99. PRESIDENT’S ADDRESS. 
principle in as strict a sense as the law of the Conservation of Energy, is 
one that we should expect to hold for a collection of a large number of 
machines of any type, provided that we could not directly affect the indi- 
vidual machines, but could only observe the average effects produced by 
an enormous number of them. On this view the Second Law, as well as 
the First, should be incapable of saying that the machines were of any 
particular type: so that investigations founded on thermodynamics, 
though the expressions they lead to may suggest—cannot, I think, be 
regarded as proving—the unit structure of light energy. 
It would seem as if in the application of thermodynamics to radia- 
tion some additional assumption has been implicitly introduced, for these 
applications lead to definite relations between the energy of the light of 
any particular wave length and the temperature of the luminous body. 
Now a possible way of accounting for the light emitted by hot bodies 
is to suppose that it arises from the collisions of corpuscles with the 
molecules of the hot body, but it is only for one particular law of force 
between the corpuscles and the molecules that the distribution of energy 
would be the same as that deduced by the Second Law of Thermo- 
dynamics, so that in this case, as in the other, the results obtained by the 
application of thermodynamics to radiation would require us to suppose 
that the Second Law of Thermodynamics is only true for radiation 
when the radiation is produced by mechanism of a special type. 
Quite apart, however, from considerations of thermodynamics, we 
should expect that the light from a luminous source should in many 
cases consist of parcels possessing, at any rate to begin with, a definite 
amount of energy. Consider, for example, the case of a gas like sodium 
vapour, emitting light of a definite wave length; we may imagine that 
this light, consisting of electrical waves, is emitted by systems resembling 
Leyden jars. The energy originally possessed by such a system will be 
the electrostatic energy of the charged jar. When the vibrations are 
started, this energy will be radiated away into space, the radiation 
forming a complex system, containing, if the jar has no electrical 
resistance, the energy stored up in the jar. 
The amount of this energy will depend on the size of the jar and the 
quantity of electricity with which it is charged. With regard to the 
charge, we must remember that we are dealing with systems formed 
out of single molecules, so that the charge will only consist of one or two 
natural units of electricity, or, at all events, some small multiple of that 
unit, while for geometrically similar Leyden jars the energy for a given 
charge will be proportional to the frequency of the vibration ; thus, the 
energy in the bundle of radiation will be proportional to the frequency 
of the vibration. 
We may picture to ourselves the radiation as consisting of the lines 
of electric force which, before the vibrations were started, were held 
bound by the charges on the jar, and which, when the vibrations begin, 
