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THE POPULAR SCIENCE MONTHLY. 



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 char- 

 coal (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 open- 

 ing, 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 quick- 

 silver distilling into the pot of water. This 

 process is founded on the fact that the sul- 

 phur of the cinnabar is retained by the 

 ashes, and perhaps, also, by the iron of the 

 inner surface of the pot, the mercury evap- 

 orating by the heat. This quicksilver is, 

 however, not pure, but always contains a 

 small quantity of foreign metals (lead, cop- 

 per, etc.). 



Action of Light on Selenium. The ac- 

 tion of light in modifying the electrical con- 

 ducting power of selenium was first ob- 

 served by May, a telegraph -operator at Va- 

 lencia, Ireland, who communicated the facts 

 to Willoughby Jones in 1873. The latter 

 haviug 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 inde- 

 pendently of the other. The results ob- 

 tained by Siemens are set forth in a lecture 

 delivered at the London Royal Institution 

 by his kinsman, C. W. Siemens. He ex- 

 hibited the action of light by a contrivance 

 of Dr. Werner Siemens, in which the sele- 

 nium 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 with- 

 out 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 firm- 

 ness to the whole. The two protruding 

 ends of the spirals serve to insert this sele- 

 nium element in a galvanic circuit. Mr. 

 Siemens calls this disk his " sensitive ele- 

 ment." The whole arrangement is no 

 larger than a sixpence. Its action was 

 shown in this way : It was placed in a gal- 

 vanic circuit, at one end being a Daniell 

 cell, and at the other a delicate index gal- 

 vanometer. 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 elec- 

 tricity 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 condi- 

 tion 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 



