THE HAMILTON ASSOCIATION. Sid 
This experiment having destroyed the prevalent theory of heat, 
was naturally looked to for hints as to a new one, and about the 
middle of this century a number of physicists almost simultaneously 
suggested that the heat which is formed when motion is hindered, 
might itself be considered as motion of some sort. Bernoulli’s gas 
theory was eagerly seized upon as affording the necessary basis for 
re-construction, and from the day that heat was ‘“‘explained” to be 
the “energy of molecular motion” may be dated the modern revival 
of the Kinetic Mheorys 4 
It was now easy to see why the pressure exerted by a gas 
should increase when the gas was warmed, for if the molecules flew 
faster (and that was the new explanation of the rise in temperature) 
they could not fail to strike harder.on the walls of the prison. A 
calculation of the rate at which these bodies must be moving, if the 
pressure of about 15 pounds per square inch exerted at ordinary 
temperatures by one ounce of air confined in a space of one cubic 
foot is to be accounted for by the bombardment of its molecules 
against the walls, gave 525 yards per sec. a result which was not only 
surprising in itself but which seemed to conflict with well known 
facts. Ought not the perfume of a plant or the noxious odours of a 
chemical laboratory to spread with incredible swiftness across the 
small space of a room if the molecules of the gases composing them 
were really moving at so unheard of a rate? ‘This discrepancy was, 
however, soon seen to be the result of a misunderstanding ; the 
motion of the molecules might well be very swift and yet their pro- 
gress in a straight line—hindered as it must necessarily be by the 
numerous collisions of one against the other—comparatively slow. 
A mathematical investigation of the question showed that, in order 
to reconcile the calculated velocity of the individual molecules with 
the observed rates of diffusion of one gas into another, the ‘“‘ mean 
free path ” or space through which an air molecule may hope to 
travel before running against a neighbor must be extremely short, 
on the average about halfa millionth inch, or in other words, in a 
second it must undergo between four and five thousand million col- 
lisions and as many changes of direction. 
The chance of a collision is, however, obviously greater, the 
greater the diameter of the particles (imagining them for simplicity’s 
sake to be spherical in form), and an enquiry into the whole subject 
