5^ 



NATURE 



[March ii, igjo 



conductor equals its radius, together with the capacity 

 of the condenser formed by the spherical surface on 

 one side and the images in it of all external 

 objects connected in parallel on the other. This 

 theorem leads at once to relations between the 

 capacity coefficients of a system of two spheres and 

 the capacities of certain spherical condensers which 

 lesse'ns very appreciably the labour involved in com- 

 puting the values of these coeflficients which are 

 required in practical work. The mutual coefficient 

 also is given in terms of the capacity of a spherical 

 condenser, and other relations between the various 

 capacities used by engineers and physicists are proved. 

 Finally, a method of finding the approximate value 

 of the capacity between a sphere and distant large 

 conductors is given. — C. Cuthbertson and Maude 

 Cuthbertson : The refraction and dispersion of carbon 

 •dioxide, carbon monoxide, and methane. The refrac- 

 tivitv of the above-named gases has been measured 

 at eight points in the visible soectrum between 

 AA 6708 — 4800. The work was undertaken with the 

 object of ascertaining the refractive power of the 

 carbon atom, on the assumption of the validity of tlie 

 additive la\v. Bv deducting the refractivity of the 

 oxvgen or hvdrogen atoms from that of the carbon 

 compound values are obtained from which the refrac- 

 tivitv of carbon can be exnressed in the form 



For 11=0 the exoressions obtained are: 



From carbon dioxide 

 °° carhon | jg, 



Carbon monoxide Methane 



I I '088 . I 1-672 



—:^— =0000195 __^ = ooooi57 



16042 I 10213 ' 10623 



There are thus wide differences,' not only between the 

 quotients, which give the refractivity, but also between 

 the numerators, which should be proportional to the 

 number of "dispersion electrons," and the denomina- 

 tors, which give the squares of the hypothetical free 

 frequencies. The result affords a further proof that 

 the "additive law" is untrustworthy except as a 

 rough guide. — A. A. Griffith : The jihenomena of rup- 

 ture and flow in solids. Difficulties which had been 

 experienced in predicting the fracture of machine 

 parts under certain types of loading suggested the 

 desirability of a fundamental inquiry into the 

 mechanism of rupture. A theoretical criterion of 

 the rupture of an elastic solid, based on the "theorem 

 of minimum energy," is enunciated in the paper. 

 This has been shown experimentally to be true in 

 the case of a ^lass plate which contains a crack when 

 unstrained. The calculation involves the surface ten- 

 sion of the material. In the experiments the maxi- 

 mum stress in the glass was estimated to be more 

 than ten times the normal tenacitv of the material. 

 Tt is shown that this result is compatible with the 

 general criterion of rupture unless the material is 

 weakened bv discontinuities of flaws the dimensions 

 of which are at least of the order ten thousand times 

 the molecular spacing. Evidence is adduced to show 

 that the strength of other substances, including metals 

 and liquids, is governed by similar considerations, 

 and that an enormous increase in the tenacitv of 

 materials would be possible if the flaws could be 

 eliminated. Experiments are described showing how 

 the elimination mav be performed in the case of glass 

 and fused silica, it having been found possible to 

 prepare samoles of these materials with nearlv fiftv 

 times their normal tenacitv. The strong phase of 

 these materials is, however, unstable, and changes 

 soontaneouslv in a few hours to the normal modifica- 

 tion. It is shown that manv of the phenomena asso- 

 ciated with the mechanieal properties of materials, 

 including- those described in the present i^aper, are 



NO. 2628, VOL. 105] 



capable of explanation in general terms if it be sup- 

 posed that intermolecular attraction is a function of 

 the relative orientation of the attracting molecules. 

 Some consequences of this theory are discussed in Ihe 

 paper. The paper concludes with a short discussion 

 of the bearing of the work on engineering practice. 



Geological Society, February 20.— Mr. G. \\ . 

 Lamplugh, president, in the chair. — Annual general 

 meeung. — G. VV. L,amplugn : Presidential aadress : 

 Some leatures of the Pleistocene giaciation of Eng- 

 land. 1 he address dealt principally with the changes 

 brought about by the ice in the surface-features of 

 our country. More than five thousand square miles 

 of English land, or about one-tenth of the whole 

 country, would vanish if the drifts were removed, as 

 the ••solid" rocks lie below sea-level in tracts of this 

 extent. .V further area of about ten thousand square 

 miles is overspread by drift of sufficient thickness 

 wholly to mask the "solid" land-forms, so that rather 

 more than one-quarter of the country owes its present 

 shape to Glacial and post-Glacial deposits. Another 

 twenty thousand square miles was glaciated, and 

 more or less modified, but without losing the 

 dominating features of its rocky framework. The 

 remainder of the country was affected only by the; 

 intensification of the atmospheric agencies, whereby 

 its original features were accentuated. In a general 

 sense, the hill-districts have not been greatly changed, 

 but the lowlands have been in most parts completely 

 altered. The source of the huge mass of material 

 contained in certain of the lowland drift-sheets was 

 considered, and the opinion was expressed that a 

 large portion of this was an addition to the land, 

 brought in by the ice from outside our present coast- 

 line. Comment was made on the curious rarity of 

 peat or other land-detritus in Boulder Clay known 

 to have been derived entirely from the land, and this 

 was thought to indicate that the conditions for a long 

 period before the actual giaciation had been unfavour- 

 able for the growth of timber or peat-producing 

 vegetation. 



February 25. — Mr. R. D. Oldham, president, in 

 the chair.— H. C. Sargent: The Lower Carboniferous 

 chert-formations of Derbyshire. The chert-formations 

 occurring in the Carboniferous Limestone and asso- 

 ciated rocks of Derbyshire may be classified under 

 two heads : (i) Those which' owe their silica 10 

 gaseous or aqueous emanations from igneous rocks. 

 (2) Those which derived their silica from the land by 

 ineans of chemical denudation. The author considers 

 that in both cases the silica was precipitated direct, 

 and did not, to any considerable extent, pass through 

 an intermediate stage of secretion by organisms with 

 subsequent solution and redeposition. He adduces 

 evidence to show that simultaneous deposition of silica 

 and calcium carbonate often took place, and it is 

 believed that, in such cases, segregation ensued, and 

 sometimes resulted in the formation of nodules and 

 lenticular masses of chert. It is suggested that the 

 bedded cherts of terrestrial origin resulted from 

 heavier precipitation of silica, comparatively free from 

 calcium carbonate, and spread over the sea-floor by 

 gentle currents. Metasomatic replacement of lime- 

 stone and calcareous organisms by silica has taken 

 place at their contact with the chert. Impurities in 

 the silica have tended to limit such replacement. 

 Organisms existing in the sea or on the sea-floor 

 would be entangled in the precipitated silica, and 

 their presence in the chert is thus explained. The 

 blackness of some chert is shown to be due to the 

 presence of carbonaceous matter. Ferrous iron mav 

 possiblv have operated sometimes in the same way. 



