_Apparatus._ The glass tumbler, G T, (No. 65); porous cup, P C, (No. 73); the strip of zinc (No. 60), well amalgamated (§ 257), or the amalgamated zinc rod (No. 74); piece of sheet copper (No. 75), bent so that it will surround P C; copper wires, C W, with connectors; a saturated solution of copper sulphate, commonly called blue vitriol or blue stone (See § 283); dilute sulphuric acid (See § 258). With the above, set up the two-fluid cell (See § 281). The galvanoscope, G V, complete, is also needed, and if quantitative work is to be done, a pair of scales weighing to 0.1 gram is necessary. (See App. Book, Chapter I, for Home-Made Two-Fluid Cells.) [Illustration: Fig. 87.] =281. Setting Up the Two-Fluid Cell.= Fig. 87. Stand the amalgamated zinc rod, Zn, in P C, then place P C in the tumbler, G T; put in the copper plate as shown. Pour diluted acid (§ 258) into P C until it stands about 2-1/2 in. deep; then at once pour the copper solution (§ 283) into G T, on the outside of P C, until it stands at the same height as the acid _in_ P C. As soon as the liquids have soaked into P C the cell will be ready for use; but it is better to connect it with the galvanoscope at once and note the increase of current during the first few minutes while the liquids soak through P C. A crystal of copper sulphate should be put outside of P C to keep the solution full strength. This is a form of the well-known Daniell cell. Fig. 87 shows a form of home-made two-fluid cell as described in Apparatus Book. If you have the one furnished, use the rod instead of sheet zinc, and use connectors on the plates. =282. Care of Two-Fluid Cell.= This experimental cell should be taken apart when not in use. It should not be left in open circuit, even for half an hour. Even after the plates are removed, copper may be deposited upon P C in case there are any metallic impurities on it. Remove the plates and P C, and thoroughly wash them. The copper solution should be put into a bottle to prevent evaporation; the dilute acid may be thrown away to be replaced by fresh acid for the next experiment. =283. Copper Sulphate Solution= is made by adding the blue crystals to water until no more will dissolve--that is, the solution should be "saturated," extra crystals always being left in the stock bottle. An ounce of the crystals to half a tumbler of water will be about right, but a pint or so should be made at a time and be kept bottled to save time. =284. Directions.= (A) In case you have access to a pair of scales that weigh to within 0.1 gram, carefully weigh both copper and zinc before proceeding. They should be washed and dried with a cloth. See that there are no drops of mercury upon the zinc that may fall off during the experiment.