the two glasses must be increased to force the ring of the order n to come and take the place of the central spot, and that in the different prismatic colours. According to the theory this quantity is equal to n times half the wave-length of the light employed. The apparatus used by me, in the laboratory of instruction of the Faculty, to verify this point of the theory, was constructed with much care by M. Laurent-Duboscq.

The rings are formed as usual, between a lens and a plane. The lens is fixed, or has only revocable movements; the plane moves with regularity under the action of a good micrometric screw, the channel of which was ordered equal to half a millimetre. Now, if some parts quite at the extremities of this screw be excepted, whatever the part which is found in the nut, we may assure ourselves at first that the angular course necessary for causing sixty consecutive rings to enter into or issue from the central spot is constant.

This first point verifies the exactness of the screw, and that, as is evident, by a process simple, rapid, and, it seems to me, capable of being applied in many cases.

Secondly, I have observed that this angular course corresponding to sixty rings is 12°-85.

We have, then, if the channel is quite a millim.,

The light employed was that of gas salted; and it is pretty generally admitted that the wave-length of the rays in the vicinity of the line D is 0.000589.

It is easy to perform analogous experiments by illuminating the glasses with sensibly homogeneous rays from the different regions of a good spectrum; and I have ascertained that the spectrum of a Drummond lamp is quite intense enough for this purpose.

The number of rings which pass with an entire turn of the screw is 1683 when the operation takes place with the light in the vicinity of the line D. Now, as the button with which the action is performed is only 4 centims. in diameter, it is clear that the hand must be moved very slowly in order that several rings at once may not be made to enter the central spot. When we wish to limit ourselves to observing the passage of fifteen rings only, it is preferable to conduct the principal motor screw with a tangent micrometric screw itself. In my apparatus, an entire turn of the tangent screw corresponds to only two and a half rings; consequently one can easily divide into more than 100 equal parts the movement of the ring which answers to a displacement of 0.000592 in the movable glass.-Comptes Rendus de l'Acad. des Sciences, January 26, 1874.

ON EXTRAORDINARY DEVELOPMENTS OF LIGHT IN THE GRINDING

OF HARD STONES. BY DR. JACOB NÖGGERATH.

It is known that a number of minerals and stones become phosphorescent by friction, especially on being rubbed against each other

in the dark. This is particularly the case with hard siliceous stones, such as rock-crystal, quartz, chalcedony, &c. The phenomenon takes place both in air and under water. But only the surface of the stones appears to phosphoresce. For many years, however, 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 5 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 entire operation properly. This position gives him the necessary purchase. 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 preliminary operations.

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

&c.) I omit to describe, as these manipulations do not touch my purpose*.

My experiments on the luminous phenomena in question were performed in bright daylight at noon, the temperature of the air being about 14° R. I had only one hour for them, and could only make use of the few sorts of stone that were at hand.

The phenomena I observed were essentially of two kinds. As soon as a stone of about the hardness of quartz was pressed against the revolving grindstone, there was produced between them an intense red light, which at the same time radiated round the object and emitted numerous sparks. This appearance was the same with all hard stones.

The second phenomenon occurred simultaneously with the other, but only with translucent and transparent stones, not with completely opaque ones. The stones of the former quality shine with brilliant red light slightly yellow-tinged. They appear almost like red-hot iron; and it really looks as if the hands of the grinder must be severely burned. All the stones tried became warm in grinding, but not very hot; feeling them with my hand, I estimated the increase of temperature at from 10° to 12° R.

The stones on which I experimented in this way were the following:

Chalcedony of a yellowish grey colour, a little translucent, from Uruguay, South America; a piece 2 inches thick became magnificently fiery red, and therewith transparent.

Chalcedony, white and opalescent, said to be oriental; behaviour as in the preceding experiment.

Chalcedony of a reddish colour, translucent, from Idar; gave the same result.

Chrysoprase from Silesia; a thick piece, pale green, very slightly translucent at the edges, hardly useful as a jewel, gave a little red light.

Rock-crystal, colourless, perfectly transparent, from Brazil. The luminous appearance was very magnificent; but the fiery red, in comparison with the above experiments, was more subdued, almost rose-red.

Rock-crystal, smoke-grey, transparent (the so-called smoky topaz), from the Swiss Alps. Pretty nearly the same result, only somewhat less beautiful.

Carnelian of a beautiful red colour, intensely translucent, from India; gave the most magnificent red light, as the colour of it was saturated with the natural colour of the carnelian.

Amethyst, crystals, intensely translucent, from Idar. The light was pale violet, because here also the violet-blue colour of the stone blended with the fiery red of the light, beautifully transparent.

Of perfectly opaque stones were tried red agate jasper from Idar, black lydite with thin white stripes of quartz, artificially blackcoloured chalcedony from Uruguay, opaque heliotrope from India,

*A very precise account, illustrated with figures, will be found in Collini, Journal d'un Voyage qui contient différentes observations minéralogiques, &c. Mannheim: 1776.

and, lastly, unweathered melaphyre from the Oberstein Railway Station. 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 observations 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 electricity, 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 temperature 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 substantially this to call attention to the singular opportunity offered by the numerous agate-grinderies of Oberstein and Idar for the purpose 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 Körper unter allen Umständen untersucht und erläutert: Nürnberg, 1820, p. 510 segq.