184 



KNOWLEDGE. 



JlTNR, 1915. 



base lines for triangulation work in surveying, invar tajics 

 are now largely employed. This has led to increased 

 accuracy, and has notably decreased the time required 

 in carrying out such work. Invar standards are also 

 employed in accurate laboratory work, while the alloy 

 has been used to some extent in electric lighting in con- 

 nection with the seals of lamps. 



ELECTRO-MAGNETIC INKRTI.\ AND ATOMIC 

 WEIGHT.— On February 26th, 191.S, Professor Nicholson 

 read a paper with the above title before the Physical Society 

 of London. Me subjected to mathematical analysis the view 

 that the mass of an atom is of electro-magnetic origin. On 

 this theory the sum of the masses of two charged particles 

 is a function of their distance apart and of the velocities 

 of the particles. The added mass due to the mutual action 

 of the particles I'rofessor Nicholson terms the mutual 

 mass, and he calculated that this is proportional to the 

 product of the charges, and inversely proportional to their 

 distance apart when thev move along the line joining them 

 with equal velocities, small when compared with the velocity 

 of light. 



These considerations are of primary importance in the 

 theoi-^' of the constitution of the atom. When, for instance, 

 an atom parts with a charged atom of helium in a radio- 

 active change, on this view the change in mass is not merely 

 that due to the separate mass of the helium atom, but 

 the loss of mutual mass is also to be taken into account. 

 It should be noted that, unlike ordinary mass, this mutual 

 electro-magnetic mass may be either positive or negative, 

 depending on whether the charges are alike or dissimilar. 



THE ELECTRIFICATION OF SURFACES AS 

 AFFECTED BY HEAT.— Dr. P. E. Shaw has recently 

 communicated to the Physical Society of London the results 

 of some interesting experiments on the effect of heat in 

 changing the sign of the electrification of surfaces. Ordin- 

 arily, when smooth glass is rubbed with silk, the glass 

 becomes charged positively and the silk negatively. If the 

 glass be passed to and fro in a flame for a few seconds the 

 sign of the effects is reversed. If the glass be subjected to 

 long-continued rubbing \vith silk or cotton it returns to its 

 normal state : while the abnormality is readily removed by 

 passing the rod of glass through the hand or a sheet of 

 indiarubber. Shaw found that this abnormal effect can 

 be produced \\-ith all the common hard solids. The kind 

 of flame used is of no moment, and the effects can be pro- 

 duced by heating in an electric furnace ; the glass having 

 been rendered anomalous in its behaviour remains so, if 

 left alone, for an indefinitely long time. The condition is 

 not removed by wa.shing with water. Subjection to intense 

 cold — as, for instance, that due to liquid air — has no 

 influence on fused silica, glass, brass, or sealing wax, 

 whether these were in their normal or abnormal state. We 

 hope that these and other researches on frictional elec- 

 tricity will be continued, and that the reproach that 

 modern science knows little more of the origin of frictional 

 electrification than the ancients will soon be removed. 



THE NATURE OF THE ULTIMATE MAGNETIC 

 PARTICLE.— In a letter to Science. April 23rd, 1915, 

 Compton and Trousdale give an account of the results of 

 an investigation undertaken with a view to settling this 

 question. The ferromagnetic crj'Stals, magnetite and 

 haematite, were examined by .a--ray photographs taken 

 through the crystals while magnetised and unmagnetised. 

 The resulting diagrams indicate that the atoms do not 

 leave their positions of equihbrium during magnetisation. 

 According to the authors, these results show that mag- 

 netism cannot be a molecular phenomenon. 



So far as these results go, they certainly appear decisive. 

 In the case of iron, nickel, and cobalt, and the Heusler 

 alloys, the view that the phenomenon is molecular seems 

 still unassailed. In the case, for instance, of an alloy made 

 up of copper, manganese, and aluminium (all sensibly 



non-magnetic metals), it is difficult to attribute the notably 

 magnetic character of the alloy to any other cause than to 

 the formation of magnetic molecule'. 



PROFESSOR W. GRYLLS ADAMS, F.R.S.— By the 

 death of Professor Adams (recorded in Natttre, April 22nd, 

 1915, to which we are indebted for the following particulars), 

 which occurred on April lOth, 1915, at the age of .seventy- 

 nine, physics has been deprived of a versatile man of 

 science. He succeeded Clerk Maxwell in the Chair of Physics 

 at King's College, London, fifty years ago. It was ten years 

 later that he introduced his now wull-known experimental 

 method of investigating the equipotential lines wliich result 

 when electric currents are passed through sheets of 

 tinfoil by exploring the surface with a pair of needles 

 connected with a galvanometer. In the same year he read 

 a paper before the Royal Society on the change of resistance 

 produced by magnetism in iron and steel. 



Adams took up the subject of the fall of the electrical 

 resistance of selenium on illumination soon after the dis- 

 covery of the effect by Mayhew, and proved that it was not 

 a secondary result due to heat ; he also showed that the 

 yellow-green were most effective rays. The physical 

 optics of crystals, terrestrial magnetism, lighthouse illumina- 

 tion, and electric lighting were some other of the subjects 

 to which he contributed. 



RADIO-ACTIVITY. 



By Alex.\nder Fleck, B.Sc. 



ABSORPTION OF HOMOGENEOUS /3 RAYS.— In 

 considering the absorption of o and /3 rays by matter 

 the general idea is that the two types of rays are very 

 different in their behaviour in this respect. The ionising 

 power of an a ray, after traversing a quantity of matter, 

 increases somewhat, and then suddenlv falls to zero. The 

 distance in air through which the a ray must travel before 

 this quick fall takes place is called the " range." Radiation 

 from one source of ^ rays is, however, absorbed exponen- 

 tiallv. Such radiation is not homogeneous, but consists, as 

 is proved by deviating a pencil of 3 rays by a magnetic 

 field to produce a " magnetic spectrum," of a collection of 

 j3 rays travelling with various speeds. R. W. Varder con- 

 tributes a paper to the May Philosophical Magazine, where 

 he describes experiments in which he separated a pencil of 

 § rays bj' means of a magnet into its component parts of 

 homogeneous rays. By altering the strength of the mag- 

 netic field he could make any set of such rays fall on a given 

 mica window. After the rays had penetrated through the 

 window, various thicknesses of aluminium were interposed 

 in their path, and so an absorption curve was found for 

 an homogeneous pencil of )i rays. A linear relation between 

 the ionisation and the thickness of the aluminium was 

 obtained, and by producing this straight line until it cut 

 the axis we get the thickness of aluminium, after which 

 fi rays of a particular velocity cease to ionise. By analogy 

 to the range of the a rays this distance is termed the " range 

 of the ^rays. " The difference in behaviour of these two 

 types of rays is therefore not so great as is often imagined. 



THE SPECTRUM OF LEAD OF RADIO-ACTIVE 

 ORIGIN. — The difference from the normal of the atomic 

 weight of lead of radio-active origin has been mentioned in 

 recent months in these columns. T. R. Merton, in a recent 

 paper published in the Proceedings of the Royal Society, 

 has supplemented these experiments by comparing the 

 spectrum of ordinary lead with that derived from the 

 radio-active mineral, pitchblende. Lead from this source 

 has been shouTi to have an atomic weight of 206 in place of 

 the normal value of 207. In agreement ^^^th the prevailing 

 theory of isotopes, Merton found that the two spectra of 

 the different leads were in every way identical. If there is 

 any difference in wave-length, it may be concluded that 



o 



such difference is not greater than 0-03 of an Angstrom 

 unit. 



