412 Molecular Physics in High Vacua. [June, 
It is not easy to make clear the kinetic theory, but 
I will try to simplify it in this way: — Imagine that I 
have in a large box a swarm of bees, each bee inde- 
pendent of its fellow, flying about in all manner of 
directions and with very different velocities. The bees are 
so crowded that they can only fly a very short distance 
without coming into contacft with one another or with 
the sides of the box. As they are constantly in collision, 
so they rebound from each other with altered velocities 
and in different directions, and when these collisions take 
place against the sides of the box pressure is produced. If 
I take some of the bees out of the box, the distance which 
each individual bee will be able to fly before it comes into 
contact with its neighbour will be greater than when the 
box was full of bees, and if I remove a great many of the 
bees I increase to a considerable extent the average distance 
that each can fly without a collision. This distance I will 
call the bee’s mean free path . When the bees are nume- 
rous the mean free path is very short ; when the bees are 
few the mean free path will be longer, the length being 
inversely proportional to the number of bees present. 
Let us now imagine a loose diaphragm to be introduced 
in the centre of the box, so as to divide the number of 
bees equally. The same number of bees being on each 
side, the impacts on the diaphragm will be equal ; and 
the mean speed of the bees being the same, the pressure 
will be identical on each side of the diaphragm, and it will 
not move. 
Let me now warm one side of this division so as to let 
it communicate extra energy to a bee when it touches it. 
As before, a bee will strike the diaphragm with its normal 
mean velocity, but will be driven back with extra velocity, 
the reaction producing an increase of pressure on the 
diaphragm. It will be found, however, that although the 
diaphragm is free to move, the extra strength of the 
recoil on the warm side does not produce any motion. 
This at first sight seems contrary to the law of aCtion and 
reaction being equal. The explanation is not difficult to 
understand. The bees which fly away from the diaphragm 
have drawn energy from it, and therefore move quicker 
than those which are coming towards it ; they beat back the 
crowd to a greater distance, and keep a greater number from 
striking the diaphragm. Near to the heated side of the 
diaphragm the density is less than the average, while be- 
yond the free path the density is above the average, and 
this greater crowding extends to all other parts of the box. 
