January 22, 1920] 



NATURE 



535 



stalling- this process is high, and bears a large 

 proportion to the market value of the annual 

 production. 



It follows, therefore, from these conclusions that 

 the industrial demand for nitric acid in the future 

 will probably be met by means of synthetic pro- 

 cesses. It is conceivable that the marketing of 

 large quantities of synthetic sulphate of ammonia 

 and cyanamide, made in Germany, will influence 

 the future price of combined nitrogen, and may 

 even control it. The Committee thinks that the 

 producers of combined nitrogen may eventually 

 have to face a competitive price of 7L or 8/. per 

 metric ton for ammonium sulphate, and 5/. or 7/. 

 per metric ton for cyanamide. The by-product 

 ammonia industry in this country may thus be 

 seriously menaqed. It can scarcely recoup itself 

 by raising the price of the other by-products to 

 any serious extent, and considering the relation 

 of combined nitrogen to the food of the country, 

 public opinion, whilst willing to tolerate, up to a 

 certain extent, the protection of "key " industries, 

 would strongly resent any action which seriously 

 interfered with the productive capacity of the land. 

 1^0 he continued.) 



THE MICROSCOPY OF METALS. 



A T the very successful symposium on the micro- 

 •^* scope organised by the Faraday Society 

 on January 14 in the rooms of the Royal Society, 



discussion. Sir Robert Hadfield's introductory 

 address surveyed the history of microscopical in- 

 vention, and was illustrated by portraits of some of 

 the pioneers in the art — Jansen, Lipperhey, 

 Leeuwenhoek, Sorby, and Dallinger. We were 

 reminded of the fact that, so far back as 1665, 

 Robert Hooke described in his " Micrographia " 

 the appearance of the point of a needle and the 

 edge of a razor, and his faithful drawings of these 

 two objects, revealing most accurately the features 

 which could be observed under a low magnifica- 

 tion, are reproduced in the paper. The next in- 

 stance of the application of the microscope to the 

 examination of metals is that by R6aumur, whose 

 work on steel, published in 1722, contains many 

 drawings of the magnified fractures of iron and 

 steel bars. Such a method, however, gave little 

 information, and did not lead to any further 

 development. In 1808 Widmanstatten studied 

 the structure of meteoric irons by polishing- a plane 

 section and heating until the constituents became 

 differentiallv coloured by oxidation, thus introduc- 

 ing the method now familiar as "heat-tinting." 

 The structure of these irons is so coarse that mag- 

 nification is unnecessary, but the method gave 

 hints to later workers, of whom Sorby is the chief. 

 The work of Sorby, in view of its great import- 

 ance, was dealt with by the president in a separate 

 note. 



Henry Clifton Sorby, one of those amateur 

 scientific investigators who have contributed so 



Fig. t.— Ma 



lificatii 



12 mm. Achrom.it. 



Fig. 2. — Magnillcation 600. 3 unit. Apuchromat. 

 Phot .micrographs showing the effect of magnification without resolution The steel used in this experiment had the following composition ; 



C 0-48, Si 0-17, S 0029, P 0-034, Mt) I'oo per cent. 



about one-half of the papers presented dealt with 

 the microscopical examination of metals, a strik- 

 ing indication of the importance which this branch 

 of microscopy has now acquired. It was therefore 

 appropriate that the president, himself a distin- 

 guished worker in this field, shouki deal historic- 

 ally with the development of micro-metallography, 

 as well as contributing an original paper to the 

 NO. 2621, VOL. 104] 



wonderfully to the progress of science in this 

 country, was led to devise the modern method of 

 microscopic petrography by seeing sections of 

 bone, teeth, etc., rendered transparent by affixing 

 one surface to glass and then grinding down to 

 an extreme thinness, as practised by the botanist 

 Williamso'i. Sorbv treated rocks in the same 

 way, and in 1849 he prepared the first rock slice 



