------- 34. RIESS’S EXPERIMENT. ELECTRIC IMAGES. _Riess’s Reibungs._ vol. 2, § 739.—He laid a coin upon a plate of glass and charged the same electrically about one-half of an hour or more. Upon removing the coin and sprinkling the plate with dust, an engraving of the coin was visible upon the glass. § 13. A suitable dust is licopodium powder. 35. SANFORD AND MCKAY’S EXPERIMENT. ELECTROGRAPHS. ORIGINAL CONTRIBUTION BY PROF. MCKAY OF PACKER INST., Brooklyn, May, ’96.—The picture of the coins in Fig. IX, was produced by the apparatus shown in Fig. VIII, _t_, _t_, tinfoil, _p_, photographic plate with coins on sensitive side, all wrapped in black paper. Fig. VIII represents the general arrangement for taking electrographs. This particular one was made by removing the upper tinfoil and touching each coin successively with wire from one of the poles, while the other wire was connected with tinfoil on the opposite side. The condenser thus formed is charged and discharged many times by a Holtz machine or induction coil. This is not a new discovery, it was first described by Prof. Sanford, I think, of Leland Stanford University, two or three years ago. Other claimants of earlier date probably exist. 36. LICHTENBERG’S EXPERIMENT. DUST FIGURES. PICTURES DRAWN WITH ANODE AND CATHODE. _Göttingen_, 1778-79. MOTUM FLUIDI ELECTRICITI.—He drew two independent superposed pictures upon a flat surface of an insulating material, for example, rosin. One picture was drawn with one terminal of a charged Leyden jar. Another picture was drawn with the other terminal of a charged Leyden jar. He sprinkled upon the surface over the two pictures, a dust made of a mixture of red lead and sulphur powder. The former became attracted to the picture drawn with the cathode, and the latter to that made with the anode, so that the two figures were clearly visible. Before sprinkling the powders upon the surface it is necessary to stir them together whereby they become oppositely electrified. [Illustration: VIII ARRANGEMENTS FOR TAKING ELECTROGRAPHS. § 35, p. 19. ] [Illustration: FROM ELECTROGRAPHS OF COINS. § 35, p. 19. Taken by Prof. McKay. ] [Illustration: X ] The sulphur arranges itself in tufts with diverging branches and the red lead in small circular patches. The particular materials, namely, the sulphur and red lead were first used by Villarsy. In case only one powder is employed, for example, licopodium, it adheres to both the positively and negatively electrified portion of the insulating plate, but in larger quantities upon the latter portions. Fig. X, shows rosin disc covered with licopodium powder after touching the disc with the knob of a Leyden jar.