164 
MR. W. CROOKES OK LIKES OE MOLECULAR PRESSURE. 
in which case the average molecule is allowed to obey its own motions or laws without 
interference; and if the mean free path is comparable to the dhnensions of the vessel, 
the properties which constitute gaseity are reduced to a minimum, and the matter 
becomes exalted to an ultra-gaseous or molecular state, in which the very decided 
but hitherto masked properties now under investigation come into play. 
The phenomena in these exhausted tubes reveal to physical science a new world— 
a world where matter may exist in a fourth state, where the corpuscular theory of 
light may be true, and where light does not always move in straight lines, but where 
we can never enter, and with which we must be content to observe and experiment 
from the outside. 
