PROGRESS IN PHYSICS—THOMSON. 197 
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 command more delicate apparatus. Had he discovered the pres- 
sure of hght, 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 color 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 centimeter of gas at standard tempera- 
ture 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 vibra- 
tors 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 
illustrate by the case of the first law of thermodynamics, the prin- 
ciple 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 
principle in as strict a sense as the law of the conservation of energy, 
