The next tube, seen in outline in Fig. 18, shows the dark space.  Like the first it is a closed cylinder of glass, with a central indentation forming a kind of hanging pocket and almost dividing the tube into two compartments.  This pocket, silvered on the air side, forms a hollow glass diaphragm that can be connected electrically from the outside, forming the negative pole, A; the two ends of the tube, also outwardly silvered, form the positive poles, B B. I pass the current, and you will see the dark space distinctly visible.  The pressure here is 0.076 millimeter, or 100 M. The next stage, dealing with more rarefied matter, is that of phosphorescence.  Here is an egg-shaped bulb, shown in Fig 19, containing some pure yttria and a few rough rubies.  The positive electrode, B, is on the bottom of the tube under the phosphorescent material; the negative, A, is on the upper part of the tube.  See how well the rubies and yttria phosphorescence shows under molecular bombardment, at an internal pressure of 0.00068 millimeter, or 0.9 M.

[Illustration:  FIG. 18.—­PRESSURE = 0.076 MM. = 100 M.]

A shadow of an object inside a bulb can also be projected on to the opposite wall of the bulb by means of an outside pole.  A mica cross is supported in the middle of the bulb (Fig. 20), and on connecting a small silvered patch, A, on one side of the bulb with the negative pole of the induction coil, and putting the positive pole to another patch of silver, B, at the top, the opposite side of the bulb glows with a phosphorescent light, on which the black shadow of the cross seems sharply cut out.  Here the internal pressure is 0.00068 millimeter, or 0.9 M.

[Illustration:  FIG. 19.—­PRESSURE = 0.00068 MM. = 0.9 M.]

[Illustration:  FIG. 20.—­PRESSURE = 0.00068 MM. = 0.9 M.]

[Illustration:  FIG. 21.—­PRESSURE = 0.001 MM. = 1.3 M.]

Passing to the next phenomenon, I proceed to show the production of mechanical energy in a tube without internal poles.  It is shown in Fig. 21 (P = 0.001 millimeter, or 1.3 M).  It contains a light wheel of aluminum, carrying vanes of transparent mica, the poles, A B, being in such a position outside that the molecular focus falls upon the vanes on one side only.  The bulb is placed in the lantern and the image is projected on the screen; if I now pass the current, you see the wheels rotate rapidly, reversing in direction as I reverse the current.

Here is an apparatus (Fig. 22) which shows that the residual gaseous molecules when brought to a focus produce heat.  It consists of a glass tube with a bulb blown at one end and a small bundle of carbon wool, C, fixed in the center, and exhausted to a pressure of 0.000076 millimeter, or 0.1 M. The negative electrode, A, is formed by coating part of the outside of the bulb with silver, and it is in such a position that the focus of rays falls upon the carbon wool.  The positive electrode, B, is an outer coating at the other end of the tube.  I pass the current, and those who are close may see the bright sparks of carbon raised to incandescence by the impact of the molecular stream.