240 Intelligence and Miscellaneous Articles. 



and, lastly, un weathered inelaphyre from the Oberstein Eailway Sta- 

 tion. Within none of these stones was any light perceptible ; only 

 the light on the ground surface remained constant. 



It deserves to be mentioned that J. H. PL Heinrich, as stated in 

 his copious work on phosphorescence*, has made similar observa- 

 tions with far inferior appliances. For his experiments (which 

 were all made in the dark) he employed an ordinary grindstone of 

 14| inches diameter, turned by hand, and making two revolutions 

 in a second. The extent of surface, therefore, which in one second 

 passed over the object applied to the grindstone was only 7 feet 

 7 inches, while in the agate-grindery it was from 47 feet 1 inch to 

 51 feet 10 inches. He thereby caused onyx, carnelian, chalcedony, 

 rock-crystal, &c. to shine with fiery red light. He does not state, 

 however, whether the shining was only superficial, or penetrated to 

 the interior of the stones ; and just as little does he say that under 

 grinding they appear transparent. On the contrary, he says that 

 the light is not so bright as when two stones of the same sort are 

 rubbed against one another. Hence the light obtained by him can 

 only have been very faint. He alleges, however, that a luminous 

 arc appeared round the circumference of the grindstone. This I 

 have not seen ; and it is perhaps only to be observed in the dark. 



If we inquire after the fundamental causes of the phenomenon 

 of the illumination of the grinding surface, these may well be no 

 other than the combination of friction and the development of elec- 

 tricity, of which the rise of temperature of the stone is a necessary 

 consequence. The red halo round the stone which is being ground, 

 and the scattering of sparks, can only arise from the illuminated 

 particles separated from the two stones by the friction. 



The second phenomenon, the magnificent fiery red illumination 

 of the stone, seems to be a mere consequence of the radiation of the 

 light through from the surface of contact of the stone to be ground 

 and the grindstone, although transparent and merely translucent 

 stones of 5 or 6 inches length are perfectly alike in diffusing the 

 fiery red light. Hence also no light can be produced in completely 

 opaque stones. 



I was not able to make closer investigations. Much more might 

 be said in respect of my experiments from the physical point of 

 view. In the first place, immediately after the grinding of the 

 stones, electroscopic experiments and trials of their increase of tem- 

 perature might be made ; also spectral analysis of the light would 

 be of importance ; lastly, my investigations should be supplemented 

 by such with many other kinds of stone. 



The intention of this publication of my experiments is substan- 

 tially this — to call attention to the singular opportunity offered by 

 the numerous agate-grinderies of Oberstein and Idar for the pur- 

 pose in question, and to invite physicists to further investigations 

 in these localities. I know not, being in my eightieth year, if I 

 am destined to be able to complete my experiments. — Poggendorff's 

 Annalen, 1873, No. 10, pp. 325-331. 



* Die Phosphorescenz der Korper unter alien Umstanden untersucht und 

 erlautert : Niirnberg, 1820, p. 510 segq. 



