238 Intelligence and Miscellaneous Articles. 



in the dark. This is particularly the case, with hard siliceous 

 stones, such as rock-crystal, quartz, chalcedony, &c. The pheno- 

 menon takes place both in air and under water. But only the sur- 

 face of the stones appears to phosphoresce. For many years, how- 

 ever, I have known that under very powerful friction, such as can 

 only be produced by corresponding machinery, such stones become 

 throughout splendidly luminous with red light and at the same time 

 transparent. This takes place at the agate-polishing establishments 

 at and near Oberstein and Idar in the Oldenburg principality of 

 Birkenfeld-on-the-Nahe. It has not been expressly mentioned by 

 any writer on the agate industry of that region. Collini, who a 

 century ago described with great accuracy all the arrangements and 

 manipulations in operation there, just as they exist unaltered at the 

 present time, makes only the general remark that polishing agates 

 on the rapidly revolving grinding-stones produces " fire and sparks." 



On a recent visit, for a mineralogical purpose, to Oberstein and 

 Idar, I made some observations on these phenomena, which I will 

 here communicate. 



The establishment is situated on a water-course, which drives an 

 undershot water-wheel, the axle of which reaches into the room, 

 and here, by means of two cog-wheels, moves a horizontal shaft, on 

 which are four or five grindstones rotating vertically. Over each 

 is brought a gutter so that a constant small stream of water pours 

 over the rotating stone and between it and the stone to be ground. 



The grindstones consist of very solid fine-grained sandstone from 

 the Bunter formation, and are obtained in the neighbouring Rhenish 

 Palatinate. They must be faultless throughout, without flaws, 

 clay-cells, and such like, as otherwise they are very likely to crack 

 with the rapid rotation. There have been many instances, even 

 recently, of stones (which had unobserved defects) cracking when 

 in rotation, flying asunder in fragments, killing the workmen, and 

 making great havoc in the work-room, even shattering the roof and 

 the walls. The stones are from 5 to 5j feet in diameter, and 14 

 inches thick at the grinding-surface. The average velocity is three 

 revolutions in a second, therefore 180 in a minute, or 10,800 in an 

 hour. Hence the grinding-surface that passes over the object 

 pressed against it amounts to from 169,646 to 186,613 feet or from 

 7 to 8 (German) geographical miles (28 to 32 English) per hour. 



When grinding, the workman lies with his belly and partially 

 his chest on a semicylindrically hollowed-out stool raised somewhat 

 obliquely in exactly the direction of the grindstone, his feet stretched 

 out behind and resting against a post fixed in the floor. In this 

 position he presses the stone to be polished, with both hands, firmly 

 against the grindstone. "With his head a little raised he can see to 

 the placing of the object on the grindstone and to perform the en- 

 tire operation properly. This position gives him the necessary pur- 

 chase. As the muscular exertion of the workman is very great, 

 the polishing is performed with intermissions, so that hours of work 

 alternate with equal times of rest mostly employed in the preli- 

 minary operations. 



The further working of the agates (boring, colouring, polishing, 



