PERRIN'S EXPERIMENTAL VERIFICATION OF BROWNIAN MOVEMENT LAWS All these theories lend themselves to verification, as soon as spherules of measurable diameter can be prepared. The law of displacement is verified as follows : The successive horizontal dis placements of a given particle are measured by observation in the camera lucida (with known magnification) of the successive positions of the particle at equal time intervals.
Three patterns have been reproduced on the adjoining diagram, on a magnified scale of 16 divisions to 5o microns; they were obtained by tracing the horizontal projections of the consecutive positions at half-minute intervals of a single particle of mastic, of radius 0.53 micron. It will be noticed that this diagram readily gives the projection on any horizontal axis (these will be ab scissae, or ordinates, given by the use of squared paper). In cidentally, a diagram like this, giving a large number of displace ments on an arbitrary scale, furnishes only an extremely inade quate idea of the extraordinary complexity of the actual path. If plottings were made at time-intervals Ioo times ;porter, each segment would be replaced by a zig-zag outline relatively as com plicated as the whole diagram.
Perrin's measurements have shown that the expression which must be equal to Avogadro's number N in accordance with Einstein's equation, has a value 64X which is independent of the nature of the emulsion. This approaches the value already found for N. Many different measurements were made, varying the experimental conditions within wide limits, especially the viscosity and the size of the particles. Finally, even the formula for rotational Brownian movement was verified, giving N equal to 65X (See ELECTRON, THE, for Millikan's value.) Summarizing, Brownian movement is an experimental fact, which reveals evidence of the existence of molecules which are themselves inaccessible under the microscope. From the philo sophic standpoint, emphasis must again be laid on the fact that Brownian movement proves to us that rest and equilibrium can only be an outward semblance which masks a state of disorder and unrest ; it prepares us for a profound alteration in the aspect of the universe as soon as we alter the scale of our observations.