186 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
trating rays from radioactive substances, or, as I suggest that we call 
them, from radiants; the atomic heat of elements, so admirably han- 
dled by Debye; the residual energy at low temperatures; and the 
constitution of the atom. 
Space prevents us from considering more than the last of these. 
The first step toward the new method was taken by Planck when 
he saw the necessity of explaining why the energy of short-wave 
radiation is some hundred millionth part of that demanded by class- 
ical dynamics. He made the supposition that energy is not indefi- 
nitely divisible, but he did not assume that it was atomic. He 
actually imagined that energy was emitted from oscillators in exact 
multiples of hn, where n is the frequency of the oscillation and h is 
a universal constant (Planck’s) with a value 6.5 10~ erg second. 
The magnitude of the energy quantum is thus proportional to the 
frequency. 
This quantum hypothesis has spread like fire during a drought. 
It pervades the scientific journals. No physicist has pretended to 
explain or understand it, for, as Jeans says, the lucky guess has not 
yet been made. Nevertheless, it appears that ‘‘h” has truth under- 
lying it, and that it has come to stay, for the applications of the 
quantum hypothesis have already achieved a great and unexpected 
measure of success. In the meantime it is necessary to proceed with 
caution, checking every theory by experiment, for there is no other 
criterion to guide the investigator, whether to hold to the old or try 
the new. 
7. The first steps toward the idea of the modern or Rutherfordian 
atom rest on an experimental basis, and are not, therefore, open to 
suspicion. 
Rutherford and Geiger found that when the alpha particles from 
a radiant, such as radium or polonium, met a thin gold leaf, the bulk 
of the alpha particles passed through with slight deflection, but 
about 1 in 8,000 bounced back, or returned toward the side of their 
source. Both large and small deviations of the alpha particles in 
passing through matter were satisfactorily explained by ordinary or 
Newtonian dynamics, with the law of repulsion inversely as the 
square of the distance between similar electric charges. One charged, 
particle was the alpha particle with a positive charge twice as large, 
numerically, as that of an electron. The other charged particle was 
the nucleus of the atom of gold, and the magnitude of this charge 
was about 4A where A is the atomic weight of gold. This view was 
subjected to a searching series of experimental tests and emerged 
triumphant. 
8. About this time C. T. R. Wilson skillfully obtained photographs 
of the mist-ladened, charged air molecules, marking the track of a 
recent alpha particle, in an expansion chamber. Some of these 
