largely through forests and
past vegetable banks, takes up a large quantity of albuminous matter,
which is so great in quantity that the atmosphere, or the oxygen in the
air, cannot purify it by the time it reaches us, so that if any
astringent matter like oak, or birch, or beech, or even alum, is put in
the water it will cause the albumen to precipitate. In the district of
La Gironde, France, the waters of the Landes are naturally very impure
from these causes, but since the cutting and floating down of the
immense oak forests, the water has been made sweet and wholesome."

"Isn't all this curious and wonderful to think about?"

The work of preparing and putting into practical form the primary
electric battery was going forward steadily, and at the Professor's
suggestion a number of cells were made, which it might be well to
describe briefly.

As the clay was the only available material, each cell had to be made
rather heavy and clumsy in appearance, and was baked when completed.
Each was ten inches deep and three by six and a half inches within. The
electrodes, made of zinc, were each one-half inch thick, six inches
wide, and nine inches long. The copper electrodes were the same
dimensions, except that they were a quarter inch thick. These were stood
in the cell, a short distance apart, and held in position by means of
notched wooden blocks.

When all this was completed the cells were filled with sulphuric acid
that had been made from the copper ore. It was, of course, much diluted
with water, so as not to make it too strong.

"What is the object in making so many cells?"

"So as to get the voltage."

"Does the voltage depend on the number of the cells?"

"Each cell gives practically two volts, so that if we have 20 cells
there will be 40 volts; 30 cells, 60 volts, and so on."

"But where do the amperes come in?"

"That depends on the size of the plates forming the battery. Surface is
required for amperage, and quantity of plates for voltage."

"Suppose I had plates the size of this table, wouldn't I get more
electricity than if I had the plates cut up into smaller pieces?"

"Electricity means both volts and amperes. There is no such thing as
electricity with one of those qualities alone. A current may have 2
amperes and 40 volts, or 40 amperes and 2 volts. Multiplying the volts
by the amperes gives what is called watts, and there would be 80 watts
in each case."

[Illustration: _Fig. 27. Complete Bat