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SCIENCE 



[N. S. Vol. XXX. No. 766 



probable explanation the phenomenon is of 

 unusual interest. In 1827 the English bot- 

 anist Brown observed by means of a micro- 

 scope that minute particles like spores of 

 plants introduced into a fluid were always 

 in a state of continuous irregular agitation, 

 dancing to and fro in all directions at con- 

 siderable speeds. For a long time this 

 effect, known as the Brownian movement, 

 was ascribed to inequalities in the tempera- 

 ture of the solution. This was disproved 

 by a number of subsequent investigations, 

 .and especially by those of Gouy, who 

 showed that the movement was spontaneous 

 and continuous and was exhibited by very 

 small particles of whatever kind when im- 

 mersed in a fluid medium. The velocity of 

 agitation increased with decrease of diam- 

 eter of the particles and increased with 

 temperature, and was dependent on the vis- 

 cosity of the surrounding fluid. "With the 

 advent of the ultra-microscope it has been 

 possible to follow the movements with more 

 certainty and to experiment with much 

 smaller particles. Exner and Zsigmondy 

 have determined the mean velocity of par- 

 ticles of known diameter in various solu- 

 tions, while Svedberg has devised an in- 

 genious method of determining the mean 

 free path and the average velocity of par- 

 ticles of different diameter. The experi- 

 ments of Ehrenhaft in 1907 showed that 

 the Brownian movement was not confined 

 to liquids, but was exhibited far more 

 markedly by small particles suspended in 

 gases. By passing an arc discharge be- 

 tween silver poles he produced a fine dust 

 of silver in the air. When examined by 

 means of the ultra-microscope the sus- 

 pended particles exhibited the character- 

 istic Brownian movement, with the differ- 

 ence that the mean free path for particles 

 of the same size was much greater in gases 

 than in liquids. 



The particles exhibit in general the char- 

 acter of the motion which the kinetic the- 



ory ascribes to the molecules themselves, 

 although even the smallest particles exam- 

 ined have a mass which is undoubtedly 

 very large compared with that of the mole- 

 cule. The character of the Brownian 

 movement irresistibly impresses the ob- 

 server with the idea that the particles are 

 hurled hither and thither by the action of 

 forces resident in the solution, and that 

 these can only arise from the continuous 

 and ceaseless movement of the invisible 

 molecules of which the fluid is composed. 

 Smoluchowski and Einstein have suggested 

 explanations which are based on the kinetic 

 theory, and there is a fair agreement be- 

 tween calculation and experiment. Strong 

 additional confirmation of this view has 

 been supplied by the very recent experi- 

 ments of Perrin (1909). He obtained an 

 emulsion of gamboge in water which con- 

 sisted of a great number of spherical par- 

 ticles nearly of the same size, which showed 

 the characteristic Brovniian movement. 

 The particles settled under gravity and 

 when equilibrium was set up the distribu- 

 tion of these particles in layers at different 

 heights was determined by counting the 

 particles with a microscope. The number 

 was found to diminish from the bottom of 

 the vessel upwards according to an expo- 

 nential law, i. e., according to the same law 

 as the pressure of the atmosphere dimin- 

 ishes from the surface of the earth. In 

 this case, however, on account of the great 

 mass of the particles, their distribution was 

 confined to a region only a fraction of a 

 millimeter deep. In a particular experi- 

 ment the number of particles per unit vol- 

 ume decreased to half in a distance of 

 0.038 millimeter, while the corresponding 

 distance in our atmosphere is about 6,000 

 meters. From measurements of the diam- 

 eter and weight of each particle, Perrin 

 found that, within the limit of experi- 

 mental error, the law of distribution with 

 height indicated that each small particle 



