April 13, 1899J 



NA TURE 



559 



Corundum deposits are known to occur all along the 

 southern flanks of the Appalachian Chain, from the 

 State of New York to that of Alabama, but it is in only a 

 few localities, principally in North Carolina and Georgia, 

 that the corundum has been extracted on any consider- 

 able scale. In 1871 attention was first drawn to the 

 deposits in North Carolina as a possible source of geins, 

 and in 1S7S mining operations were commenced to ex- 

 tract the abundant corundum of the district as an abrasive 

 material. A great deal of secrecy has been maintained 

 respecting the nature and extent of the corundum in- 

 dustry in the United States ; but there appears to be no 

 doubt that since 1878 a steady increase in the output of 

 the corundum mines has been maintained. 



The discovery of valuable deposits of corundum in the 

 third of the localities noticed above, that of British 

 Ontario, dates only from 1896. But already there seems 

 to be promise that the counties of Hastings, Renfrew and 

 Peterborough in Eastern Ontario, may, at no distant 

 date, yield large supplies of corundum to the manu- 

 facturer. 



The Indian corundum is usually found among the 

 gneissose and schistose rocks ; the e.xact conditions 

 under which the mineral makes its appearance will be 

 better understood when the investigations,upon which Mr. 

 Holland has been engaged for some years, are fully pub- 

 lished. He has already shown that in some cases the 

 corundum is found in connection with nepheline-bearing 

 rocks, and a precisely similar association has been 

 demonstrated for the corundiferous deposits of Eastern 

 Ontario. The corundum of the Appalachian belt of the 

 United States, however, as shown by Dr. J. II. Pratt, 

 would appear in all cases to occur in the Peridotites 

 (Dunites, Serpentine, &c.), which are intrusive in 

 crystalline schists, and especially in the zones of contact 

 on the outer limits of those intrusive masses. 



Corundum, the crystallised oxide of aluminium, has 

 been prized from the earliest times on account of its 

 hardness — which exceeds that of all other natural sub- 

 stances, with the exception of the diamond. In India, 

 blocks of corundum and fragments mounted in tools have 

 been used for grinding, perforating and engraving gems. 

 For general abrasive purposes elsewhere, the rock emery 

 (especially that of Naxos and the adjoining islands and 

 mainland of Asia Minor) has long been preferred to 

 corundum itself. The reason of this is that although 

 emery has a far less " effective hardness," or power of 

 abrading hard materials, than pure corundum, yet the 

 ease with which it can be reduced to powder greatly 

 facilitates its use. 



Pure corundum, when freed from its adhering matrix 

 of softer materials (mica, chlorite, &c.), is crushed be- 

 tween rollers and sifted, the " corundum sand " thus 

 formed having far more abrasive power than crushed 

 emery. The chief use of corundum sand is for making 

 corundum wheels ; the cementing materials employed in 

 making these wheels seem to be very varied. Shellac 

 alone, or with the so-called "oxidised linseed oil," is one 

 of the commonest materials employed, as in the so-called 

 "red wheels." Silicate of soda is employed in the 

 "silicate wheels," and india-rubber and other substances 

 in the " vulcanite" or "black wheels" ; while the cement- 

 ing material in the "union wheel" is oxychloride of 

 magnesia, and in the " tanite wheel " some form of a 

 so-called " solution of leather," the process of manufacture 

 being kept secret. 



Pure corundum wheels are said to be at least twice as 

 effective and durable as emery wheels. Corundum wheels 

 are made in India, with the lac-resin as the cementing 

 material. 



Emery and corundum wheels may be regarded as 

 rotary files, whose cutting points never grow dull. They 

 are rapidly replacing files for cutting down metal surfaces, 

 and taking the place of grindstones for sharpening tools. 



NO. 1537, VOL. 59] 



The corundum grains throughout the wheel retain their 

 cutting power, so that it can be worked until quite 90 per 

 cent, of its weight has been worn off, while a file is use- 

 less before it has lost 5 per cent, of its weight. It has 

 been estimated that to remove one pound weight of iron 

 with a file costs is. 6</., while the same amount of work 

 can be done with an emery or corundum wheel in about 

 one-eighth of the time and at one-seventh of the cost. 

 Compared with grindstones in grinding tools, experiments 

 by some English firms show that the cost of the emery 

 wheel is about one-fifth, and the time only one-half of 

 that required by the use of the old grindstone, and at the 

 same time the danger of bursting during rapid revolution, 

 which is such a cominon accident with the latter tool, is 

 practically abolished. 



The corundum wheel is said to be twice as effective as 

 the emery wheel, while its cost is only 15 percent, more. 



It will be seen irom these statements that there cannot 

 fail to be a great future for wheels made from corundum 

 and similar materials. 



As is pointed out by Mr. Holland, corundum is the 

 richest ore of the valuable metal aluminium. So long, 

 however,as abundant supplies of bauxite (impure hydrated 

 oxides of aluminium) can be obtained, it is scarcely likely 

 that the hard and intractable corundum will be used for 

 the extraction of the metal. The time may, however, 

 come when such a use will be made of the material, 

 which is now almost wholly sought for abrasive purposes. 



MULTIPLE VISION. 

 T T is well known that, owing to what is termed irregular 

 ■'• astigmatism, a small bright object, for which the 

 eye is not accommodated, often presents a multiform 

 appearance, the number of separate images perceived 

 varying in difterent cases from about six to fifteen. 



Irregular astigmatism, to which every one is in some 

 degree a victim, can be easily demonstrated in the 

 following manner. With the point of a fine needle a 

 very small hole is pricked in a sheet of tin-foil ; this is 

 held up to the light, and the hole is looked at with one 

 eye, the other being closed. Even at the distance of 

 most distinct vision — ten inches, or thereabouts — there 

 will probably be a ragged appearance about the per- 

 foration, as if it were not perfectly round. But if the 

 tin-foil be brought an inch or two nearer to the eye, the 

 perforation will not seem to be even approximately 

 circular ; it will generally assume the form of a little star 

 with six or more pointed rays. The form of the star is 

 not often the same for the right eye as for the left ; but 

 if several holes be pricked in the tin-foil, all the stars as 

 seen by the same eye will appear to be formed after the 

 same model, though some may be larger or brighter 

 than others. 



If the luminosity of the source of light is sufficiently 

 diminished by screening with a coloured glass, or other- 

 wise, the star will be seen to consist of several distinct 

 images of the hole superposed upon an irregular nebulous 

 patch. Seven such images can generally be perceived — 

 a central one surrounded by six others, but sometimes 

 there may be more. This and other allied phenomena 

 exhibited by a healthy eye are commonly attributed to 

 the fact that the crystalline lens is composed of several 

 sections connected by radial sutures, six or more in 

 number, which occur upon the two surfaces of the lens. 



Some observations described in a recent communication 

 to the Royal Society {Proc. Roy. Soc, January 1899) 

 indicate that under certain conditions the number of 

 independent images due to a single luminous point is 

 far greater than could be accounted for in this way ; 

 there may, in fact, be several hundreds, and their form- 

 ation probably arises from the cellular structure of the 

 transparent media of the eye. 



