n8 PEDESIS OR BROWNIAN MOVEMENT [Ch. IV 



the circulation of the blood. The truth of what has just been said 

 can be easily demonstrated in studying the circulation in the gills 

 of Necturus, or in the frog's foot, by using first a low power in which 

 the field is actually of considerable diameter (fig. 29; Table, § 49) 

 and then using a high power. With the high power the apparent 

 motion will appear much more rapid. For spiral, serpentine, and other 

 forms of motion, see Carpenter-Dallinger, p. 433. 



§ 208. Pedesis or Brownian movement. — ■ Employ the same 

 object as above, but a 4 mm. or higher objective in place of the 16 

 mm. Make the body of the microscope vertical so that there may be 

 no currents produced. Use a small diaphragm and light the field 

 well. Focus and there will be seen in the field large motionless masses, 

 and between them small masses in constant motion. This is an in- 

 definite, dancing, or oscillating motion. 



This indefinite but continuous motion of small particles in a 

 liquid is called Brownian movement or Pedesis; also, but improperly, 

 molecular movement, from the smallness of the particles. 



The motion is increased by adding a little gum arabic solution or a 

 slight amount of silicate of soda or soap; sulphuric acid and various 

 saline compounds retard or check the motion. One of the best ob- 

 jects is lamp-black ground up in water with a little gum arabic. Car- 

 mine prepared in the same way, or simply in water, is excellent; and 

 very finely powdered pumice-stone in water has for many years been 

 a favorite object. Pedesis is exhibited by all solid matter if it is finely 

 enough divided. 



Compare the pedetic motion with that of a current by slightly 

 inclining the tube of the microscope. The small particles will con- 

 tinue their independent leaping movements while they are carried 

 along by the current. The pedetic motion makes it difficult to obtain 

 good photographs of milk globules and other small particles. The 

 difficulty may be overcome by mixing the milk with a very weak 

 solution of gelatin and allowing it to cool (10% gelatin is good). 



Until recently no adequate explanation of this movement had been 

 offered. At the present time it is believed to be due to the kinetic 

 activity of matter, and in itself to be one of the best proofs of that 

 activity. This is what is said by Rutherford: "The character of the 



