162 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



provement, as well as notices of experiments about to be 

 made in Vienna, Paris, and London. 23 A, August 22, 18*73, 

 779. 



LIGHT OBSERVED IX GRINDING HARD STONES. 



At the agate -polishing establishments in Oldenburg, a 

 phenomenon has been observed for the past century that has 

 as yet, perhaps, attracted too little attention from scientific 

 men. Under very powerful friction, such as can only be 

 produced by the machinery at those works, hard stones be- 

 come splendidly luminous and transparent throughout. In 

 this establishment the axis of an undershot water-wheel 

 reaches into the Qrindinsj-room, where four or live armd- 

 stones rotate vertically; over each is brought a gutter, so 

 that a constant small stream of water pours upon the stone. 

 The grindstones themselves are entirely faultless. They are 

 about five feet in diameter, and make three revolutions in a 

 second, so that the grinding surface that passes the object 

 pressed against it amounts to thirty-two English miles per 

 hour. When grinding, the workman lies on his belly, his 

 chest on a semicvlindrical hollowed stool, his feet stretched 

 out behind, braced against a post fixed in the floor. With 

 both hands he presses the stone to be polished firmly against 

 the grindstone, bringing his whole weight to bear upon it. 

 The muscular strength brought to bear is very great, and 

 the work is performed with intermissions of equal times of 

 rest and work. Some experiments were made in this work- 

 shop by Noggerath in broad daylight. He observed that as 

 long as a stone of the hardness of quartz was pressed against 

 the revolving grindstone there was produced an intense red 

 light, which at the same time radiated around the object and 

 emitted numerous sparks. This experience was the same 

 with all hard stones, which appear almost like red-hot iron, 

 and it really looks as if the hands must be severely burned. 

 All the stones became warm in grinding, but not very hot. 

 Chalcedony gave a magnificent fiery red light; chrysoprase 

 a feeble red light ; rock crystal a beautiful rose red ; coralline 

 a superb red light, the color being evidently increased by the 

 natural hue of the stone. Amethyst gave a pale violet, while 

 numerous opaque stones gave no light at all. 7 A, XL VII., 

 237. 



