X. ON CHEMICAL RAYS AND THE STRUCT URE AND LIGHT OF THE SKY. A DISCOURSE. DELIVERED IN THE ROYAL INSTITUTION OF GREAT BRITAIN On Friday, January 15, 1869. "This is a very mysterious and a very beautiful phenomenon when observed by the aid of a polariscope, consisting of a tourmaline plate, with a slice of Iceland crystal or nitre, cut at right angles to the optic axis, and applied on the side of the tourmaline farthest from the eye. In a cloudless day, if the sky be explored in all parts by looking through this compound plate, the polarized rings will be seen developed with more or less intensity in every region but that nearest the sun and that most distant from it-the maximum of polarization taking place on a zone of the sky 90° from the sun, or in a great circle, having the sun for one of its poles, so that the cause of polarization is evidently a reflection of the sun's light on something. The question is, on what? Were the angle of maximum polarization 76° we should look to water or ice for the reflecting body. But though we were once of this opinion (art. Light, Encycl. Metropol. § 1143), careful observation has satisfied us that 90°, or thereabouts, is the correct angle, and that therefore, whatever be the body on which the light has been reflected, if polarized by a single reflection, the 'polarizing angle' must be 45°, and the index of refraction, which is the tangent of that angle, unity; in other words, the reflection would require to be made in air upon air! The only imaginable way in which this could happen would be at the plane of contact of two portions of air differently heated, such as might be supposed to occur at almost every point of the atmosphere in a bright sunny day; but against this there seems to be an insuperable objection. The polarization is most regular and complete, as we have lately been able to satisfy ourselves under the most favorable possible atmospheric conditions, after sunset, in the bright twilight of a summer night, with the sun some degrees below the horizon, and long after all the tremor and turmoil of the air, due to irregular heating, must have completely subsided. On the other hand, if effected by several successive reflections, what is to secure a large majority of them being in one plane (in which case only their polarizing effect would accumulate); and of those which become ultimately effective, what is there to determine an ultimate deviation of 90° as that of the maximum? The more the subject is considered, the more it will be found beset with difficulties; and its explanation, when arrived at, will probably be found to carry with it that of the blue color of the sky itself." SIR JOHN HERSCHEL. X. ON CHEMICAL RAYS AND THE STRUCTURE AND LIGHT OF THE SKY. 1 THE first physical investigation of any importance in which, jointly with my friend Professor Knoblauch, I took part, bore the title: "The Magneto-optic Properties of Crystals, and the Relation of Magnetism and Diamagnetism to Molecular Arrangement." This investigation compelled me to reflect upon the structure of crystals, on their optical properties in relation to that structure, and more particu larly on the striking phenomena exhibited by many of them in the field of a sufficiently powerful magnet. These were evidently due to the manner in which the molecules of the crystals were built together by the force of crystallization; and it was natural, if not necessary for me, to employ such strength of imagination as I possessed in obtaining a mental picture of this molecular architecture. The inquiry gave a tinge and bias to my subsequent scientific thought, rendering, as it did, the conceptions and pursuits of molecular physics pleasant to me. Its influence is to be traced in most of my scientific work. The first lecture, for example, which I ever delivered in this theatre, was ❝ On the Influence of Material Aggregation on the Manifestations of Force;" by "material aggregation" being meant the way in which, by Nature or by Art, the molecules of mat 1 Philosophical Magazine, July, 1850. ter are arranged together. In 1853 I also published a paper "On Molecular Influences," in which common heat was made the explorer of organic structure. In the “Bakerian Lecture," given before the Royal Society in 1855, the same idea and phraseology crop out. The Bakerian Lecture for 1864 bears the title "Contributions to Molecular Physics." And all through the investigations which have occupied me during the last ten years, my wish and aim have been to make radiant heat an instrument by which to lay hold of the ultimate particles of matter. The labors now to be considered lie in the same direction. In the researches just referred to, tubes of glass and brass were employed, called, for the sake of distinction, 66 experimental tubes," in which radiant heat was acted upon by the gases and vapors subjected to examination. Two or three months ago, with a view of seeing what occurred within these tubes on the entrance of the gases or vapors, it was found necessary to intensely illuminate their interiors. The source of illumination chosen was the electric light, the beam of which, converged by a suitable lens, was sent along the axis of the tube. The dirt and filth in which we habitually live were strikingly revealed by this method of illumination. For, wash the tube as we might with water, alcohol, acid, or alkali, until its appearance in ordinary daylight was that of absolute purity, the delusive character of this appearance was in most cases revealed by the electric beam. In fact, in air so charged with suspended matter as that which supplies our lungs in London, it is not possible to be more than approximately clean. Vapors of various kinds were sent into a glass experimental tube, a yard in length, and about three inches in diameter. As a general rule, the vapors were perfectly transparent; the tube, when they were present, appearing as empty as when they were absent. In two or three cases, however, a faint cloudiness showed itself within the tube. |