42: 



NATURE 



[August 23, 1906 



each molecule touches six neighbours. But it may be con- 

 jectured that some of them may take up pyramidal piling 

 (touching twelve others) under the compulsion of strong 

 forces — such forces, for example, as act on the superficial 

 molecules of a surface that is being polished. 



If this also occurs at a surface of slip, it gives us a clue 

 to several known facts. It at least assists in explaining 

 the familiar Result that metal is hardened by straining in 

 the sense of being made less plastic. Again, it accounts 

 for the general increase of density which is found to take 

 place in such an operation as wire drawing. Further, if 

 a local increase of density occurs in the interior of a grain 

 through piling of some molecules in the closer manner 

 where repeated slips are going on, the concentration of 

 material at one place requires it to be taken from another ; 

 in other words, the closer piling tends to produce a gap or 

 crack in the neighbourhood where it occurs. This is con- 

 sistent with what we know of the development of cracks 

 through repealed alternations of strain. 



Recourse to the model shows that with pyramidal piling 

 the polar axes point' in so random a manner that the 

 aggregate may fairly be called amorphous. To illustrate 

 this a group is shown witli centres fixed at the corners of 

 equilateral triangles. 



It is obvious that any pyramidal piling at a surface of 

 slip tends to bar further slip at that particular surface. 

 Hence not only the augmented hardness due to strain, but 

 the tendency in repeated alternations to lateral spreading 

 of the region on which slip occurs. The hardness due to 

 straining is, of course, removed when we raise the metal 

 to such a temperature that complete recrystallisation occurs, 

 normal piling being then restored in the new grains. 



Taking a previously unstrained piece, it is clear that 

 the facility with which slip will occur at anv particular 

 surface of slip in any particular grain depends not only 

 on the nature of the metal and on the orientation of the 

 surface in question to the direction of the stress, but also 

 on the amount of support the grain receives from its 

 neighbours in resisting slip there. In other words, for a 

 given orientation of surface the resistance to slip may be 

 said to consist of two parts; one is inherent in the surface 

 itself, and the other is derived from the position of the 

 grain with reference to other grains. 



To make this point clear, think of a grain (under stress) 

 in which there is a gliding surface oriented in the most 

 favourable direction for slipping. Slip on this surface can 

 take place only when its yielding compels the neighbours 

 (which are also under stress) to yield with it. and the 

 surfaces in these on which slip is compelled to occur are, 

 on the whole, less favourably situated. Hence the original 

 grain cannot yield until the stress is considerablv in e.xcess 

 of that which would suffice to make it yield if it stood 

 alone, or had neighbours equally favourably inclined. 



.Apply this consideration to the case of steel, where there 

 .are two classes of grains : the ferrite, which is simply 

 iron, and the pearlite. which is a harder structure. Slip 

 on any ferrite grain is resisted partly by the strength of 

 the surface itself, and partly by the impossibilitv "of its 

 yielding without forcing slip to take place on neighbour- 

 ing (harder) grains. Now suppose the structure is a very 

 gross one, such as Mr. Stead has shown mav be found in 

 steel that is seriously overheated. On the large grains of 

 ferrite in overheated steel the resistance to slip will be 

 but little greater than it would be in iron, and, con- 

 sequently, under an alternating stress fatigue of strength, 

 leading to rupture, may be produced by a very moderate 

 amount of load. Mr. Stead ' has shown how the effects 

 of overheating can be removed by the simple expedient of 

 raising the steel to a temperature sufficient to cause re- 

 crystallisation — a homceopathic remedv that transforms the 

 gross structure of the overheated metal into an ordinarily 

 fine structure, where no ferrite grain can yield without 

 compelling the yielding of many pearlite grains. Hence 

 we find, as Rogers - has demonstrated by experiment, that 

 steel cured by reheating from the grossness of structure 

 previously produced by overheating, has an immensely in- 



1 See especially a paper bv J. E. Stead and A. W. Richards on "The 

 Restoration of naneemusly Crystalline Steel by Heat Treatment," /urird. 

 o/the Iron and St'-et r,nl.. No. 2, loo^. 



- K. Rogers, " Heat Treatment and Fatigue of Steel," /oiirii. Iron, and 

 Steel Inst., TAo. I, 100^. 



NO. 192 I, VOL. 74] 



creased power to resist the deteriorating effects of often 

 repeated stress. 



I trust you will not feel I have abused the license of the 

 Chair in presenting contributions to molecular theory that 

 are for the most part in the nature of speculative sugges- 

 tions, thrown out in the hope that they may some time 

 lead to fuller and more definite knowledge. Remote as 

 they may seem to be from the concerns of the workaday 

 engineer, they relate to the matter which it is his busi- 

 ness to handle, and to the rationale of properties, without 

 which that matter would be useless to serve him. We 

 have attempted to penetrate into its very heart and sub- 

 stance in order the better to comprehend the qualities and 

 functions on which the practical work of engineering relies. 

 The man whose daily business leads him through familiar 

 tracks in a forest does well to stray from time to time into 

 the shady depths that lie on either hand. The eyes of his 

 imagination will be opened. He will at least learn his 

 own limitations, and, if he is fortunate, he may gain 

 some clearing on a hilltop which commands a wider view 

 than he ha= ever had before. 



SECTION I. 



FHYSIOLOGY. 



Ope.mng .Address by Prof. Kr.^ncis Gotch, M..A., D.Sc, 

 F.R.S., Waynflete Professor of Physiology in the 

 University of Oxford, President of the Section. 

 " The investigators who are now working with such 

 earnestness in all parts of the world for the advance of 

 physiology have before them a definite and well-understood 

 purpose, that purpose being to acquire an exact knowledge 

 of the chemical and physical processes of animal life and 

 of the self-acting machinery by which they are regulated 

 for the general good of the organism."' 



In this admirable and concise manner the late Sir John 

 Burdon-Sanderson described the aims and methods of 

 physiology. The words were spoken in 1881, when the 

 British .Association last met in this historic city. At that 

 time the subjects of .Anatomy and Physiology formed a 

 subsection of the Section of Biology, and it was presided 

 over by this distinguished man, whose recent death has 

 deprived not only physiology but natural science of one of 

 its most honoured leaders. His continuous work, extend- 

 ing over a period of fifty years, was remarkable from 

 many points of view, but in none more than the extent 

 of its scope. Sanitary science, hygiene, practical medicine, 

 botany, pathology, and physiology have all been illuminated 

 and extended by his researches. His claim for being in- 

 eluded among the great names in English science does 

 not rest merely upon his acknowledged eminence as an 

 original and exact investigator, but also upon the influence 

 which, for four decades, he exerted upon other workers 

 in medical science, endowing their investigations with 

 purpose and materially helping to give English physiology 

 and pathology their proper scientific status. Many circum- 

 stances contributed to make this influence widely felt : 

 among these were the peculiar charm of his manner, his 

 striking and commanding personality, the genuine enthu- 

 siasm with which he followed the work of others, the 

 devotion with w'hich he advocated the use of experimental 

 methods, his scientific achievements, and his extensive 

 knowledge. All these qualities of mind and character 

 marked hiin as one of those great masters who inspire 

 the work and mould the thought of a generation. It is 

 in tribute to his memory that, as one of his pupils and 

 his successor in the Oxford Chair of Physiology, I utilise 

 this occasion for recalling such fruitful features of his 

 scientific conceptions as are expressed in the felicitous 

 phrase which I have quoted. 



Probably the most important of the many services which 

 Burdon-Sanderson rendered to English medical science was 

 that of helping to direct physiological and pathological 

 inquiry towards its proper goal. It will be admitted by 

 all who knew him intimately that among his most 

 characteristic scientific qualifications were the insight with 

 which he realised the essence of a physiological problem, 



I Address to the Subsection of Anatomy and Physiology, by J. Burdon- 

 Sanderson, British .Association Report, York, 1881. 



