the potential difference need not be greater than 1.8 volts, the current
being of any desired amount, according to the surface of plates used.

The electrical measurements taken by Dr. John Hopkinson during these
experiments for the Electrical Purification Association, to whom I had
sold my patents, entirely corroborated my contentions as to E.H.P. used,
and agreed with the measurements of the managing electrician, Mr.
Octavius March.

The process was then thoroughly investigated by Sir Henry Roscoe, who
had control of the works for one month. He reports as follows:

"The reduction of organic matter in solution is the crucial test of the
value of a purifying agent, for unless the organic matter is reduced,
the effluent will putrefy and rapidly become offensive.

"I have not observed in any of the unfiltered effluents from this
process which I have examined any signs of putrefaction, but, on the
contrary, a tendency to oxidize. The absence of sulphureted hydrogen in
samples of unfiltered effluent, which have been kept for about six weeks
in stoppered bottles, is also a fact of importance. The settled sewage
was not in this condition, as it rapidly underwent putrefaction, even in
contact with air, in two or three days.

"The results of this chemical investigation show that the chief
advantages of this system of putrefaction are:

"First.--The active agent, hydrated ferrous oxide, is prepared within
the sewage itself as a flocculent precipitate. (It is scarcely necessary
to add that the inorganic salts in solution are not increased, as in the