BROWNIAN MOVEMENT (originally called BRUNONIAN MOTION or MOVEMENT), an irregular agitation seen when minute solid particles, suspended in a liquid, are viewed under a high magnifying power. The nomenon is named for the Scottish botanist and agriculturist, Robert Brown (1773-1858), who observed it in 1827. It had been previously noted by Buffon and Spallanzani, but they did not comprehend its true nature, and thought that the moving particles which they saw were rudimentary animalcules. Brown had the advantage of improved achromatic scope objectives. The Brownian movement is not to be confused with the tremor that results from vibration of the microscope stand, which affects all the particles in the visible field in the same manner, so that they execute their tions simultaneously and in parallel directions. Nor is it to be confused with such motions as may be produced by convection currents in the liquid in which the particles are suspended. Movements due to currents of this kind affect neighboring particles in the same way, whereas in the true Brownian movement the particles show wholly independent movement even when they are almost in contact with one each pursuing its own course without the least relation to its neighbors. The movement is shown by particles of all kinds, and it has been observed in liquid carbonic acid in a cavity in the interior of a transparent quartz crystal, where it probably has been going on ously for millions of years. It cannot be explained by any peculiarity of chemical sition, nor as a phenomenon dependent upon the gradual solution of the particles. Moreover, experiment shows that it does not depend upon light (as the motion of the radiometer does), because its activity is not modified by ing the intensity of the light in the microscope field a thousand fold. It may be conveniently observed by examining, under a powerful croscope, water to which a very small amount of Chinese ink has been added.
Brown's discovery attracted no special atten tion at the time it was announced, and little is to be found with regard to it in scientific literature until 1888, when Gouy emphasized the inadequacy of the summary explanations of the phenomenon that had been offered pre vious to that time. Quite recently the subject has received exceedingly careful study from Prof. Jean Perrin of the Sorbonne, who gives the results of his investigations in his book 'Les Atomes,' and also in 'The Brown ian Movement and Molecular Reality> (trans lated by Soddy). According to present views, which have been so completely verified by experiment that no important future modifica tion is probable, the movement is due to the bombardment of the visible solid particles by the molecules of the liquid in which they are suspended. This bombardment is sensibly the same in all directions, if our observation ex tends over any considerable time; but on account of the nature of the molecular motion the balance cannot be perfect at every instant.
An extremely small suspended particle is there fore impelled hither and thither, as the force of the bombardment preponderates, momen tarily, now on one side of it and now on an other side; and the Brownian motion is the visible evidence of this action. Large suspended masses are subject to the same irregularity of pressure (or impact), but they do not show any sensible movement because the preponderance of the bombardment in any one direction is of too brief a duration to accelerate large masses to a measurable extent.
If the Brownian movement is caused as here described, much may evidently be learned from it with regard to molecular physics; because microscopic solid particles will behave in many respects like molecules of enormous size; and their behavior and distribution in the liquid in which they float will conform, very closely, with the laws which govern the inter diffusion of the molecules of liquids.
In his methodical study of the subject, Per rin undertook to make emulsions containing minute solid particles of spherical shape and uniform size. For this purpose he dissolved resinous substances in alcohol, and subsequently precipitated them as emulsions by pouring their alcoholic solutions into considerable quantities of water. Of the 11 different resins tried, two (namely, mastic and gum gamboge) gave ex cellent results. To secure uniformity of size in the spherules he subjected the emulsions to powerful centrifugal action for a long time, thereby effectively separating the larger par ticles from the smaller ones. The work was tedious, but after several months of labor he obtained, from a kilogram of gum gamboge, a satisfactory emulsion containing several decigrams of the gum in the form of sus pended spherules of remarkably uniform size and sphericity. Consonant determinations of the density of these spherules were obtained by three methods,— one of which consisted in dis solving bromide of potassium in the emulsion until the centrifugal machine no longer affected the spherules and then determining the den sity of the solution. To ascertain the diameters of the particles three methods were also used. One of these consisted in allowing a single layer of spherules to settle in contact with one another on the surface of a microscope slide, and then counting the number of spherules included within a known length or covering a known area.
From a study of the way in which the density of distribution of the spherules in his standard emulsions varied with height when a state of equilibrium was attained, Perrin draws import ant conclusions concerning the relation be tween the masses of his resin spherules and the masses of actual molecules; and as he had already determined the average mass of the spherules, he is therefore able to calculate the mass of an actual molecule. In this way he finds that 682 thousand million million million hydrogen atoms would have a combined mass of one gram. See GASES, MOLECULAR THEORY OF; and MOLECULAR THEORY.