boundaries of the known spectrum have been 

 considerably extended since the war broke out. In 

 the ultra-violet Lyman has extended the region first 

 investigated by Schumann to a wave-length of about 

 500 Angstrom units, and Richardson and Bazzoni have 

 very recently further extended this to 420 A.U. The 

 longest X-ray so far measured by Siegbahn has a wave- 

 length of 12 A.U. Rutherford has recently given evi- 

 dence for believing that the wave-length of the hardest 

 7 rays from Ra-C is in the region of 1/ 100' A.U. We 

 are thus now familiar with a range of more than ten 

 octaves of X- and y rays without a break — not at all a 

 bad record for so young a subject. There still remain 

 about five octaves to be explored in the region be- 

 tween X- and ultra-violet rays, a region which con- 

 tains the characteristic X-rays of the light elements 

 from hydrogen to neon. 



And now to turn to quite a different topic. At the 

 moment we are all reproaching ourselves for our past 

 neglect of science in this country. We are paying the 

 penalty of our indifference, despite the wonderful adapt- 

 ability and resource which this war has shown we 

 possess as a nation. The country is slowly learning 

 its lesson. Willy-nilly, we are being led to see at last 

 that our system of education misdirects much genius 

 into unproductive channels, and we are awakening to 

 the importance of research, both pure and applied. 



The value of applied science to industry is now 

 accepted throughout the country, and British industr)- 

 should begin to feel the benefit, especially now that 

 the principle of State-aided research is established. 



But we must not forget that it is the pure academic 

 research, unrestricted and unprescribed, which has been 

 the prime cause of all the radical changes in industrial 

 methods. Research in pure science is rarely appre- 

 ciated by the general public or manufacturer, for it 

 cannot be done to order. One must put faith in the 

 research worker that he may continue to have faith in 

 himself. Much of what he will do will be discon- 

 tinuous and abortive, but he must not be hampered 

 by utilitarian notions being continually rammed down 

 his throat. If he does not solve the original problem 

 he will probably solve some other which has sprung 

 from it, and one successful discovery may outweigh 

 by far all his failures. 



The equal importance of the applied research worker, 

 who is responsible for turning to account the dis- 

 coveries of the pure investigator, must not be lost 

 sight of for a moment. There is no line of demarca- 

 tion between the two divisions of research. Each in- 

 volves study, hard work, and thought. The methods 

 of both branches are questioning and searching; the 

 common end is knowledge, to which there is no 

 heaven-sent road. 



What has \mh^x\ the reward of the research worker 



