Sronsy—- The known part of Nature's Work. 81 
Gas, a gas consists of an enormous swarm of little missiles, all 
alike in each kind of gas, though differing from one gas to another. 
These molecules dart about among oneanother with almost incredible 
activity, and are, to use Maxwell’s simile, like the individuals of a 
swarm of bees which furiously make short flights in every direction, 
while the swarm as a whole is either stationary or quietly sailing 
along. In a gas eacn molecule dashes forward in an almost? 
straight line till it gets close to another molecule. Then an 
encounter takes place: the molecules struggle together for an 
excessively brief period, after which they fling asunder in two new 
directions. ‘The average velocity with which the molecules dart 
about had been known before Maxwell’s investigation. It is about 
500 metres per second in the air which we breathe. It was also 
known that, except in very high vacua, the molecules are so crowded 
that their journeys between their encounters can be but short; but 
the length of these journeys was not known. What Professor 
Maxwell effected was an actual determination in certain gases of 
the average length of these “free paths.” He did this by showing 
that upon this average depends what is called viscosity in a gas— 
that property which gradually brings a gas to rest after it has 
been disturbed and currents established in it. He further showed 
that the average length of the free paths is what determines the 
rate at which gases diffuse into one another. Accordingly, from 
experiments on viscosity made by Sir George Stokes, and from 
Graham’s experiments on diffusion, he was able to ascertain what the 
average length of the free paths must be to produce the observed 
amount of effect. He thus found it to be about six eighthets’ of 
a metre—that which would be represented arithmetically by 
0:000,000,06 of a metre—in atmospheric air, at the temperature 
and pressure of the experiments, which we may take to have been 
a barometric pressure of 760 millimetres of mercury and a tem- 
perature of about 17° centigrade. This length is smaller than 
any interval which the microscope can show, and yet it isa length 
which must be regarded as very large among molecular magnitudes. 
1 The gravitation of the molecules towards the Earth must bend the free paths, but 
the curvature is insensible until, near the boundary of the atmosphere, the attenuation 
of the air far exceeds any that can be reached in artificial vacua. 
2 Subsequent experiments by Maxwell himself on the viscosity of air (Phil. Trans., 
1866, p. 258) assign a length of 10-6 eighth-metrets to the average free path. The mean 
of all the determinations is 7°6 eighth-metrets. 
SCIEN. PROC. R D.S., VOL, IX., PART I. G 
