MOLECULAR PRESSURE, AKD THE TRAJECTORY OF MOLECULES. 
139 
When the gas is dense the disturbance extends a short distance only from the metal; 
but as rarefaction proceeds the layer of molecular disturbance increases in thickness. 
In air at a pressure of '078 millim. this molecular disturbance extends for at least 
8 millims. from the surface of the disk, forming an oblate spheroid around it. Had the 
pole been a point the spheroid would have become a sphere 16 millims. diameter. 
b. The diameter of this dark space varies with the exhaustion ; with the kind of gas 
in which it is produced ; with the temperature of the negative pole ; and, in a less 
degree, with the intensity of the spark. For equal degrees of exhaustion it is greatest 
in hydrogen and least in carbonic acid as compared with air. 
c. The shape and size of this dark space do not vary with the distance separating 
the poles; nor, except very slightly, with alteration of battery power, or with intensity 
of spark. When the power is great the brilliancy of the unoccupied parts of the tube 
overpowers the dark space, rendering it difficult of observation ; but on careful scrutiny 
it may still be seen unchanged in size, nor does it alter even when, with a very faint 
spark, it is scarcely visible. On still further reduction of the power it fades entirely 
away, but without change of form. 
499. I consider, therefore, I am justified in assuming that if the molecular dis¬ 
turbance communicated to the surrounding gas by the metal were to be further 
reduced in intensity, the spheroid would still remain the same for the same pressure, 
although invisible. If instead of exciting the disturbance in the metal pole by electri¬ 
cal means, I effect the same by heating it to redness, the spheroid of molecular 
disturbance in the surrounding gas would probably be there ; and if the disturbance 
is only produced by allowing light to shine on the metal plate, we may still imagine 
the molecular disturbance to extend around the plate, although vastly reduced in 
intensity. 
500. The vibrations communicated by the induction spark to a metal plate are 
doubtless different in kind as well as in intensity from those caused by heating the 
plate, for the electrical excitement induces luminosity in the gas, although the metal 
pole is only slightly heated ; while igniting the metal to bright redness in the absence of 
electrical excitement from the induction coil causes no luminosity in the surrounding 
gas. 
501. Numerous experiments have been tried with the object of seeing if this visible 
layer of molecular disturbance was the same as the invisible layer of molecular pressure 
or stress, the investigation of which has formed the subject of many papers presented 
to the Royal Society". Small exploring flies similar to the one described in a former 
paper (4l7)t were moved about in various parts of the bulb, both inside and outside 
the spheroid, but the results were most contradictory, and were difficult to obtain at 
all owing to the electrification of the fly. 
* Phil. Trans., Yol. 164, p. 501; Vol. 165, p. 519; Yol. 166, p. 325; Vol. 166, p. 355; Vol. 169, p. 243; 
Yol. 170, p. 87. 
t Ibid., Yol. 170, p. 103. 
T 2 
