79 
bonate of lime, appeared to me somewhat less active in 
comparison. 
In varying the liquid, however, by dissolving different 
salts therein, I was soon struck by the fact that the purest 
distilled water alone gave the movement in the highest 
perfection. With a few exceptions, soon to be noticed, all 
acids, alkalis, or salts tended to diminish the movement in 
a manner wholly independent of their peculiar chemical- 
qualities. 
The inquiry was much facilitated by discovering that 
the microscopic movement is closely connected with the 
suspension of fine powder in water.* Clay and pounded 
glass which are most active in the microscope are also 
capable of remaining long in suspension. All acids, alkalis, 
or salts which checked the motion under the microscope 
were found also to have a power which has not been 
sufficiently noticed, of precipitating suspended matter. At 
the same time gum arabic, which possesses a most extra- 
ordinary power of exciting the molecular movement, is also 
capable of maintaining powder in suspension, and has long 
been used for this purpose in the manufacture of ink. The 
molecular motion does not directly affect the gravity of the 
particles, but it prevents the particles from aggregating 
together into larger bodies and thus overcoming the resist- 
ance of the liquid. 
That the motion is due to electricity I was soon con- 
vinced, by the close analogy with the circumstances in 
which electricity is produced by the hydro-electric machine. 
Armstrong and Faraday found that pure water in this 
machine alone produced much electricity, and that almost 
any salt, acid, or alkali prevented the action by rendering 
the water a conductor. Ammonia, however, is a remarkable 
* I have since found that the microscopist Dujardin noticed this connec- 
tion. See “ Manuel Complet de l’observateur au Microscope.” Paris, 1843, 
p. 60. 
