W. Crookes— Radiant Matter. 243 
a Fourth state or condition, a condition as far removed from 
the state of gas as a gas is from a liquid. 
Mean Free Path. Radiant Matter. 
ing through an exhausted tube, a dark space is seen to sur- 
round it. This dark space is found to increase and diminish 
p 
rally infer that the dark space is the mean free path of the 
Molecules of the residual gas, an inference confirmed by 
experiment. 
