The air is expelled from the tube by filling with mercury.  This may be done by making a paper funnel and pouring the mercury slowly into the tube through the funnel.  When the tube is filled to within 1/2 in. of the funnel remove the funnel and tap the side of the tube gently in order to remove any small air bubbles that may be clinging to the sides of the tube.  The air bubbles will rise and come to the top.  The tube now must be filled completely, expelling all the air.  Place a finger over the end of the tube to keep the mercury in and invert the tube and set the end in the bowl of mercury.  The mercury in the tube will sink until the level will be at about 30 in., leaving 8 in. of vacuum at the top.  The next operation is to seal the tube at the half-way point between the lower platinum wire and the mercury level.

As the lower end of the tube must be kept at all times in the bowl of mercury until the tube is sealed, an assistant will be necessary for this last operation.  Have the assistant hold the tube in the mercury at a slight angle, using care to always keep the lower end in the mercury, while you hold the burner in the left hand and allow the flame to strike the tube at the stated point.  The part of the tube above this point will gradually bend over of its own weight as the glass softens.  When it reaches the angle of about 60 deg., Fig. 6, take hold of the tube with the right hand still keeping the flame on the tube, and gradually draw the softened portion out until it separates from the main tube.

The tube is now finished and when the platinum wires are attached to the terminals of a spark coil a beautiful blue light will appear in the tube with a dark space at the negative end or cathode.  —­Contributed by David A. Keys, Toronto, Can.

** Loosening Rusted Nuts [155]

Nuts that are rusted fast can often be loosened by giving a hard turn in the tightening direction.

** Cleaning Greasy Stoves [155]

Greasy stoves may be cleaned with a strong solution of lye or soda.

** How to Make a Take-Down Background Frame [156]

Many amateur photographers who desire to do portrait work at home have left the subject alone for the want of a suitable background.  A frame such as is used by the professional is entirely out of the question in most homes, says a correspondent of Camera Craft.  The frame as shown in the sketch was devised and its chief advantage lies in the fact that when not in use it can be compactly tied together and stored away in a closet.

Almost any wood may be used in constructing this frame, but yellow pine is the best, as it is easily obtained and at the same time very well suited for such work.  All pieces are to be dressed on all sides.