April , I92I. ] CURRENT TOPICS. 573

f rom one par t of the variable field to another. He is able to show that his theory is consis tent with the inverse square law, with the deflection of l ight by the sun and with the gravi ta t ional effect upon the f requency of light emit ted by an atom. " I t may be said that since ma t t e r is cer tainly par t ly electrical and since the refract ive index of the ether certainly varies near large masses, it seems certain tha t part , at least, of the observed gravi ta t ional forces mus t be due to an action of the kind considered in this paper, so that it is sa t is factory to find tha t it is possible to explain the whole a t t ract ion by means of this kind of actio'n."

G. F. S.

The Charge on the Atomic Nucleus and the L a w of Force. J. CI~ADWlCK. (Phil. Mag., December, I92O.)--" The theory of the nuclear const i tut ion of the a tom, put forward by Sir Ernes t Ruth- erford in 191I , has been confirmed by evidence ga thered f rom such var ious sources tha t it now forms, the foundat ion on which the deve lopment of a tomic physics is based. On this theory the posit ive charge associated with an a tom is concent ra ted on a massive nucleus of small dimensions, sur rounded by a distr ibu- tion of electrons extending over a distance comparab le with the d iameter of the atom, as usual ly understood.

" The physical and chemical proper t ies of an e lement are de- t e rmined by the charge on the nucleus, for this fixes the number and a r r angemen t of the external electrons, on which these prop- erties mainly depend. The mass of the nucleus influences the a r r a n g e m e n t of the electrons only to a very small degree. The nuclear charge is thus the fundamenta l constant of the a tom and the question of its actual magni tude of grea t impor tance for the deve lopment of a tomic theory ."

As the consequences and implicat ions of Ruther ford ' s theory became unders tood a t t emp t s were made to connect the nuclear charge with other a tomic constants . " van den Broek suggested tha t the nuclear charge might be equal to the a tomic n u m b e r of the element, i.e., the num ber of the e lement when all the e lements are a r ranged in order of increasing a tomic weight ." The pahna ry significance of the a tomic num ber in this connection was b rought forward by the bril l iant young Brit ish physicist , Moseley, whose un t imely death at Gallipoli was the grea tes t single loss to science inflicted by German aggression. H e showed that the X- ray spec- tra of the e lements was dependent on the square of a number which became grea ter by one when passage was made f rom one e lement to the e lement of next higher a tomic weight . " On the nuclear theory of a tomic s t ruc ture this character is t ic n u m b e r mus t be closely connected with the charge on the nucleus, and Moseley concluded that the number gave in fundamenta l units the actual value of this charge. This is one of the most impor tan t genera l iza t ions in modern physics, and gives a s tar t ing-point for

574 CURRENT TOPICS. [J. F. I.

the deve lopment of the external s t ruc ture of the a tom." The only direct method of measu r ing the charge on the nucleus is by s tudy ing the single sca t te r ing of alpha particles. W h e n a pencil of these falls on a thin sheet of mat ter , the number of the scat- tered part icles fal l ing per second on a unit area of screen at a known distance and angle with the pencil is connected with the num- ber of particles per second in the pencil by a fo rmula in which every quan t i ty is ei ther a known cons tan t or a measurab le quan- t i ty except the nuclear charge of the sca t te r ing a toms. This charge can therefore be calculated f rom the formula, when the resul ts of exper iment are inserted in it.

The direct as well as the scat tered alpha part icles were counted by the scinti l lat ions they produced upon the same zinc sulphide screen. If there were th i r ty scat tered part icles per minute , there would be about 2o,ooo direct part icles in the same time. To count this large n u m b e r a ro ta t ing disk wi th a slit was inter- posed in the pa th of the pencil before i t reached the screen, so tha t only a known fract ion of the part icles could pass. Thus the n u m b e r to be observed was made so small that it could readily be counted.

Expe r imen t s were made for three metals , p la t inum, silver and copper. In all cases the part icles counted numbered some thou- sands. N means the n u m b e r by which the fundamenta l charge of electrici ty, the charge of the electron, mus t be mult ipl ied in order to get the nuclear charge of an a tom of the metal.

Atomic Number Metal. N. of Metal .

Platinum . . . . . . . . . . . . . . . . . . . . . . . . 77.4 78 Silver . . . . . . . . . . . . . . . . . . . . . . . . . . . 46.3 47 Copper . . . . . . . . . . . . . . . . . . . . . . . . . . 29.3 29

The a tomic n u m b e r of the meta l is derived f rom its X- ray spec- tra, while N comes f rom the exper iment . The ag reemen t is strik- ing and Mose ley ' s conclusion is confirmed in a r emarkab le way. An examina t ion of fur ther exper imenta l da ta leads to the conclu- sion tha t the inverse square law holds t rue in the region of inves- t igat ion, i.e., within one hundred-bi l l ionth of a cent imeter f rom the nucleus.

G. F. S.

L ime N i t r o g e n . r a T h e claim has been made tha t l ime ni t rogen or commerc ia l calcium cyanamide produces injuries of the skin and of the resp i ra to ry tract, acts as a poison, and gives rise to a disease wi th character is t ic s y m p t o m s - - " l ime n i t rogen disease." SIEBNER (Chemiker Zeitung, I92o , xliv, 369-37 o, 382-383) has made an e labora te s tudy of the subject , and draws the fol lowing conclus ions: The injuries to the skin are due to free caustic l ime which occurs in the cyanamide dust; the injuries to the resp i ra to ry t rac t are exact ly the same as those produced by any dust. Cal-