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134 SOIOPTICON MANUAL. about J inch apart. Now coat the copper wires and the wood with melted parafime, but take care that none gets on the platina; this will insulate the copper wires and prevent the wood from absorbing any moisture. This little frame placed in the tank, immersed in acidulated water (water with a few drops of sulphuric acid), and the terminal wires attached to say two cells of Groves' battery, will show the decomposition of water admirably. A similar piece of apparatus with the terminal wires at the centre of the board, united by a vertical coil of very fine platina wire, will be found useful in illustrations of circulation by heat. Such a frame immersed in clear water will be seen on the screen as a black coil, seem- ingly hanging down from a black bar on the top of the screen; if now, by means of a pipette, some colored fluid, say a solution of permanganate of potash in water, be carried to the bottom of the tank, it will on the screen seem to spread itself out as a red stripe under the black one and enveloping the little coil; a current of elec- tricity passed through the wire will heat the little platina coil and thus heat the water in contact with it, so that currents will be established in the fluid, carrying with them the colored fluid in a very beautiful curling cloud of color. I mention to you these few examples, of how readily the needful appliances for illustrations can be improvised ; now I have frequently heard persons say that they " feared the expense entailed in the use of a lantern •" that "the lantern is so useless without a great many accessories, which are so expensive in themselves." Feeling a lively interest in your very meritorious in- vention, I have volunteered these hints, which I beg you will, if you see fit, use to your benefit. Yery truly yours, COLEMAN SELLERS.