In a leading article of the 10th of January, 1852, after an
account of the effects produced on water by radiation and the
protection afforded to plants by the ice with which ponds are
covered in winter, you go on to say that there are some
circumstances under which water-plants suffer greatly, and from a
singular cause, but one which when looked into is sufficiently
simple and intelligible. As you do not appear to have hit upon the
true reason, allow me to quote a little further, and then give my
reason for this singular effect.
You say that on a very fine but still night, water is cooled less
rapidly than the earth: under such circumstances the bottom of the
pond cools more rapidly than the surface, the plants become
colder--in fact, some degrees below the freezing-point, &c. &c.
I submit that such reasons are inadmissible, for there would be an
immediate upward current, which, as water is such an excellent
conductor of heat, would immediately equalize the temperature of
all the water above 40 degrees Fahrenheit, and stratified (if I
may use the expression) above the water of this temperature there
would be another layer of water of equal but gradually decreasing
temperature until it fell below 32 degrees Fahrenheit.
The explanation I offer is this. It is well known that if water is
kept perfectly still it may be cooled down considerably, or at
least some degrees below 32 degrees, without freezing; but the
moment it is shaken a portion of it is converted into a spongy,
porous ice, and the temperature rises to 32 degrees.
What may be the case in the rivers of the South of England I do
not know, but in the rapid streams of the North this process may
be seen on a very extensive scale in severe frosts. The water in
the still pools (before they are frozen over) is cooled down to
below 32 degrees, and so soon as this cooled water reaches the
next stream, precipitation (if I may so call it) takes place, and
the spongy ice lays hold of every projecting pebble, which serves
as a nucleus in the same way as threads and bits of stick serve in
the crystallization of salts. After a severe frost, when followed
by bright sunshine the next morning, I have seen thousands of
these bits of spongy ice rising from the stones to which they had
been attached to the surface of the water. I have seen after long-
continued frost the course of a stream completely altered by this
bottom-ice (as it is called here), and I have also seen a weir
with a wall of ice on it three feet high (raised in a single
night) by the same cause. Now apply this to the bottom-ice in
ponds (which however I must confess I never saw). The night being
calm, the water gets cool below 32 degrees, but then a breeze
springing up the water becomes agitated, precipitation takes
place, and the plants serving as nuclei become immediately clothed
with this spongy ice, and the sun shining next morning imparts so
much warmth to the plants that the ice thaws which is in contact
with them, and rises to the surface. Of course if the sun does not
shine next morning, and the frost continues, the plants may be
clothed with ice for a long time.
To the foregoing the Editor of the "Agricultural Gazette" replied
as follows:--
We cannot admit the soundness of our correspondent's explanation
of the formation of bottom-ice or ground _gore_. We are well
acquainted with the statements of Arago and other writers as to
the cause of this curious phenomenon, and after a careful
consideration of the subject believe that it is due to radiation
and not to any other cause. Bottom-ice has been observed in ponds
on perfectly still nights when there was no breeze to agitate the
surface of the water.
The waters in the pools between the rapids of rivers can hardly
ever be still enough for the water to fall below the freezing-
point and yet remain fluid; the temperature of water in such
situations is not below 33 degrees.