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Page:Popular Science Monthly Volume 20.djvu/675

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The curve representing the distribution of heat resembled that obtained from the spectrum of the sun, the invisible calorific radiation being reduced by the water from nearly eight times to about twice the visible. Could we get above the screen of atmospheric vapor, a large amount of the ultra-red rays would assuredly be restored to the solar spectrum. This conclusion has been recently established on the grandest scale by Professor Langley, who, on the 10th of September, wrote to the lecturer from an elevation of twelve thousand feet on Mount Whitney, "where the air is perhaps drier than at any other equal altitude ever used for scientific investigation." An extract from Professor Langley's letter will fitly close this summary: "You may," he says, "be interested in knowing that the result indicates a great difference in the distribution of the solar energy here from that to which we are accustomed in regions of ordinary humidity, and that, while the evidence of the effect of water-vapor on the more refrangible rays is feeble, there is, on the other hand, a systematic effect, due to its absence, which shows, by contrast, its power on the red and ultra-red in a striking light. These experiments also indicate an enormous extension of the ultra-red spectrum beyond the point to which they have been followed below, and, being made on a scale different from that of the laboratory—on one, indeed, as grand as Nature can furnish—and by means wholly independent of those usually applied to the research, must, I think, when published, put an end to any doubt as to the accuracy of the statements so long since made by you, as to the absorbent power of this agent over the greater part of the spectrum, and as to its predominant importance in modifying to us the solar energy."



THE effects of lightning on the tops of mountains are often very intense. Among them have been cited such works as the transportation of large masses to a considerable distance. They also frequently include the development of high degrees of heat. The clearest possible proof of this fact is afforded by the superficial fusion of rocks, even of such refractory ones as granite and other crystalline stones. Laussure long ago described vitrifications of this kind on the summit of Mont Blanc; Ramond has found them on the Pic-du-Midi, and Humboldt in Mexico; and the characters they exhibit are everywhere uniform.

The vitrifications are generally only a few tenths of a millimetre thick, but they sometimes extend over surfaces of nearly a square

  1. Translated from "La Nature."