1007 
2 is the mean length of path of the molecules, A is a constant 
determined at 0,873 in Guyr’s laboratory at Geneva’). Different 
investigators have performed measurements of A?, however almost 
always from the mutual differences of the times in which a particle 
travels a definite distance under the influence of an external force. 
It is, however, the question whether the thus determined values of A? 
are the same as those of equation (1). For with the movement under 
an external force the distribution in space of the molecules of the 
medium is disturbed, and it moves for a part with the particle. The 
chance to a Brownian displacement upward or downward will no 
longer be symmetrical. Only one investigation is known to me, 
that bv FLEercHER?’), in which no external force acted on the particle. 
The data obtained in this way, are, however, not numerous. 
It seemed therefore not superfluous to me, to start another inquiry 
into the validity of equation (1). In my experiments, carried out in 
the Physical Laboratory at Amsterdam (Director Prof. ZrrMan), the 
displacement of a particle was measured while the gravity and the electric 
force were in equilibrium with each other. This can be established 
with pretty great accuracy; in order, however, not to be disturbed 
by a small residual force I observed the movement in horizontal 
instead of vertical direction. 
2. 1 made use of the well-known method of MirikaN*®) and 
EHRENHAFT *). 
When v, represents the velocity of fall of the particle with mass 
M, v,' the velocity under influence of gravity and an electric force 
of equal direction, v, the velocity of rising, when this electric force 
is reversed, the following equations hold: 
1 
Mg = B Te REEN 
re ao, PEN ETT Le 
Ves 
e€ + Mg = ne ENE Ed Meike ae 
SN B 
e& is the absolute value of the electric force, e the charge of the 
particle. 
From (4a) and (45) follows: 
1) A. ScuipLor et Mlle J. MurzyNowska, Arch. de Genève 4,40,1915, p. 386 and 486, 
2) H. Frercuer, Phys. Rev. 33, 1911, p. 81. 
8) R. A. Mirrikan, Phys. Rev. 29, 1909, p. 560. 
4) See for a full description: F. EHRENHAFT. Wien. Sitz. ber IIa, 123, 1914, p. 53, 
64* 
