172 
Movement of Microscopic Particles 
'[April, 
The substance which I have found to be most convenient 
for experiments on pedesis is fine pure china clay, or kaolin. 
This clay is used in photography, and is to be purchased 
very carefully washed and prepared for the purpose. A 
small quantity of the clay shaken up witli pure water makes 
a milky liquid, a drop of which being placed under a piece 
of thin glass will show the motion in great perfection ; but 
finely-powdered glass, earthenware, and in faCt almost any 
very finely-powdered substance, will do nearly as well. 
Among vegetable substances gamboge may be mentioned as 
readily giving minute particles, which quiver and dance 
about in a wonderful way. 
In endeavouring to discover the cause of this phenomenon 
I made a certain number of experiments to test the validity 
of various explanations which had been offered. It has 
often been suggested that the motion is excited by rays of 
light or heat falling upon the liquid, and Crookes’s radio- 
meter now shows that in some cases it is possible to convert 
radiant energy direCtly into the energy of visible motion. 
But this idea was easily and completely disproved. Taking 
several aCtive substances, such as kaolin, road-dust, red 
oxide of iron, and mixing them with distilled water, I 
examined them in the microscope, causing the direCf rays of 
the sun, somewhat concentrated by a lens, to pass straight 
through the glass slide into the microscope. I then inter- 
posed a dark glass alternately between the sun and the 
specimen, and between the eyepiece and the eye. While 
tne amount of light reaching the eye remained nearly the 
same, the intensity of the rays falling on the moving parti- 
cles was probably several hundred times as great in one 
case as the other ; yet the vibrations of the particles pro- 
ceeded in exactly the same manner in comparative darkness 
and in intense light. Different coloured glass screens — 
purple, yellow, rose-coloured, &c. — were also tried, but no 
difference in the motions was apparent. The same conclu- 
sion was indirectly obtained, after the connection of pedesis 
with the supension of particles in water had been ascer- 
tained, by taking two tubes of china clay and pure water, 
putting one in a dark place, and exposing the other to the 
direCt rays of the sun for three hours. No difference in the 
rapidity of subsidence was apparent. 
It is asserted in Dr. Carpenter’s work on the microscope 
(§ 130) that this so-called molecular movement is greatly 
accelerated and rendered more energetic by heat. “ This,” 
he says, “ seems to show that it is due either direCtly to 
some calorical changes continually taking place in the fluid, 
