2O0 



NATURE 



[October 17, 1912 



state more prtcisfly the problem which Dr. Beilby 

 desires to see investigated. The fundamental ques- 

 tion, which sees beyond Quincke's hypothesis, is this, 

 whether the liquid 'metal undergoes any changes or 

 separations before actual solidification commences, and, 

 if so, whether there is really any formation of foam 

 cells or analogous structures governing the crystal- 

 lisation of the metal. In his note Dr. Beilby quotes 

 some lines of evidence from the manner in which 

 a thin film of fused salt or other substance solidifies 

 on a glass slip which appears to be strikingly con- 

 tradictorv to Quincke's views, and indeed the impres- 

 sion derived from reading Dr. Beilby 's note is rather 

 that he finds the ■' foam cell" theory less attractive 

 after thus considering it more closely. -A. practical 

 result is, however, likely to follow from Dr. Beilby's 

 interest in the matter, in the =hape of an exhaustive 

 report on our present knowledge of the passage from 

 the liquid to the solid state in metals, prepared under 

 the auspices of a committee of the Institute of Metals, 

 and this will certainly be very welcome. 



Of purelv theoretical .interest is the paper presented 

 by Dr. Rosenhain and .Mr. Ewen, of the National 

 Physical Laboratory, on the intercrystalline cohesion 

 of metals. In this paper the authors advance the 

 hypothesis that the crystals of a pure metal are held 

 together bv the action of a thin layer of metal in the 

 amorphous condition forming a species of cement 

 between the crystals. The conception of the existence 

 of such a cement has already been put forward by 

 Bengough and by Osmond, but the authors claim to 

 have used it as a working hypothesis in their own 

 laboratory before others had published their views. 

 The paper begins with a detailed discussion of the 

 general facts which lead in the first place to the idea 

 that there should be some special condition at the 

 boundary surfaces of crystals in solid metals; perhaps 

 the most striking of these facts is the strength of 

 these bounding surfaces, since it has been conclusively 

 shown that pure metals normally undergo fracture 

 through the crystals and not along the boundaries 

 between them; the cohesion across these bounding 

 surfaces is thus stronger than that across the cleavage 

 planes of the crystals themselves. 



The authors next suggest in general terms that 

 when two growing crystals approach one another, a 

 region is formed at their boundary in which the mole- 

 cules are no longer able to assume the crystalline 

 arrangement, and they further point out that if the 

 unit or element of which the crystal is built up is 

 large compared with the "liquid" molecule, then at 

 the junction of two crystals gaps must remain which 

 are too small to contain another complete crystal unit 

 and that consequently such gaps would ultimately be 

 filled by undercooled liquid metal which had been 

 unable to crystallise. This undercooled liquid would 

 then be identical with the "amorphous phase" of 

 Beilby, and would possess similar properties. The 

 paper points out that Beilby has shown that the amor- 

 phous phase is more soluble jn acids and possesses 

 greater chemical activity than the crystalline phase, 

 and it would accordingly possess a higher vapour 

 pressure under corresponding conditions of tempera- 

 ture. It follows that if two pieces of the same metal, 

 one containing a small and the other a relatively 

 large proportion of amorphous matter, were heated 

 to the same high temperature in a high vacuum, the 

 one containing the larger proportion of amorphous 

 matter would lose weight more rapidly than the other. 



This conclusion the authors have submitted to the 

 test of experiment in the following manner. Tf an 

 amorphous intercrystalline cement exists, then a 

 specimen of metal 'consisting of a few large crystals 



NO. 2242, VOL. go] 



would contain less of this amorphous material than 

 a piece of the same metal which consists of a very 

 large number of minute crystals, since the material 

 of the cement is chemically identical with the bulk of 

 the metal. Specimens possessing large and small 

 crystals respectively were prepared by the authors in 

 various wavs, and these specimens were heated in 

 high vacua (0005 mm. and under) at temperatures 

 sufficient to produce considerable volatilisalion losses. 



but still well below the melting points of the metals 

 in question. 



In the case of silver, zinc, and copper the authors 

 found their expectations verified ; the specimens 

 possessing the minute crystal structure in every case 

 losing weight at a greater rate than the coarsely 

 crystalline specimens. The differences found were, in 

 fact, considerably greater than the amount of amor- 

 phous cement which could reasonably be supposed to 

 exist in the specimens, but although this point was 



somewhat ^boured in the discussion, the authors offer 

 a fairly satisfactory explanation by showing that the 

 formation of fissures due to the evaporation of the 

 "cement" allows all the crystals in the fine-grained 

 metal to undergo volatilisation from all their surfaces, 

 so that after a time the effective evaporating surface 

 of the fine-grained specimen is much larger than that 

 of the coarse-grained. 



This explanation is supported by microscopic 

 evidence, since the widening — by evaporation — of the 



