, so that Einstein’s formula would become
This was the form which Einstein originally gave to his equation, a very simple derivation of which has been given by Langevin.[76] The essential elements of this derivation will be found in [Appendix C].
The first careful test of this equation was made on suspensions in liquids by Perrin,[77] who used it for finding
the number of molecules in a gram molecule. He obtained the mean value
, which, in view of the uncertainties in the measurement of both
