J 34 



NATURE 



[October i8, 19 17 



OUR ASTRONOMICAL COLUMN. 



The Nature of Sun-spots. — A useful summary of 

 our knowledge of the phenomena presented by sun- 

 spots is given by the Kev. A. L. Cortie in Science 

 Progress for October. A spot is regarded as an up- 

 rush of metallic vapours, which become cooled by 

 rapid expansion, so that the spot appears dark by 

 contrast with the bright solar surface. The umbra is 

 considered to rise above the level of the photosphere, 

 while the penumbra is built up by dark radial streams 

 flowing from the umbra and seeking a level 

 slightlv lower than that of the photosphere. In round 

 spots the penumbra is a shallow, saucer-like cavity, 

 the lowest portion being due to the falling-in of the 

 photospheric clouds caused by the initial uprush. This 

 falling-in and heaping together of the photospheric 

 clouds to fill the partial void produced by the ejection 

 of the umbral vapours v.-ould account for the bright 

 border which is generally seen to separate the umbra 

 and penumbra. At a high level above the spot are the 

 hydrogen flocculi, the rotation of which gives rise to 

 the appearance of the solar vortices. Friction of the 

 gyrating gases and vapours is considered competent 

 to generate electric currents, and the accompanying 

 magnetic fields which produce the Zeeman effects in 

 the spectra of spots. Father Cortie considers it doubt- 

 ful whether the umbra and penumbra of the spots 

 themselves share this gyratory motion. 



Solar Prominences in 19 15. — An account of the 

 observations of solar prominences made at Catania 

 during 19 15 has recently been given by Prof. Ricco, in 

 continuation of the admirable series commenced by 

 him in 1880 {Mem. Soc. Spett. Italiani, July-August, 

 1917). The number of days of observation was 168, 

 and the total number of prominences exceeding 15" in 

 height was 1264, giving a mean daily frequency of 7-5. 

 The activity in the northern hemisphere was the 

 greater, the respective numbers being 677 and 587. 

 The prominences were distributed almost symmetric- 

 ally in the two hemispheres, few or none appearing in 

 the polar and equatorial regions, while there were well- 

 defined maxima in the zones ±30°-35° and ±5o°-55_°. 

 Twenty-four of the prominences exceeded 100" in 

 height, and one observed on May 5 reached 286". The 

 mean latitude of all the prominences was 38-65°, which 

 was rather lower than for the preceding year. There 

 was a marked mcfease of activity as compared with 

 1914. 



A COMMERCIAL IRON OF UNUSUAL 

 PURITY. 



FOR some time past the Shelton Iron, Steel, and 

 Coal Co., of Stoke-on-Trent, has been manu- 

 facturing a commercial iron of unusual purity in the 

 basic open-hearth furnace. This iron is guaranteed 

 to be 9984 per cent, pure, and has been placed on 

 the market under the trade name " Armco Iron." It 

 differs from wrought-iron in that it has been melted 

 and cast, and thus contains much less slag, and from 

 mild steel in that its carbon content is so low that no 

 pearlite is present. It has been found, however, to 

 possess a peculiar property which militates against 

 its practical usefulness, viz. the property of a char- 

 acteristic red-shortness, or brittleness, when subjected 

 to mechanical treatment between certain limits of 

 temperature. The reason for this peculiar behaviour, 

 which is not shared by other forms of commercial 

 iron and mild steel of high quality, has been investi- 

 gated by Messrs. Brooke and Hunting, and their pre- 

 liminary results were communicated in an interesting 

 note to the recent meeting of the Iron and Steel Insti- 

 tute. 



NO. 2503, VOL. 1 00] 



Very early in the history of the process it was 

 found that this brittleness always appeared between 

 certain fixed limits of temperature, which they place 

 at from 900^-800° C. ; that on heating the iron to well 

 above AC3, and allowing it to cool, brittleness appeared, 

 first at about 900° C, and disappeared sharply at 

 about 800" C; and that above and below these tem- 

 peratures the metal possessed an unusually high de- 

 gree of ductility and malleability. In fact, they com- 

 ment on its similarity when cold to copper in respect 

 of malleability, thus emphasising also its resemblance 

 to the electrolytic iron investigated in 19 13 by Stead 

 and Carpenter. 



The authors then proceeded to carry out systematic 

 experiments on specimens of the iron quenched from 

 various temperatures. They determined the tensile pro- 

 perties, and examined its structure and the type of frac- 

 ture. Most interesting and illuminating results were 

 obtained with the photographic records. Quenched 

 from temperatures above 1000° C, the structure was 

 that of 7 iron with " martensitic" markings. As the 

 quenching temperature fell this appearance altered, 

 and the "7 iron effect changed to a more definite 

 ferrite form." The authors say that at about the 

 Ar3 point a complete change occurred, the ferrite 

 grains increased considerably in size, and at the junc- 

 tions of many of the crystal boundaries a peculiar 

 structure was observed which was "perfectly constant 

 and always possessed the same characteristic . . . 

 viz. a central structure more or less pearlitic and very 

 clearly defined, surrounded by a space composed of 

 ferrite, and the whole again surrounded by a definite 

 boundary which connects up with adjacent crystal 

 grains." This structure is clearly seen in the photo- 

 graphs published by the authors. They say that a 

 very large number of experiments have been made, 

 and that in every case this peculiar structure has 

 appeared in exactly the same manner. On lowering 

 the quenching temperature somewhat the structure 

 gradually becomes less pronounced, and at just above 

 800° C. it ceases to exist. Below 800° C. it was 

 never observed, and the structure was that of normal 

 ferrite. The temperatures at which this material is 

 precipitated and reabsorbed coincide so remarkably 

 with the beginning and end of the zone of brittleness 

 that a strong presumption has been established that 

 herein lies the origin of the characteristic red-short- 

 ness of the iron. The authors suggest that it is a 

 eutectoid, probably composed of iron carbide, phos- 

 phide, and sulphide, with possibly traces of man- 

 ganese sulphide and ferrous oxide, and that ,it is 

 thrown out of solution in a semi-liquid or plastic 

 condition, causing the grains to be very loosely held 

 together, and thus making the structure relatively 

 weak. If the iron is quenched in the 9oo°-8oo° C. 

 zone, no brittleness is observed in the static tests. 



The authors have found that a sample of Swedish 

 iron similar in purity to the above material also 

 shows a brittle zone in the same temperature range, 

 and that an iron containing carbon 006 per cent, and 

 manganese o-io per cent, shows no brittleness when 

 manufactured in the same way as "Armco" iron. 

 At one stage of the investigation they were inclined 

 to connect the appearance of the material with the 

 existence of dissolved oxide, since in one set of experi- 

 ments they found it difficult to reproduce the char- 

 acteristic structure, and this coincided with a remark- 

 able absence of spots of "oxide material." Samples 

 of metal, however, taken from the bath, just before 

 tapping, when it was known to be in a super-oxidised 

 condition, gave only a normal amount of eutectoid 

 structure. Again, when complete deoxidation was 

 attempted it still appeared. There is no reason, there- 

 fore, for connecting it with the presence of an unusuai^ 

 amount of oxide, and the nature of the " eutectoid " 



