182 
Movement of Microscopic Particles 
[April, 
Almost all soluble substances prevent both one and the other 
phenomenon ; but ammonia is one of a few exceptions — it 
allows both of eleCtric excitation and pedesis. Boracic acid 
is another exception, and gum a third one. There are diffi- 
culties, no doubt. Silicate of soda and silicic acid were not 
tested by Faraday. Tartaric, citric, and acetic acids are 
mentioned by him as giving little conducting power to water, 
but they arrest pedesis in a considerable degree. But it is 
to be remembered that the conducting power of liquids and 
the connected effeCt of electrolysis are very imperfectly un- 
derstood, and few researches on the subject have been under- 
taken since these old ones of Faraday. 
The proper way of settling the question, no doubt, is to 
make diredt experiments upon silicic acid and the other sub- 
stances which allow or promote pedesis. This I attempted 
to do, immersing two platinum points, about i-6oth of an 
inch apart, in various solutions, and connecting them with 
two of Grove’s cells and an ordinary galvanometer. The 
results did not altogether agree with the theory, but there 
are many doubtful points about the matter. It does not 
follow that the conducting power of a liquid through i-6oth 
of an inch, with the power of two of Bunsen’s cells, will 
correspond to the conducting power through i -5000th of an 
inch at a very low tension ; and there are other difficulties 
in the matter. In spite of some discrepancies 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. Various reasons may 
be given for regarding this conclusion as probable. 
There is no doubt that pure water may produce eleCtric 
tension. If pure water be put into an ordinary eleCtric 
battery, the tension produced is supposed to be the same as 
with dilute sulphuric acid. In faCt the acid is only added 
to make the water a conductor, without which the current 
is imperceptible. Then, again, there is no doubt that, when 
water is in contaCt with silicates, some chemical aCtion goes 
on, for silicic acid is invariably found in all water which 
has been in contaCt with the earth. It is perfectly well 
known that all clays, and also all rocks exposed to air and 
water, decompose slowly, as Bywater acutely remarked. 
The aCtion is very slow, so that there is nothing absurd in 
supposing that chemical aCtion may go on for eight years, 
or more, as in the case of my tubes of china clay and water. 
The silicic acid dissolved out of the clay promotes rather 
than prevents pedesis. 
In attempting to explain the exadt modus operandi, we 
