The sensibilities of ignorant or superstitious people have at various
times been alarmed by the different phenomena of so-called blood, ink,
or sulphur rains. Ehrenberg very patiently collected records of the most
prominent instances of these, and published them in his treatise on the
dust of trade winds. Some, it is known, are due to soot; others, to
pollen of conifers or willows; others, to the production of fungi and
algae.
Many of the tales of the descent of showers of blood from the clouds
which are so common in old chronicles, depends, says Mr. Berkeley, the
mycologist, upon the multitudinous production of infusorial insects or
some of the lower algae. To this category belongs the phenomenon known
under the name of "red snow." One of the most peculiar and remarkable
form, which is apparently virulent only in very hot seasons, is caused
by the rapid production of little blood-red spots on cooked vegetables
or decaying fungi, so that provisions which were dressed only the
previous day are covered with a bright scarlet coat, which sometimes
penetrates deeply into their substance. This depends upon the growth of
a little plant which has been referred to the algae, under the name
of _Palmellae prodigiosa_. The rapidity with which this little plant
spreads over meat and vegetables is quite astonishing, making them
appear precisely as if spotted with arterial blood; and what increases
the illusion is, that there are little detached specks, exactly as if
they had been squirted from a small artery. The particles of which the
substance is composed have an active molecular motion, but the morphosis
of the production has not yet been properly observed. The color of the
so-called "blood rain" is so beautiful that attempts have been made
to use it as a dye, and with some success; and could the plant be
reproduced with any constancy, there seems little doubt that the color
would stand. On the same paste with the "blood-rain" there have been
observed white, blue, and yellow spots, which were not distinguishable
in structure and character.
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