Escape of Gases from Atmospheres. 693 
what chances on some of these occasions differs enormously 
from that which occurs in most of them. Of this kind are 
the interactions between the molecules of a gas and the inter- 
fused zether, and especially those complicated s struggles between 
molecules which we call their encounters—events each of 
which, when viewed, as it ought to be viewed, from the 
molecular standpoint, is a battle lasting a long time as time 
has to be measured in molecular physics, and with an immense 
number and variety of incidents. These—the interactions 
between the molecules and the ether, and the interactions 
between molecule and molecule—are the primary events, the 
real determining events, which occur within a gas: while 
the movements of the molecules as they dart about between 
one encounter and the next; the spectrum radiated by the 
gas; the ions which present themselves after some of the 
encounters ; the compounds which result from chemical 
reactions during some of the encounters (if what we are 
dealing with happens to be a mixture of suitable gases) ; 
and finally that remarkable partition of energy between the 
events going on within the molecules and the translational 
motions of the molecules, which is effected during some of 
the encounters—all of these are subordinate events depending 
upon those which are above spoken of as the primary events. 
When dealing with such almost immeasurably intricate and 
obscure operations of nature, it behoves us with the very utmost 
caution to distinguish between what is theory and what hypo- 
thesis in the data we employ, in order to be able to ascertain 
how far any conclusions we draw follow from the one, and how 
far they involve the other with the risks inseparable from it. 
Theories are suppositions we hope to be true ; hypotheses 
are suppositions we expect to be useful. As to theories, 
they are either correct or erroneous. They may be, they 
usually are, but they by no means need be, of use to man. 
The virtue of a theory is simply to be true. On the other 
hand, hypotheses usually make use of machinery which we 
can see to be simpler than that operating in nature; and 
especially is this the case with those hypotheses to which 
we are obliged to bave recourse in mathematical investi- 
gations, which, in order to be of use, must be so great a 
simplification of the complex intricacies of nature that human 
mathematics shall be able to cope with them. 
The theory of gas universally put forward in scientific books 
when the present writer was young, was the erroneous statical 
theory that the molecules of a gas may be stationary, that 
they have a capacity for expanding and contracting, and that 
each molecule presses against its neighbour s. An illustration 

