A solution for silver plating may be prepared as follows:  Dissolve 3/4 oz. of commercial silver nitrate in 8 oz. of water, and slowly add a strong solution of potassium cyanide until no more white precipitate is thrown down.  Then pour the liquid off and wash the precipitate carefully.  This is best done by filling the bottle with water, shaking, allowing precipitate to settle and then pouring off the water.  Repeat six times.  Having finished washing the precipitate, slowly add to it a solution of potassium cyanide until all the precipitate is dissolved.  Then add an excess of potassium cyanide—­about as much as was used in dissolving the precipitate—­and make the solution up to 1 qt. with water.  This solution, with an electric pressure of 2 to 4 volts, will give a good white coat of silver in twenty minutes to half-an-hour; use 2 volts for large articles, and 4 volts for very small ones.  If more solution is required, it is only necessary to double all given quantities.

Before silver plating, such metals as iron, lead, pewter, zinc, must be coated with copper in the alkaline copper bath described, and then treated as copper.  On brass, copper, German silver, nickel and such metals, silver can be plated direct.  The deposit of silver will be dull and must be polished.  The best method is to use a revolving scratch brush; if one does not possess a buffing machine, a hand scratch brush is good.  Take quick, light strokes.  Polish the articles finally with ordinary plate powder.

The sketch shows how to suspend the articles in the plating-bath.  If accumulators are used, which is advised, be sure to connect the positive (or red) terminal to the piece of silver hanging in the bath, and the negative (or black) terminal to the article to be plated.  Where Bunsen cells are used, the carbon terminal takes the place of the positive terminal of the accumulator. —­Model Engineer.

** An Ingenious Electric Lock for a Sliding Door [89]

The apparatus shown in Fig. 1 not only unlocks, but opens the door, also, by simply pressing the key in the keyhole.

In rigging it to a sliding door, the materials required are:  Three flat pulleys, an old electric bell or buzzer, about 25 ft. of clothesline rope and some No. 18 wire.  The wooden catch, A (Fig. 1), must be about 1 in. thick

[Illustration:  Electric Lock for Sliding Door]

and 8 in. long; B should be of the same wood, 10 in. long, with the pivot 2 in. from the lower end.  The wooden block C, which is held by catch B, Can be made of a 2-in. piece of broomstick.  Drill a hole through the center of this block for the rope to pass through, and fasten it to the rope with a little tire tape.