MODERN CONCEPTIONS OF THE UNIVERSE. 67 
tion would still draw each towards the other. There is therefore 
some connexion or relation between the two molecules, and to 
speak of them as two separate systems is only a convenient 
mode of speech which does not express the whole of the 
conditions. If they were two entirely independent systems, 
the motion of one molecule away from the other would have no 
effect upon the latter. But the law of gravitation assures us 
that the second molecule would experience a distinct effect, for 
the attractive force acting on it would gradually diminish as the 
distance increased. An extension of this idea leads us to realise 
that all the molecules in the universe are so linked together by 
gravitation as to form but a single system. Yet gravitation is 
not the only link, for electric and magnetic actions between 
molecules produce their effects, whatever the distance between 
the molecules. In addition, there are other actions which are 
sometimes practically in abeyance, as when two molecules, one 
of oxygen and one of hydrogen, are too far apart for chemical 
combination to take place. Nevertheless, the power of com- 
bination remains ever ready to do its work, when the distance 
between the molecules is sufficiently reduced and certain other 
conditions are fulfilled. 
We could, of course, suppose that these actions between 
molecules arise from something inherent in the molecules 
themselves, and that the intervening space has nothing to do 
with the affair. But the facts of optics and of electro- 
magnetism compel us to recognise the existence of an 
all-pervading medium to which the name of ether has been 
given. This medium is conceived to extend through all space, 
and there are good reasons for the belief that the forces between 
electrified bodies are in reality due to stresses in it. The ether 
enables radiant energy to be transmitted from one body to 
another, as when the earth receives heat from the sun, or 
telegrams are sent, with or withous the aid of wires, from one 
station to another. There is thus a most intimate connexion 
between molecules and the ether, and hence the ether may be 
regarded as the substance, if it can be called substance, which 
binds the whole universe tovether, 
The rate at which energy is supplied to the earth by radiation 
from the sun is very great. On each square yard of illuminated 
surface, energy 1S supplied at the rate of about one-fifth of a 
horse-power. For the whole of the illuminated hemisphere, 
this amounts to something like twenty million horse-power. 
Thus we come to recognise that the whole tribe of molecules 
is linked together by the ether in such a way that they and the 
