sensibility of selenium to the various rays of the solar spectrum. As it was the opinion of one or two friends of much experience in kindred questions in physical science that heat, not light, is the active agent, and that selenium might with advantage replace the thermopile in the measurement of radiant heat, I procured the necessary apparatus for a preliminary trial. Although my experiments are hardly sufficient to justify a decided opinion in favour of or against its applicability to photometric purposes, I think that they remove all hope of selenium replacing the thermopile in the measurement of heat of low refrangibility. I exposed a rod of selenium during a sufficient interval to obtain the maximum effect from the light of a candle 3 inches distant. The diminution of resistance produced amounted to 24.3 per cent. A second trial gave a diminution of 24.2 per cent., the candle being at 4 inches distance. A vessel of hot water of about 9 or 10 inches diameter and 9 inches height produced no perceptible change when it stood for several minutes with its centre at 13 inches distance, nor even when it was brought 6 inches nearer. On the other hand, by means of the thermopile a deviation to the extent of 136 divisions of the scale of a Thomson's galvanometer was produced by the radiation from the vessel of hot water whose centre was at 13 inches distance; and when it was replaced by the candle at 4 inches distance, the deviation was increased to only 315 divisions. Thus it was shown that the two instruments were not comparable as measurers of the radiation from the two sources of heat. The change of resistance produced by exposure to the radiation from a candle when a sheet of glass was alternately interposed and removed was next measured. The effect due to absorption by the glass appeared to be small, certainly not much more, possibly even less than the average absorption of light by glass. Perhaps 90 per cent. was transmitted. Owing to the inconstancy of the light, much accuracy was not readily obtainable in this experiment. The piece of glass had been shown by the thermopile to transmit 80 per cent. of the solar rays, and under 1 per cent. of the radiant heat from a blackened tin vessel of hot water. It was now by the same means ascertained that 48 per cent. of the radiant heat from a candle was transmitted by it. A glass cell filled with a solution of alum was now taken; and it was found that, while the exposure of the selenium bar to the radiation from a naked candle at 34 inches distance produced a diminution of resistance amounting to 94 per cent., when the alum solution was interposed the decrease was still 8.95 per cent.; hence the quantity transmitted by the glass cell and the alum solution it contained was as much as 95.2 per cent., as measured by the rod of selenium. The same, measured by the thermopile, was found to be 7.06 per cent. Having satisfied myself as to the comparative, if not the absolute insensibility of a bar of selenium to radiant heat of low refrangibility, and therefore as to its being unsuitable for replacing the thermopile in the measurement of those rays, I made a few experiments with the view of finding whether it was likely to be a suitable instrument in photometry. To obtain a more constant light I substituted a paraffin lamp for the candle; and by placing in front of the horizontal selenium bar a vertical slit whose breadth could be varied at pleasure, and which could be closed before and during every alternate measurement of the resistance, also by varying the distance of the lamp, it was found that while the decrease of resistance varied as the breadth of the slit, and therefore as the length of the portion of the bar exposed to light, it was far more nearly proportional to the reciprocal of the distance, and therefore to the square root of the intensity of the incident light than to the intensity simply, between the limits within which the observations were made. The decrease of resistance observed was: The length of the bar was 24 inches. No experiments were made to ascertain how far the sensibility was affected by the temperature of the room; but the presence of moisture rendered the action feeble and uncertain as the dewpoint was approached, probably owing to the deposit of a very slight film of moisture on the surface of the bar. Another effect, probably to be ascribed to the same cause, was noticed on two occasions-a diminution of the change of resistance after a certain duration of exposure to a screening from the light. In a drier atmosphere the maximum, which was still as rapidly attained, was retained with tolerable constancy. Probably the diminution of the effect after the maximum had been reached may have been due to the slower dissipation of the film of moisture, and to its re-formation under the alternations of temperature which accompanied the alternations of light and darkness. I regret that I shall not have leisure to pursue this subject for some time, and am therefore obliged to leave off these experiments in their present incomplete state. January 1874. XXII. Note on the Composition of certain Mine Waters. By J. ARTHUR PHILLIPS, M.Inst.C.E., F.G.S., F.C.S., &c.* SINCE INCE the publication of a paper in which I attempted to show that the waters of the thermal spring at Huel Seton Mine are probably derived from the seat, two other waters from deep Cornish mines have been analyzed in my laboratory. The first of these was from the 212-fathom level at the Phoenix Mines near Liskeard, where it issues from the lode at a temperature of 65° F. This mine is in granite, and at one time produced large quantities of copper ores, but has for the last ten years been principally worked for tin, which is obtained from the same veins which were formerly wrought for copper. A large portion of the vein above the point from which the water for analysis was collected had been removed some years previously. The following results, in grammes per litre and grains per gallon, were obtained by analysis. * Communicated by the Author. † Philosophical Magazine, July 1873 Water from the Phoenix Mines. Solid matter 2130 gramme per litre, or 14.91 grains per gallon. Sp. gr. =10002.. The second water examined was collected at the 302-fathom level at Dolcoath near Camborne; this mine also formerly produced large quantities of copper ores. These have in depth gradually given place to cassiterite, and Dolcoath is at the present time the most productive tin mine in the United Kingdom. The water analyzed was collected from the roof of a short crosscut, in granite, 25 fathoms east of the engine shaft and 15 feet south of the main lode. The water issued in considerable quantities, at a temperature of 92° F.; and a large portion of the upper parts of the vein had been removed by stoping. The following results, in gramme per litre and grains per gallon, were obtained by analysis: Water from Dolcoath. Solid matter 6710 gramme per litre, or 46-97 grains per gallon. |