PRESIDENT’S ADDRESS. 21 
theory of light and to be fatal to the theory that light consists of 
undulations. 
Take, for example, the pressure exerted by light. This would follow 
as a matter of course if we supposed light to be small particles moving 
with great velocities, for these, if they struck against a body, would 
manifestly tend to push it forward, while on the undulatory theory 
there seemed no reason why any effect of this kind should take place. 
Indeed, in 1792 this very point was regarded as a test between the 
theories, and Bennet made experiments to see whether or not he could 
find any traces of this pressure. We now know that the pressure is 
there, and if Bennet’s instrument had been more sensitive he must have 
observed it. It is perhaps fortunate that Bennet had not at his com- 
mand more delicate apparatus. Had he discovered the pressure of 
light, it would have shaken confidence in the undulatory theory and 
checked that magnificent work at the beginning of the last century 
which so greatly increased our knowledge of optics. 
As another example, take the question of the distribution of energy 
in a wave of light. On the emission theory the energy in the light is the 
kinetic energy of the light particles. Thus the energy of light is made 
up of distinct units, the unit being the energy of one of the particles. 
The idea that the energy has a structure of this kind has lately 
received a good deal of support. Planck, in a very remarkable series of 
investigations on the Thermodynamics of Radiation, pointed out that the 
expressions for the energy and entropy of radiant energy were of such 
a form as to suggest that the energy of radiation, like that of a gas on 
the molecular theory, was made up of distinct units, the magnitude of 
the unit depending on the colour of the light; and on this assumption 
he was able to calculate the value of the unit, and from this deduce 
incidentally the value of Avogadro’s constant—the number of molecules 
in a cubic centimetre of gas at standard temperature and pressure. 
This result is most interesting and important because if it were a 
legitimate deduction from the Second Law of Thermodynamics, it would 
appear that only a particular type of mechanism for the vibrators which 
give out light and the absorbers which absorb it could be in accordance 
with that law. 
If this were so, then, regarding the universe as a collection of 
machines all obeying the laws of dynamics, the Second Law of Thermo- 
dynamics would only be true for a particular kind of machine. 
There seems, however, grave objection to this view, which I may illus- 
trate by the case of the First Law of Thermodynamics, the principle of the 
Conservation of Energy. This must be true whatever be the nature of 
the machines which make up the universe; provided they obey the laws of 
dynamics, any application of the principle of the Conservation of Energy 
could not discriminate between one type of machine and another. 
Now, the Second Law of Thermodynamics, though not a dynamical 
