CH. Ill ] INTER PRE TA TION OF A PRE A RANCES 99 



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

 the field is actually of considerable diameter (Fig. 37, Table, § 51) 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. 



§ 150. Pedesis or Brownian Movement. — Employ the same 

 object as above, but a 3 mm. (}i in.) or higher objective in place of 

 the 16 mm. Make the body of the microscope vertical, so that there 

 ma}* 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 indefinite, dancing or oscillating motion. 



This indefinite but continuous motion of small particles in a liquid 

 is called Pe-de sis or Brownian movement. Also, but improperly, molec- 

 iilar movement, from the smallness of the particles. 



The motion is increased by adding a-litfe 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. Carmine 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 di- 

 vided and in a suitable liquid. In the minds of most, no adequate 

 explanation has yet been offered. See Carpenter-Dallinger, p. 431 ; 

 Beale, p. 195 ; Jevons in Quart. Jour. Science, n. s., Vol. VIII (1878), 

 p. 167. In 1894, Meade Bache published a paper in the Proc. Amer. 

 Philos. Soc, Vol. XXXIII, pp. 163-167, entitled "The Secret of 

 the Brownian Movement." This paper is suggestive if not very 

 satisfactory. 



For the orginal account of this see Robert Brown, "Botanical 

 appendix to Captain King's voyage to Australia," Vol. II, p. 534. 

 (1826). 



See also Dr. C. Aug. Sigm. Schultze, "Mikroskopische Unter- 

 suchungen fiber des Herren Robert Brown Entdeckung lebender, selbst 

 im Feuer unzerstorbarer Theilchen in alien Korpern." From "Die 

 Gesellschaft fur Betorderung der Naturwissenschaften zu Freiburg. ' ' 

 1828. 



Compare the pedetic motion with that of a current hy slightly in- 

 clining the tube of the microscope. The small particles will continue 



