176 Movement of Microscopic Particles [April, 
but the brittle metals, antimony, arsenic, and bismuth, were 
readily ground into fine powder, and showed aCtive motion. 
Afterwards I obtained much motion with steel, lead, bronze, 
and standard silver and gold coin, the particles being pro- 
duced by rubbing the metals on a smooth hone. Sulphur 
could not he ground up fine enough ; but some sulphuretted 
hydrogen water having become decomposed, the precipitate 
of sulphur was found to consist of very minute particles, 
probably about i-30,oooth of an inch in diameter, in a state 
of extraordinarily active motion. It is certain that sub- 
stances of the most widely different chemical characters will 
exhibit the phenomenon, and it is difficult to establish any 
clear differences in the activity of the motion as connected 
with the chemical nature. 
As the form and chemical nature of the particles and the 
physical circumstances of the liquid seemed to throw no 
light on the cause of pedesis, it remained to consider the 
chemical character of the liquid. Striking differences in 
this respeCt were soon detected. If, instead of mixing china 
clay with pure water, we mix it with a very dilute solution 
of sulphuric acid, say one part in a thousand, an extraordi- 
nary difference results. Under the microscope the particles 
of clay are seen to aggregate together, and rapidly sink 
down upon the glass slide. Thepedetic movement is almost 
entirely destroyed, or, if any particles move at all, it is only 
exceedingly minute ones. The same effeCt is produced by 
almost any mineral acid, and, as a general rule, by all salts 
and other soluble substances, with certain significant excep- 
tions afterwards to be described. Before describing the 
experiments in this direction, however, it is necessary to 
point out the intimate connection which exists between 
pedesis and the suspension of particles in liquid. 
It seems surprising that the conspicuous phenomena 
arising from the suspension or precipitation of solid particles 
in liquids have not been more carefully studied. Besides 
being full of scientific interest, they have great practical 
importance in questions of water supply and sewage treat- 
ment. A few experimenters have touched upon the subject. 
The microscopist Dujardin, already quoted, hits the right 
nail on the head when he says (p. 60) — “ Ce mouvement 
Brownien joue un role important dans certains phenomenes 
physiques ; e’est lui qui empeche les eaux troubles de se 
clarifier promptement par le repos.” 
A few other casual remarks to the same effeCt may be 
quoted from several sources, but I am not yet aware of any 
connected series of experiments upon the suspension of 
