April 28, 192 1] 



NATURE 



265 



Letters to the Editor. 



{The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications .] 



The Internal Physics of Metals. 



I NOTICE in an article in Nature of April 14 on 

 "The Internal Physics of Metals" considerable im- 

 portance is given to the idea of the existence of an 

 amorphous or vitreous layer between the crystals 

 composing metals and alloys, and certain seasonal 

 changes in them are attributed to the presence of this 

 layer. The remark is made that until 19 19 the pheno- 

 menon of "season cracking" was considered to be 

 an isolated one, and rec©gnised only in brass. 



"Season cracking" is, however, only an extreme 

 case of the secular relief of strain which occurs in all 

 metals which have been subjected to cold working. 



It may be of interest to some of your readers to 

 learn that this state of strain in cold-worked metals 

 and its cause had been dealt with in a paper read 

 before the Faraday Society in 1904, while in the 

 May lecture to the Institute of Metals in 191 1 an 

 illustration was exhibited of the partial relief of strain 

 by cracking which had occurred within twenty-four 

 hours. In these papers, and in others communicated 

 to the Royal Society, the change from the crystalline 

 to the vitreous state brought about by mechanical 

 disturbance and "flow " was shown to occur in metals 

 and other crystalline substances. The effects of this 

 change of state on the chemical, electrical, acoustical, 

 optical, and mechanical properties of the substances 

 were dealt with, and were all shown to be associated 

 •with a condition of strain whidfi could be completely 

 relieved by the restoration of the fully crystallised 

 condition by raising the mass to a temperature far 

 short of its melting point. 



It was suggested that the changes of structure 

 which are produced by the cold working of riietals 

 could be accounted for by the occurrence of liquid- 

 like flow at all internal rubbing surfaces, followed by 

 almost instantaneous resolidification of the liquid 

 phase, thus producing a hard cement, binding together 

 the broken and distorted remains of the original 

 crystal grains. In wire-drawing, for example, an 

 entirely new "texture" is developed even in pure 

 metals'. The crystal grains are drawn out into fibres, 

 which are embedded in and cemented together by the 

 portion of the metal which had passed through the 

 liquid phase as the wire flowed through the hole in 

 the draw-plate. Owing to the greater solubility of 

 the metal which has flowed into the vitreous state, 

 the first effect of a solvent on the wire is to dissolve 

 away the cementing material and to expose the fibrous 

 structure. 



" Season cracking " seems, therefore, to depend 

 (i) on the free flow of the metal during drawing, 

 for the greater the production of the liquid phase 

 the greater will be the shrinkage at the moment of 

 resolidification, and the greater will be the resulting 

 state of strain in the hardened metal ; and (2) on the 

 subsequent action of a solvent which, by removing or 

 breaking up the vitreous skin and cementing material, 

 will enable the elasticallv strained fibres or layers to 

 spring apart. The solvent may be mercury, or an acid 

 or saline liquid, or acid vapours or even water 

 vapour in the atmosphere. A piece of hard rolled 

 metal foil is thoroughly springy and resilient, but 

 this resilience is completely removed and the foil 



NO. 2687, VOL. 107] 



beconaes soft if the vitreous layer on the surfaces of 

 the foil is removed by a solvent. 



George Beilby. 

 April 21. 



Sir George Beilby 's work on the generation of 

 amorphous metal as the result of " how " during 

 plastic straining or surface polishing of metals is so 

 well known and appreciated that a contribution from 

 him to the discussion of "season cracking" is very 

 welcome. At the general discussion on this subject 

 the fullest reference to his work in first indicating 

 the existence of metal in an amorphous condition was 

 made. Sir George Beilby 's letter, however, appears to 

 be based mainly upon the brief article in Nature of 

 April 14 rather than on the full discussion of the 

 subject, of which that article could not give more 

 than a very brief account from one particular point 

 of view. No doubt for that reason Sir George Beilby 

 has apparently missed some of the main points of 

 the discussion, and has made a suggestion with regard 

 to " season cracking " which is not easily reconciled 

 with the known facts. 



Thus, one reason why special importance is attached 

 to an amorphous inter-crystalline layer or "cement" 

 which exists in entirely unstrained metals is that frac- 

 ture in "season cracking" follows the boundaries of 

 the original crystals, and does not follow the lines of 

 flow or slip within the crystals upon which Sir George 

 Beilby 's amorphous metal is formed. Further, it has 

 now been clearly shown that fracture essentially of 

 the nature of "season cracking " can and does occur 

 under the prolonged application of external stress in 

 fully annealed, or even cast, metal in which there has 

 been no formation of Sir George Beilby 's amorphous 

 metal as the result of plastic strain. It follows, there- 

 fore, that the amorphous metal generated by plastic 

 strain must be regarded as playing only an indirect 

 part in the phenomena of "season cracking," that 

 part being so to stiffen and harden the metal that it 

 can carry an internal stress high enough to bring 

 about the gradual separation of the crystals along 

 their original boundaries. 



With regard to the statement that prior to 19 19 

 " season cracking " had been regarded as an isolated 

 phenomenon confined to brass, this is true in the sense 

 that until the publication of Rosenhain and Arch- 

 butt's paper it had not been recognised that this type 

 of inter-crystalline fracture under prolonged stress 

 could occur in other metals than brass, and possibly 

 nickel-silver, whereas it was then shown that it also 

 occurs in lead, in aluminium alloys, and even in steel. 

 The Writer of the Article. 



Biological Terminology. 



Mr. Cunningham writes (Nature, February 24, 

 p. 828) : " It is a mere matter of terms and synonyms. 

 The modern biologist would say that the normal 

 hand was hereditary, or innate, or due to certain 

 factors or genes in the chromosomes which usually are 

 handed on unchanged ' down the germ-tract ' ; that 

 the sixth digit was a mutation, due to some change 

 in the genes in the chromosomes, and therefore 

 gametogenic ; and that the scar was due to an injury 

 which resulted in regenerative processes producing 

 new tissue. . . . Sir Bryan Donkin writes that like 

 exactly begets like when parent and child develop 

 under like conditions; if we say, then, that the 

 differences due to unlike conditions are acquired 

 characters, what is the objection ? " 



The objection is that what is true of individuals 

 is not necessarily true of characters, and that 

 Mr. Cunningham's thoughts drift to and fro. 



