Ch. IV] I'KDESIS OR BROWNIAN MOVEMENT 119 



Brownian movement irresistibly impresses the observer 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 con- 

 tinuous and ceaseless movement of the invisible molecules of which 

 the fluid is composed." "Whatever may be the exact explanation 

 of this phenomenon, there can be but little doubt that it results from 

 the movements of the molecules of the solution, and is thus a striking, 

 if somewhat indirect, proof of the general correctness of the kinetic 

 theory of matter." Nature, Vol. 81, 1909, pp. 257-263; Science, 

 N. S., Vol. 30, 1909, pp. 289-303. 



By the aid of the ultra-microscope it has been shown that the 

 particles in smoke, etc., exhibit the pedetic movement even more 

 strikingly than do those in liquids. 



§ 209. Demonstration of pedesis with the polarizing microscope 

 (Ch. VIII). — -The following demonstration shows conclusively that 

 the pedetic motion is real and not illusive (Ranvier, p. 173). 



Open the abdomen of a dead frog (an alcoholic or formalin speci- 

 men is satisfactory). Turn the viscera to one side and observe the 

 small whitish masses at the emergence of the spinal nerves. With 

 fine forceps remove one of these and place it on the middle of a clean 

 slide. Add a drop of water, or of water containing a little gum arabic. 

 Rub the white mass around in the drop of liquid and soon the liquid 

 will have a milky appearance. Remove the white mass, place a cover- 

 glass on the milky liquid, and seal the cover by painting a ring of 

 castor oil all around it, half the ring being on the slide and half on 

 the cover-glass. This is to avoid the production of currents by 

 evaporation. 



Put the preparation under the microscope and examine with first 

 a low power, then a high power (4 mm.). In the field will be seen 

 multitudes of crystals of carbonate of lime; the larger crystals are 

 motionless, but the smallest ones exhibit marked pedetic movement. 



Use the micro-polariscope (Ch. VIII), light with great care, and 

 exclude all adventitious light from the microscope by shading the 

 object (§ 140) and also, by shading the eye. Focus sharply and ob- 

 serve the pedetic motion of the small particles, then cross the polarizer 

 and analyzer, that is, turn one or the other till the field is dark. Part 



