NA TURE 



[June 28, 1900 



crystalline pattern completely. Experiments have a.io been 

 made at ioo° C. and 150° C, leading to the general result that 

 crystalline growth will occur at any temperature from that of an 

 ordinary room, i.e. 15° C. or 20° C, up to the melting point of 

 1 ead, and that in general the higher the temperature the more 

 rapid is the initial rate of change. No numerical data can be 

 given, as the crystals are quite irregular, both in size and shape. 



A comparison of micro-photographs of the same specimen at 

 various stages reveals the fact that the growth of an individual 

 crystal occurs, not in uniform layers all round it, but by the 

 formation of arms and branches that invade the neighbouring 

 crystals, the intervening i)ortions sometimes changing at a later 

 stage. This action is analogous to the formation of skeleton 

 crystals in a metal during solidification from the liquid state, 

 the space between the branches filling in as solidification 

 proceeds. 



A marked feature observed in several specimens was the large 

 and rapid growth of one or two individual crystals ; in several 

 instances such individuals grew until they were some hundreds 

 of times larger than their neighbours. Generally the most 

 aggressive crystals were found near the edges of the specimen. 

 It is noticeable that at times a crystal which has already grown 

 considerably is swallowed up by a more powerful neighbour. 



Some light is thrown on the nature of these actions by the 

 fact that this growth only occurs in crystals that have been 

 subjected to severe plastic strain. By casting the metal in a 

 chill mould, specimens of lead can be obtained having a crystal- 

 line structure quite as minute as that found in a severely strained 

 specimen, but this structure remains unchanged at temperatures 

 which produce rapid change in a strained specimen. 



The investigation of the effects of such comparatively moderate 

 temperatures was extended to other metals, viz. tin, zinc and 

 cadmium. In tin, the various phenomena of crystallisation 

 from the fluid state are strikingly illustrated on a large scale by 

 the thin layer of that metal which constitutes the surface of 

 commercial tin-plate. The effects of rapid and slow solidifica- 

 tion in producing small or large crystals respectively are well 

 marked, and an examination of the etched surface of tin-plate 

 under the microscope reveals beautiful geometrical markings or 

 pits, whose oriented facets produce the well-known .selective 

 effect of oblique illumination. The study of the crystalline 

 structure affords an explanation of the nature and method of 

 production of patterns in " moiree metallique," a process which 

 has long been in use for the decoration of articles manufactured 

 of tin-plate. 



The final section of the paper deals with an hypothesis, which 

 is advanced as an attempt to explain the mechanism of the 

 growth of crystals in apparently solid metal. ^ According to this 

 hypothesis, the metallic impurities which are present in a metal 

 play an important part in the action. When a metal solidifies 

 from the fluid state, the metallic im.purities ultimately crystallise 

 as a film of eutectic alloy in the inter-crystalline junctions ; 

 when fairly large quantities of such eutectics are present, the 

 microscope reveals their presence as an inter-crystalline cement, 

 such as that formed by " pearlite" in slowly cooled mild steel ; 

 very minute quantities of eutectic, however, will be invisible 

 and yet capable of forming a thin film of fusible cement. The 

 authors conceive that the changes of crystalline structure which 

 go on while the piece is in the solid state are accomplished 

 by the agency of eutectic films between the crystals, in dis- 

 solving metal from the surfaces of some crystals and 

 depositing it on others. When a metal is severely strained, 

 these films of eutectic will be also strained and in many 

 places broken, thus allowing the actual crystals to come into con- 

 tact with one another. The difference in the rate of etching of 

 adjacent crystals and the phenomena of the electrolytic transfer, 

 in an acid solution, of lead from one crystal to another in the 

 same mass of metal, support the supposition that there is a 

 difference of electric potential between the crystal faces which 

 are brought into contact by severe strain. If it be assumed that 

 a film of eutectic alloy when fluid, or even when in the pasty 

 condition that precedes fusion, can act as an electrolyte, we may 

 regard any two crystals thus in contact, with a film of eutectic 

 interposed in places, as a very low resistance circuit, and the 

 growth of the positive crystal at the expense of the negative 

 would resu It. Moreover, such growth would be more rapid at 

 higher temperatures, and its rate at a given temperature would 

 vary in different specimens according to the nature and quantity 

 of the im purities present. That an alloy can act as an electro- 

 1 It is proper to say that this hypothesis is due to Mr. Rosenhain.— J. A. E. 



NO. l6C0, VOL. 62] 



lyte has not been established experimentally, but the assumption 

 is supported by the close general analogy between alloys and 

 salt solutions. This analogy extends to the very question of 

 the growth of crystals, as Joly has shown that when crystals of a 

 salt are immersed in their mother-liquor, growth of one at the 

 expense of others will take place. 



It should be added that solution of one crystal into the inter- 

 vening film of eutectic, along with deposit on the neighbouring 

 crystal from the eutectic, may occur as a consequence of 

 differences of orientation, producing differences of "solution 

 pressure" apart from actual electrolysis, but the fact that growth 

 has not been observed to occur except in strained crystals 

 favours the view that the action is electrolytic. 



Some further results which have been deduced from the 

 above hypothesis have been verified by experiment. It follows 

 from the hypothesis that an inter-crystalline boundary contain- 

 ing no eutectic would be an impassible barrier to crystalline 

 growth, but if the eutectic could in any way be supplied, growth 

 across the boundary might take place. In an absolutely 

 pure specimen of lead, there would be no eutectic at the 

 inter-crystalline junctions, but as extremely minute traces 

 of impurity would suffice to set up the action, it is almost 

 hopeless to verify the hypothesis in this way. Some experi- 

 ments on the cold welding of lead have, however, borne out 

 these conclusions. Two clean, freshly-scraped lead surfaces will 

 unite under great pressure in the cold state, and if a piece so 

 welded be annealed, the crystalline growth due to the anneal- 

 ing, with very rare exceptions, never crosses the inter-crystalline 

 boundary formed by the welding surface. To test whether the 

 presence of some eutectic would allow growth to take place, 

 small quantities of a P'ore fusible metal were scattered over the 

 freshly -scraped surfaces of lead before squeezing them together. 

 Then, after a cold weld had been made by pressure, on anneal- 

 ing by exposure to 200° C. it was found that crystal growths fre- 

 quently crossed the line of the weld, as the above theory led one 

 to expect. This experiment has been repeated many times with 

 the uniform result that whenever a small quantity of eutectic, or 

 of an impurity capable of forming a eutectic with the lead, was 

 scattered over the clean surfaces before welding, a distinct 

 growth of crystals across the boundary took place as a result of 

 annealing. On the other hand, a large number of welds were 

 made without introducing any impurity, and with very rare ex- 

 ceptions they showed no growth across the boundary, even after 

 the annealing process was continued for some weeks. In rare 

 exceptions a minute amount of growth across the boundary was 

 observed, but these may fairly be accounted for by the almost 

 unavoidable presence of traces of impurity. The result as a 

 whole goes far to confirm this solution theory of crystalline 

 growth in annealing. 



June 14. — " Static Diffusion of Gases and Liquids in Relation 

 to the Assimilation of Carbon, and Translocation in Plants." 

 By Horace T. Brown, F.R.S., LL.D., and F. Escombe,B.Sc., 

 F.L.S. 



This paper is intended to be the first of a series descriptive 

 of the work carried out by the authors in the Jodrell labor- 

 atory on the fixation of carbon by green plants, and deals 

 mainly with the purely physical processes by which atmospheric 

 carbon dioxide gains access to the active centres of assimilation. 



The new evidence which F. F. Blackman brought forward 

 in 1895 in favour of the gaseous exchanges of leaves taking 

 place exclusively through the stomatic openings, presents at 

 first sight certain difficulties of a physical nature, which have 

 led to an examination of the whole question of the free diffusion 

 of carbon dioxide at very low tension, and under a set of con- 

 ditions very different from those under which the previous deter- 

 minations of the coefficient of diffusion of carbon dioxide and 

 air have been made by Loschmidt and others, where the gases 

 were initially of equal tension, and the ratios of mixture de- 

 parted widely from those of ordinary atmospheric air. The 

 inquiry has led to the discovery of some new facts connected 

 with the static diffusion of gases and liquids, which are of con- 

 siderable interest, not only from the physical point of view, but 

 from the explanations they suggest of certain natural processes 

 which are primarily dependent on diffusivity. 



The method employed in the first instance for the deter- 

 mination of the diffusivity of atmospheric carbon dioxide was 

 one of static diffusion down a column of air of a definite 

 length towards an absorptive surface at the bottom of the 

 column When a static condition has been established, there 

 is a steady flux of the carbon dioxide down the air column 



