{22 AE: te TRANSACTIONS OF THE Peart 
than the mass of the hydrogen atom and whose electric 
charge, according to the most recent measurements, 
always equals 4.7 x 10° electrostatic units we want to 
know how this negative electron enters into the structure 
of the atom. The answer to this question is not complete. 
We are now in a tangle of theory and experimental data 
which will require the keenest minds to unravel. It is 
my purpose to present the problem as seen by the world 
of science today. Naturally the solution is sought by 
investigating the phenomena which exhibits a source of 
negative electrons. 
The photo-electron effect is the emission of negative 
electrons by matter when illuminated. Practically all 
substances exhibit this effect in varying degrees. Sodium 
and potassium are very marked in this respect. When ua 
surface of sodium is illuminated with violet light negative 
electrons are shot out with velocities increasing as the 
frequency of the light increaszs. In other words the 
shorter the wave tength of light the greater the velocity 
with which the electrons leave the metal surface. The: 
intensity of the light in no way afiects this velocity but 
affects only the number of negative electrons shot out. 
The relation between the velocity of emission and the 
frequency of electron emission is stated by the equation 
14 m V?=h+P 
where P is a constant depending on the energy required 
to pull the electron out of the surface, m is the mass of 
the electron and h aconstant. It appears, therefore, that 
for light of any given frequency f an amount of energy hf 
must be absorbed before an electron can be expelled 
That is, the energy must be absorbed in units not smaller 
than hf. The constant h has a value 6.547 x 107’. 
We find this constant h annearing from an entirely 
different direction. When Max Plauck attempted to 
derive an equation which would express a relation 
between the temperature of a body and the emitted wave 
length which had the maximum energy he assumed that 
radiation energy was affected only in units depending on 
the frequency of the wave. The equation which he 
derived fits the experimental facts better than any other. 
It is a most remarkable fact that the constant by which 
Plauck multiplied the frequency to find the units in 
