XOTES AND MEMORANDA. 
147 
the resisting liquid ; pedetic motion prevents the formation of groups, 
and keeps the minute particles apart so that each encounters the 
separate resistance of the fluid. 
Pure water exhibits pedesis in the highest perfection, even the 
air and carbonic acid usually dissolved in it producing a per- 
ceptible difference. If, however, instead of mixing china clay with 
pure water, it is mixed with a very dilute solution, say one part 
in a thousand of sulphuric acid, the pedetic movement is almost 
entirely destroyed, the same effect being produced by almost any 
mineral acid, and as a general rule by all salts and other soluble 
substances. To this general rule, however, there are certain remark- 
able exceptions, such as pure caustic ammonia (but not its compounds), 
boracic acid, and silicate of soda — gum arabic even possessing the 
power of increasing the motion. Comparing the substances which do 
not prevent the motion with those which do, it becomes apparent that, 
with some doubtful exceptions, they differ widely in the power of 
making water a conductor of electricity. Faraday found that some 
acids, such as the sulphuric, phosphoric, oxalic, and nitric, increase the 
conducting power of water enormously, whilst others, as the acetic 
and boracic acids, produce no change ; gum and ammonia producing 
no effect, whilst its carbonate does, and sulphate of soda and many 
soluble salts producing much effect. The argument in the case of 
pedesis is, the Professor considers, exactly analogous to that which 
Faraday employed in his inquiry into the production of electricity 
by the Armstrong electrical boiler, which he found must be supplied 
with pure distilled water to yield much electricity. The smallest 
drop of sulphuric acid or a little crystal of sulphate of soda prevented 
the evolution of electricity, as also did the addition of any of the 
saline or other substances which give conducting power to water. As 
ammonia increases the conducting power of water only in a small 
degree, Faraday concluded that it would not take away the power of 
excitement, and accordingly, on introducing some to the pure water, 
electricity was still evolved, but the addition of sulphuric acid by 
forming sulphate of ammonia took away all power. “ The analogy 
of these circumstances to that of pedesis is so remarkable,” the Pro- 
fessor writes, “ that little doubt can be entertained that the same 
explanation applies. It is perfectly pure water which produces elec- 
tricity and pedesis ; almost all soluble substances prevent both one and 
the other phenomenon, but ammonia is one of the few exceptions — 
it allows both of electric excitation and pedesis. Boracic acid is 
another exception, and gum a third. ... In spite of some discre- 
pancies and failures, I still think the analogy between pedesis and 
Armstrong’s electrical machine so strong, as to leave little doubt 
that pedesis is an electrical phenomenon.” In attempting to explain 
the exact modus operandi, we can only speculate that the action uj)on 
a minute irregular fragment will never be exactly equal all round. 
In order that a particle shall rest motionless in a non-conducting 
fluid, it must be in exactly equal chemical and electric relation to the 
fluid on all sides. That this should happen is almost infinitely impro- 
bable, and a condition of unstable equilibrium within limits is the result. 
The Professor concludes by pointing out that there is probably a 
close connection between pedesis and the phenomena of osmose. 
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