CH. Ill,] INTERPRETATION OF APPEARANCES. 89 



slight amount of silicate of soda or of 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 pow- 

 dered 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 explana- 

 tion has yet beai offered. See Carpenter-Dallinger, p. 373 ; Beale, p. 

 x 95 ; 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 wholly satisfactory. 



For the original account of this see Robert Brown, " Botanical appen- 

 dix to Captain King's voyage to Australia," Vol. II, p. 534. (1826). 



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

 ungen iiber des Herreu Robert Brown Entdeckung lebsnder, selbst im 

 Feuer unzerstorbarer Theilchen in alien Korperu. ' ' From ' ' Die Gesell- 

 schaft fur Beforderung der Naturwissenschaften zu Freiburg." 1828. 



Compare the pedetic motion with that of a current by slightly inclin- 

 ing the tube of the microscope. The small particles will continue their 

 independent leaping movements while they are carried along by the 

 current. 



§ 142. Demonstration of Pedesis with the Polarizing Micro- 

 scope. — The following demonstration shows conclusivel}' that the pe- 

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



Open the abdomen of a dead frog (an alcoholic specimen will do if it 

 is soaked in water for some time, but a fresh specimen is more satisfac- 

 tory). 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 ap- 

 pearance. 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 then a high power (3 mm. or }i in.). In the field will be seen 

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

 tionless but the smallest ones exhibit marked pedetic movement. 



Use the micro-polariscope, light with great care and exclude all ad- 



