CH. ///] INTERPRETATION OF APPEARANCES 109 



object moves across the field it may appear to move with great 

 rapidity, whereas if one measures the actual distance passed and 

 notes the time, it will be seen that the actual motion is quite slow. 

 One should keep this in mind in studying the circulation of the 

 blood. The truth of what has just been said can be easily demon- 

 strated 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. 42, Table, 58) 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. 



163. Pedesis or Brownian Movement. Employ the 

 same object as above, but a 3 mm. ( l /3 in.) 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 indfinite, dancing or oscillating motion. 



This indefinite but continuous motion of small particles in a 

 liquid is called Pe-d'e' sis or Brownian movement. Also, but im- 

 properly, 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 objects is lamp-black ground up 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 and in a suitable liquid. In the minds of most, no adequate 

 explanation has yet been offered. 



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 ob- 

 tain good photographs of milk gobules 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 (see Ch.IX). 



