Wind a strip of blotting-paper or wrapping paper 2 inches wide around the bottle at one side of the line along which you wish to cut. Make three or more thicknesses and then tie the paper with cord within ½ inch of the edge to be cut. Wrap another similar piece on the opposite side of the place to be cut and ³⁄₁₆ inch from the first piece (Fig. 88).
FIG. 89
HEATING THE BOTTLE
Now stand the bottle in a pail of water until the paper is thoroughly wet (about five minutes), take it out, rotate it in a horizontal position and direct the blowpipe flame against the glass between the papers (Fig. 89).
Continue this for four or five minutes, then if the bottle has not dropped apart, plunge it vertically into the pail of water.
The bottle will break into two parts along the line between the two papers (Fig. 90). If it does not do so, repeat the operation until it does. Smooth the rough edges outside and inside with the file. You cannot do this with the flame because the glass is too brittle.
FIG. 90
THE BOTTLE CUT IN TWO
Experiment 68. To grind glass.
Rough edges of glass can be ground smooth by means of emery paper. For example, to smooth the edges of the glass bottle you have just cut in two, use the file for the rough work, then lay a piece of emery paper on a plate of glass, emery side up, pour a little kerosene on it and rub the rough surface on the emery with a rotary motion (Fig. 91). Finish with fine emery paper, and smooth the edges inside and out with the fine paper.