426 
Molecular Physics in High Vacua. 
TJune, 
only to vibrate and become temporarily luminous while the 
discharge is going on, but to produce an impression upon 
the glass which is permanent. The explanation which has 
gradually evolved itself from this series of experiments is 
this : — The exhaustion in these tubes is so high that the 
dark space, as I showed you at the commencement of this 
Lecture, that extended around the negative pole, has 
widened out till it entirely fills the tube. By great rarefac- 
tion the mean free path has become so long that the hits in 
a given time may be disregarded in comparison to the 
misses, and the average molecule is now allowed to obey 
its own motions or laws without interference. The mean 
free path is in faft comparable to the dimensions of the 
vessel, and we have no longer to deal with a continuous portion 
of matter, as we should were the tubes less highly exhausted, 
but we must here contemplate the molecules individually. At 
first this was only a convenient working hypothesis. Long- 
continued experiment then raised this provisional hypothesis 
almost to the dignity of a theory, and now the general opinion 
is that this theory gives a fairly correct explanation of 
the fadls. In these highly exhausted vessels the mean free 
path of the residual molecules of gas is so long that they 
are able to drive across from the pole to the other side of the 
tube with comparatively few collisions. The negatively 
eledtrified molecules of the gaseous residue in the tube there- 
fore dash against anything that is in front, and cast shadows 
of obstacles just as if they were rays of light. Where they 
strike the glass they are stopped, and the produdtion of 
light accompanies this sudden arrest of velocity. 
Other substances besides English, German, and uranium 
glass, and Becquerel’s luminous sulphides, are also 
phosphorescent. I think, without exception, the diamond 
is the most sensitive substance I have yet met for 
ready and brilliant phosphorescence. I have here a tube, 
similar to those already exhibited, containing a mica 
screen painted with powdered diamond, and when I 
turn on the coil, the brilliant blue phosphorescence of 
the diamond can be seen, quite overpowering the green 
phosphorescence of the glass. Here, again, is a very 
curious diamond, which I was fortunate enough to meet 
with a short time ago. By daylight it is green, produced, 
I fancy, by an internal fluorescence. The diamond is 
mounted in the centre of this exhausted bulb (Fig. io), 
and the negative discharge will be diredled on it from 
below upwards. On darkening the theatre you see the 
diamond shines with as much light as a candle, phosphor- 
escing of a bright green. 
