Attach the rod to the discs by the screws, and slip the discs on to the ends of the shutter spindles. (The free end of the rod should be upwards.) Press the shutters against the front so that they cannot open, adjust the discs at an angle of 45 degrees to the front in an upward direction, and solder them firmly to the spindles.

The upper end of the connecting rod should be turned over to form a finger rest, or be sharpened off to take a knob.  The last operation is the fitting of the spring to close the shutter.  A spiral spring attached at one end of the crank on the lower spindle and at the other to a nail projecting from the side of the front is the most convenient arrangement.  If you have not got a spiral spring, you can easily make a. fairly efficient substitute out of hard brass wire wound a few times round a large wire nail.

An alternative method of springing is to add an arm, a, to the connecting rod, as shown by dotted lines in Fig. 189, and to use the projection for engaging a spring, made by winding hard brass wire a few times round a nail.  A screw passed through the coil holds it to the front.

The tension of the spring must be just sufficient to close the shutter smartly and prevent it rebounding far enough to pass any light.

XXXIX.  A MINIATURE GASWORKS.

The most primitive method of making coal gas on a small scale is to fill a tin—­which must have folded, not soldered, joints—­with small coal, punch a hole in the bottom, and place it lid downwards in the fire.  Gas soon begins to issue, but, owing to the quantity of moisture and impurities present, it will not ignite until some minutes have elapsed.  The flame, when it does make its appearance, is very smoky and gives little light, because, in addition to the coal gas of commerce, there are present ammonia gas, sulphuretted hydrogen, carbonic acid, tar vapour, etc., which prevent brightness of flame.

[Illustration:  Fig. 190.—­General view of gas-making apparatus.]

A miniature gasworks, if it is to be worthy of its name, must obviously endeavour to separate the troublesome components from the useful gas.  The doing of this involves several processes, all simple enough in principle, and requiring but simple apparatus for demonstration on a small scale.  To take them in order the processes are—­

(l) The formation of gas in a retort;

(2) The condensation of the tar;

(3) The condensation of steam;

(4) The removal of the ammonia gas;

(5) The removal of the sulphuretted hydrogen and carbonic acid.

The last two processes are, in a real gasworks, usually separated, but for simplicity’s sake we will combine them.  Finally, the storage of the gas has to be provided for.