Into a pint gem-jar put water 10 parts, sulphuric acid 1 part, potass, bichromate 1 part. Have jar three quarters full. Cut a piece of board 4 in. square, bore two holes in it, and through the holes thrust two pieces of electric light carbon, 5 in. or 6 in. long. The outer edges of the carbons should not be more than two inches apart. With a saw, cut a slit in the board between the holes and insert a strip of zinc 2 in. by 7 in. previously rubbed over with mercury. Set the three elements in the jar, connect the two carbons to one wire, and the zinc to another. One cell of this kind will run a small motor, operate a telegraph sounder, make a simple electro-magnet, or ring an electric bell; two cells will decompose water: three will heat a piece of fine iron wire red-hot. DECOMPOSITION APPARATUS 1. Cut the neck end from a pickle bottle. Get a No. 1 stopper, (rubber) with two holes in it and insert a piece of platinum foil 2 in. by 1/8 in. into each hole so that 1/2 in. projects above and below. Insert a tight plug beside each strip, thus holding it fast and making the stopper watertight. Insert the stopper into the neck of the jar. Pour into the vessel thus formed enough water to cover the platinums, and add a few drops of sulphuric acid. Touch the wires from the battery to the lower ends of the strips. Note bubbles of gas arise from the platinums. These may be collected in test-tubes and found by test to be oxygen and hydrogen. 2. Fasten a strip of platinum 1 in. by 1/8 in. to each wire from the battery and dip these into some acidulated water contained in a tumbler. The decomposition of the water into two gases can be seen, but the gases cannot be collected so readily as in 1 above. Bits of electric light carbon will do instead of platinum if the current is not too weak. PNEUMATIC TROUGH When oxygen or other gas is to be collected over water, use a milk pan or similarly shaped vessel. SPIRIT-LAMP Use an ink-bottle to contain the alcohol and several strands of string for the wick; make a hole in a piece of tin and draw the wick through; then let the tin rest on the neck of the bottle to support the wick. BAROMETER A siphon barometer takes less mercury than a cistern barometer. To the open end of the barometer tube attach a piece of strong rubber tubing 4 in. long and to this a piece of glass tubing 3 in. long. Fill the tube thus formed with mercury to within 3 in. from the top. Holding the short glass tube open end up, turn the long tube closed end up. (A tube of 1/8 in. bore needs only one quarter of the mercury required to fill a tube 1/4 in. bore.) HYGROMETER For a hygrometer, suspend two dairy thermometers side by side against the wall, cover the bulb of one with thin muslin, and let the muslin hang down and dip into water in some small vessel placed about three inches below the bulb on a little shelf. HINTS To avoid explosions, a spirit-lamp should be kept filled. Toy rubber balloons answer well for sheet rubber.