In 1913 the German physicist Franck began a series of experiments by methods which made it possible to regulate accurately the velocity of the electrons, and to determine the kinetic energy before and after collisions with atoms. He first applied the methods to mercury vapour, where the conditions are particularly simple, since the mercury molecules consist of only one atom. Franck bombarded mercury vapour with electrons all of which had the same velocity. He then showed that if the kinetic energy of the electrons was less than 4·9 volts the collisions with the atoms were completely “elastic,” _i.e._, the direction of the electron could be changed by the collision, but not its velocity. If, however, the velocity of the impinging electrons was increased so much that it was somewhat larger than 4·9 volts, there was an abrupt change in the situation, since many of the collisions became completely inelastic, _i.e._, the colliding electron lost its entire velocity and gave up its entire kinetic energy to the atom. If the initial velocity was even greater, so that the kinetic energy of the colliding electron was 6 volts, for instance, then when the collision took place there would always be lost a kinetic energy of 4·9 volts, since the electrons would either preserve their kinetic energy intact or have it reduced to 1·1 volt (cf. Fig. 30). [Illustration: FIG. 30.—Schematic drawing of Franck’s experiment with electron collisions. _G_ is a glowing metal wire which emits electrons. If between _G_ and the wire net _T_ there is a difference of potential of 6 volts, the electrons will pass through the holes of the net with great velocity out into the space _R_, where there is mercury vapour. _a_ represents a free electron _F_ and a mercury atom _Hg_ before the collision, while _b_ represents them after the collision; with the collision _F_ loses a kinetic energy corresponding to 4·9 volts; at the same time a bound electron _B_ in the atom goes over to a larger stationary orbit.]