May 13, 1897] 



NATURE 



43 



fissures form along the surface, from which pour out the yellow 

 vapours of sodium. Occasionally, the mixture on the top of 

 the furnace is not sufficiently porous to allow the rapid escape 

 of the gases. The result is, that the latter accumulate until the 

 pressure is so great that, at some weak point in the mixture 



' < ive, a path is forced open and the gases rush out violently. 

 i^ mainly for the purpose of avoiding this "blowing '' that the 

 udust is put in the mixture, since the former, by making the 

 mixture porous, allows the gases to escape freely. 



At the end of about twenty-four hours the current is cut off 

 from the furnace, and it is allowed to cool for a few hours. 

 Then the side walls are taken down and the unchanged mixture 

 raked off the top of the furnace, until the outer crust of 

 amorphous carborundum is reached. This crust is cut through 

 with large steel bars, and can then be easily removed from the 

 inner crust of amorphous carborundum. The inner crust is 

 next removed with a spade, and the crystalline carborundum 

 exposed. 



A cross-section of a carborundum furnace presents an interest- 

 ing and beautiful appearance (Fig 2). In the centre is the core, 

 which, on examination, is found to be very different in some of 

 its physical characteristics from the coke of which it was 

 originally composed. It no longer possesses a bright metallic 

 appearance. Many of the kernels are quite soft, and can be 

 squeezed between the fingers, 

 leaving on them a mark like 

 black lead. In fact, the high 

 temperature to which the 

 core has been raised has 

 driven off all impurities 

 from the coke, leaving 

 nothing but pure carbon, 

 either in the amorphous or 

 graphitic form From the 

 core radiate beautifully- 

 coloured carborundum cry- 

 stals to a distance of 10 or 

 12 inches. \ single furnace 

 yields over 4000 pounds 

 of cr}'Stalline carborundum. 

 Most of these crystals are 

 not remarkable as regards 

 their size, but in places 

 where hollows have formed, 

 large hexagonal crystals are 

 found, sometimes measuring 

 ^ inch on a side. At the 

 distance of 10 or 12 inches 

 from the core the crystals 

 suddenly cease, and, instead, 

 we find a thin layer of a 

 light-green colour, which is 

 the inner crust of amorphous 

 carborundum. Beyond this 

 is the outer crust of amor- 

 phous carborundum, and 

 this also ends aljrupily in 



unchanged mixture. Other curious substances are some- 

 times produced in the furnaces ; for example, silica, which 

 has the appearance of spun glass. On opening a fur- 

 nace and cutting down to ilie core, a layer is found that 

 appears at first sight to consist of very dull black carbor- 

 undum crystals. On closer examination, however, it is 

 found that though this material has the exact form of the 

 carborundum crystals, it is nothing but pure carbon in the 

 graphitic form. 



After the carborundum has been removed from the furnace it 

 is taken to a crusher, which consists of a large iron pan, rotated 

 in a horizontal plane by means of a vertical shaft. A horizontal 

 shaft, carrying two heavy rollers, is attached to a collar sur- 

 rounding the vertical shaft, thus permitting a free vertical 

 motion of the rollers which rest in the pan. The latter, in 

 revolving, causes the carborundum to pass under the rolls, 

 which break the mass of crystals apart. From the crusher 

 the carborundum is taken to large wooden tanks, where 

 it is treated for several days with dilute sulphuric acid to 

 remove impurities. It is then thoroughly washed, dried, and 

 graded. 



Carborundum is ajiparently infusible ; for after a certain 

 temperature has been reached, decomposition commences, 



NO. 1437. VOL. 56] 



without fusion, and the crystals are broken up into carbon 

 and silicon. It is quite insoluble in water or any acid. 

 Its hardness lies somewhere between 9° and 10% probably 

 very close to 10°, which is the hardness of diamond. An 

 attempt was made to obtain some idea of the relative 

 hardness of diamond, corundum, and carborundum, by the 

 following experiment. 



A series of lines was scratched on a small plate of glass with 

 each of the three materials, and the scratches examined with a 

 microscope. The appearance of the lines made by the diamond 

 and the carborundum crystal was indistinguishable ; but that 

 made with the piece of corundum was quite different, being 

 rough, and not presenting the clean-cut outlines of the 

 other scratches. This seems to show that carborundum 

 is much nearer diamond than corundum in hardness, al- 

 though it is not as tough. The specific gravity of car- 

 borundum is 3 23, which is less than that of emery, \\ 

 pounds of the latter being equal in volume to i pound of 

 the former. 



Carborundrum is chiefly used at present as an abrasive, for 

 which purpose it possesses many advantages over emery and 

 corundum. The Carborundum Company produced during the 

 year 1896, in round numbers, 1,191,000 pounds of crystalline 

 carborundum. 



Furnace opened to show formation of carborundum around the core. 



UNIVERSITY AM) EDUCATIONAL 

 INTELLIGKiXCi:. 



OxioKi). — Prot E. B. Elliott and Prof H. H. Turner have 

 been appointed Electors to the Savilian Professorship of 

 Geometry. 



The Board of the Faculty of Natural Science have givers 

 notice that henceforth in the Final Honour School of Chemistry 

 the use of books in the examination in (^)uantitative Analysis, 

 and in any part of the examination, shall be at the discretion of 

 the Examiners. 



Trinity College has decided to build a laboratory adjoining the 

 existing laboratory of Balliol College, and communicating with 

 it. Such action in favour of Oxford science on the part of a 

 College, far from rich, is especially commendable. 



This term Mr. II. 11. Champion, of Cambridge, is lecturing, 

 for Prof Turner, on " Lunar Theory." Prof. Kay Lankester 

 will lecture on Arthropoda, and Mr. G. C. Bourne on 

 Mechanical Theories of Development. Dr. Benham and Mr. 

 Bourne are conducting the annual summer course of Practical 

 Embryology. 



Prof. E. B. Tylor's subject for this term is the Anthropology 

 of Political and Social Institutions. Mr. Barclay Thompson 



