CHAPTER IV. 



PHENOMENA OF MOTION. 

 Brownian-Zsigmondy Movement. 



UPON examining a drop of milk under the microscope, we are at 

 once struck by the appearance of the fat droplets on account of their 

 strong refraction (a dark ring with a brilliant center). It is seen 

 that they exhibit a certain oscillation (trembling). This character- 

 istic oscillating movement is more intense with the smaller droplets 

 (Fig. 5), whereas those having a diameter of more than 4 ^ do not 

 show it at all. The phenomenon is named after the English botan- 



FIG. 5. Brownian movement of milk globules. (From O. Lehman.) 



ist, ROBERT BROWN, who discovered it as early as 1827, in an aqueous 

 suspension of plant pollen. It may be observed in every suspension 

 or emulsion which is sufficiently fine. Particles of I IJL diameter show 

 a radius of translation of 1 ju. The " dance of the motes," the rush- 

 ing hither and thither of the bright particles observed in the ultra- 

 microscope, 1 is nothing else than an enormously exaggerated Brownian 

 movement, due to the fact that the particles are much smaller than 

 those that may be seen under the microscope. Particles of 10 to 50 ju/-i 

 have a speed of more than 100 IJL per second. These movements 



1 R. Lorenz correctly calls attention to the fact that the great advance in our 

 science does not date from Brown who observed the "oscillations" of microscopic 

 particles but from R. Zsigmondy who recognized that particles of molecular 

 dimensions are in a similar mobile state. (Frankforter Ztg. 4.6.11, I Morgenbl.) 



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