Page:Popular Science Monthly Volume 10.djvu/126

116 powdered by means of an iron, boat-shaped mortar, with a circular knife. It is then washed to remove the foreign matter, and to obtain the cinnabar in a finely-powdered state. This is, after being dried, mixed with an equal weight of half-burned charcoal (half coal and half ashes), and the whole is put into an iron pot, which is carefully covered with a round iron cover. This cover has in the middle a round opening, into which a curved tube of iron is fixed and cemented with a mixture of loam, salt, and a little water, the other extremity of the tube ending in a pot filled with cold water. The whole tube is wrapped in some fibrous substance, and kept cool by aid of cold water. The whole is generally heated on a small open charcoal furnace, the quicksilver distilling into the pot of water. This process is founded on the fact that the sulphur of the cinnabar is retained by the ashes, and perhaps, also, by the iron of the inner surface of the pot, the mercury evaporating by the heat. This quicksilver is, however, not pure, but always contains a small quantity of foreign metals (lead, copper, etc.).

Action of Light on Selenium.—The action of light in modifying the electrical conducting power of selenium was first observed by May, a telegraph-operator at Valencia, Ireland, who communicated the facts to Willoughby Jones in 1873. The latter having fully confirmed the observations of May, the attention of physicists, both in England and Germany, was drawn to the subject. Within the last twelve months it has been made matter of special inquiry by Prof. Adams and by Dr. Werner Siemens, each carrying on his investigations independently of the other. The results obtained by Siemens are set forth in a lecture delivered at the London Royal Institution by his kinsman, C. W. Siemens. He exhibited the action of light by a contrivance of Dr. Werner Siemens, in which the selenium was in a form in which the surface-action of light can produce its maximum effect. Two spirals of thin wire (iron or platinum) are laid on a plate of mica in such a way that the wires lie parallel without touching. While in this position a drop of fluid selenium is made to fall upon the plate, filling the interstices between the wires; and, before the selenium has bad time to harden, another thin plate of mica is pressed down upon it so as to give firmness to the whole. The two protruding ends of the spirals serve to insert this selenium element in a galvanic circuit. Mr. Siemens calls this disk his "sensitive element." The whole arrangement is no larger than a sixpence. Its action was shown in this way: It was placed in a galvanic circuit, at one end being a Daniell cell, and at the other a delicate index galvanometer. The "disk" was first inclosed in a dark box; the circuit was "made," but no electricity passed through—no movement of the index was seen. The "disk" was then exposed to light; still no action was apparent. Another disk was taken that had been kept in boiling water for an hour, and gradually cooled. In the dark box it gave a slight passage to electricity as indicated by the index, but as soon as the light was admitted the index registered a great passage of electricity. Another disk heated to 210° C, and allowed to cool, was then used, and a greater action still was apparent with this. Dr. Werner Siemens has worked at the meaning of this, but without tables and diagrams it is not possible to convey an adequate idea of his results. The basis of the change in condition seems to lie in the fact of the extent to which the selenium is heated, for, when again allowed to cool, its behavior depends on the extent to which it has been heated. The experiment was shown of the effect of different parts of the spectrum on a disk. The actinic ray produces no effect, but the influence increases as we approach the red end. A selenium photometer was also shown in action, the principle of which is to compare the relative effects of two lights in affecting the conditions for the passage of electricity. At the end of the lecture a most interesting little apparatus was put at work, which Mr. Siemens calls a selenium "eye." There is a small hollow ball, with two apertures opposite to each other. In one is placed a small lens, one and a half inch diameter, and at the other a "disk." The disk is connected with a Daniell cell and a galvanometer, and this represents the retina. There are two slides