MOLECULAR PRESSURE, AND THE TRAJECTORY OE MOLECULES. 
137 
cells, but I could detect no alteration in the size of the dark space so long as the pressure 
remained constant. The only effect was to cause an increase or decrease in luminosity. 
Air was let in and exhaustion recommenced several times. At the same exhaustion 
the dimensions of the dark space were always the same. It is indeed easy to tell the 
degree of exhaustion by measuring the thickness of the dark space. On afterwards 
taking the pressure in the usual way the agreement is seen to be close enough for 
ordinary observations. 
When only one pole of the coil is connected with one pole of the tube the same sized 
dark space is seen round the pole as when the two poles are connected, but the light is 
faint, and the room requires to be darkened for accurate observations. 
491. The sliding pole was now removed; one-half was coated with lampblack as shown 
in fig. 8, and it was replaced. Exhaustion was recommenced, and at a convenient 
Fig. 8. 
pressure the coil was attached. The dark space round the pole was now seen to have 
lost its uniform thickness ; it was 1 millim. thick over the metallic half and 1 '5 millims. 
thick over the lampblack half. On continuing to exhaust, the dark space became 
larger ; that over the lampblack keeping about the same proportion in excess of that 
over the bare metal. As the exhaustion increased the size of the dark space over the 
platinum gained a little more upon that over the bare metal. 
492. A tube was made, as shown in fig. 9 ; a b are the two ends of a long platinum 
wire, coiled into a close spiral, and sealed in the bulb. The coils of the wire are not 
Fig. 9. 
in contact, and the ends are sealed in separately, so that the spiral can be made red 
hot by connecting the wires a and b with a battery, c and d are two flat aluminium 
poles. An apparatus was connected with the pump whereby different gases could be 
introduced into the bulb and experimented with at different pressures. 
493. The following experiments were tried with air :—At a pressure of 3”25 millims. 
the dark space round the negative pole was 1'75 millims. from the pole. The size and 
appearance were the same whether the negative pole was c or d, or whether the spiral 
a b was made negative. The induction spark was passed between a b (negative) and c 
MDCCCLXXIX. 
T 
