downy sward of infinitesimal pond weed. I count on this modest
vegetation to keep the water in a reasonably wholesome state,
without driving me to frequent renewals which would disturb the
work of my colonies. Sanitation and quiet are the first conditions
of success. Now the stocked pond will not be long in filling
itself with gases unfit to breathe, with putrid effluvia and other
animal refuse; it will become a sink in which life will have killed
life. Those dregs must disappear as soon as they are formed, must
be burnt and purified; and from their oxidized ruins there must
even rise a perfect life-giving gas, so that the water may retain
an unchangeable store of the breathable element. The plant effects
this purification in its sewage farm of green cells.

When the sun beats upon the glass pond, the work of the water weeds
is a sight to behold. The green-carpeted reef is lit up with an
infinity of scintillating points and assumes the appearance of a
fairy lawn of velvet, studded with thousands of diamond pin's
heads. From this exquisite jewelry pearls break loose continuously
and are at once replaced by others in the generating casket; slowly
they rise, like tiny globes of light. They spread on every side.
It is a constant display of fireworks in the depths of the water.

Chemistry tells us that, thanks to its green matter and the
stimulus of the sun's rays, the weeds decompose the carbonic acid
gas wherewith the water is impregnated by the breathing of its
inhabitants and the corruption of the organic refuse; it retains
the carbon, which is wrought into fresh tissues; it exhales the
oxygen in tiny bubbles. These partly dissolve in the water and
partly reach the surface, where their froth supplies the atmosphere
with an excess of breathable gas. The dissolved portion keeps the