5o6 



NATURE 



[September 23, 1897 



due to the fact that the silver gradually shades off towards the 

 clear glass. Occasionally, however, each band, in passing from 

 the glass-air to the glass- silver surface, is joined to both the 

 nearest bands on the silver. It was due to this cause that 

 Quincke was unable to say whether an acceleration or retardation 

 was produced. 



Wernicke states that a retardation is only produced when the 

 silver is of a friable nature, and could be readily rubbed off the 

 glass. The accompanying photograph was obtained with a 

 silver film that easily bore polishing, and showed no want of 

 adherence to the glass. E. Edser. 



H. Stansfielp. 



MICRO-STRUCTURE OF ALLOYS. 

 A T the Royal Society's conversazione this year, Mr. J, E. 

 ■^^ Stead exhibited a series of photographs illustrating the 

 micro-structure of various alloys. In many cases the structure 

 portrayed was very complex and interesting, and in some cases 

 beautiful. 



Many series were illustrated by Mr. Stead, but it would take 

 much more space than is available in a short notice to more than 

 point out briefly the main features of a few. 



The photographs showed that when the antimony exceeded 

 7 '5 per cent, in antimony-tin alloys, the excess over that amount 

 separated out with an equal atomic proportion of tin as more 

 or less perfectly formed cubes. That they were crystals of 

 definite chemical atomic composition Mr. Stead had verified by 

 several careful analyses after having dissolved away the eutectic, 

 or what was once the mother liquor, with nitric acid, which left 

 the crystals intact. The photographs of alloys of tin containing 

 phosphorus and arsenic had the appearance of very straight 

 bright lines, which cut up the surface into irregular figures. 

 These lines are the edges of flat plates, which, when separated 

 by dissolving away the tin, have been proved by analysis to have 

 the composition of SnjPg and SnjAso, respectively. The photo- 

 graphs of the separated compounds indicate that they had both 

 the same crystalline form of hexagonal plates. A photograph 

 of one of the free ends of a plate showed several pointed crystals 

 having angles of 6o°. 



The structure of tin-copper alloys rich in tin was illustrated 

 by several photographs, which showed that in alloys containing 

 from 2 to O'lo per cent, copper acicular crystals were present, 

 and that with each addition of copper the separated compound 

 assumed a more plate-like structure, until when 35 per cent, 

 copper was present, apparently it was all in the form of plates. 

 All these compounds have been separated and analysed by Mr. 

 Stead, with the following results : — 



65"3 >> 34'7 !, ■■• 56'i2 ,, 43'88 ,, SnCu2 ,, 



It will be seen that although the compound separated from 

 the 2 per cent, alloys approximates to the composition of SnCu, 

 each addition of copper to the alloy results in the formation of a 

 compound which after separation proves to contain a greater 

 proportion of copper than that from the alloy containing less 

 copper. 



It has not yet been proved whether these compounds are 

 amorphous mixtures or combinations of one or more atomic 

 chemical constituents. 



It appears that in all the solid alloys of lead and antimony 

 the elements are in a free state. There is a eutectic which 

 contains 127 per cent, antimony. Those having more than that 

 quantity of antimony contain large crystals of free antimony, 

 which until 50 per cent, is present are found at the upper part of 

 the alloy if Jhe cooling of the liquid alloy has been sufficiently 

 slow, but belween these crystals the eutectic is clearly visible. 

 WTien the antimony is increased to 50 per cent, the white 

 crystals and dark eutectic occupy nearly equal areas, and with 

 each addition of antimony the dark areas diminish until when 

 100 per cent, is present the surface presents a homogeneous 

 white appearance free from the dark eutectic. 



With alloys containing less than 127 per cent, antimony the 

 polished and etched surfaces clearly show the presence of den- 

 dritic crystals of lead. 



NO. 1456, VOL. 56] 



The eutectic has the very peculiar structure similar to that of 

 nodular radiated pyrites. On treating this compound with 

 dilute nitric acid for a long period, a coherent dark-coloured 

 mass is left free from lead, and which appears to consist when 

 broken up as very fine bright plates, exceedingly thin and easily 

 broken up, with the slightest pressure, into what appears to be 

 an amorphous powder. . 



Krii 75 per cent. ; Sb, 20 per cen 



Mr. Stead and Mr. Charpy have simultaneously investigated 

 the alloys of tin-antimony, tin-copper, and lead-antimony, and 

 the results of their micro-examinations are almost identical; but 

 Mr. Stead has supplemented his micro- research with chemical 

 examination, which greatly increases their value. 



The micro-structures of ternary alloys are of very much 

 greater interest than those of two metals only, for Mr. Stead 



Fig. 2. — Magnolia metal. Magnified 200 diameters. 

 Pb, 80 per cent. ; Sb, 15 per cent. ; Sn, 5 per cent. 



has shown that it is possible to detect two, and sometime 

 three, distinctly different compounds in the same microscopic 

 field. Sometimes two of the elements combine and crystallise 

 together ; sometimes three will so unite. Examples of tin- 

 copper-antimony, and tin-antimony-arsenic (Fig. i), and lead- 

 antimony-tin (Fig. 2), and tin-antimony-phosphorus were shown 

 at the Royal Society. 



