I have occasionally been astonished in the tropical climates of south
america, to observe the variable intensity of the zodiacal light. As i
passed the nights, during many months, in the open air, on the shores of
rivers and on ilanos, i enjoyed ample opportunities of carefully examining
this phenomenon. When the zodiacal light had been most intense, i have
observed that it would be perceptibly weakened for a few minutes, until it
again suddenly shone forth in full brilliancy. In some few instances i have
thought that i could perceive -- not exactly a reddish coloration, nor the
lower portion darkened in an arc-like form, nor even a scintillation, as
mairan affirms he has observed -- but a kind of flickering and wavering of
the light.*
[footnote] *Arago, in the 'Annuaire', 1832, p. 246. Several physical facts
appear to indicate that, in a mechanical separation of matter into its
smallest particles, if the mass be very small in relation to the surface,
the electrical tension may increase sufficiently for the production of light
and heat. Experiments with a large concave mirror have not hitherto given
any positive evidence of the presence of radiant heat in the zodiacal light.
(Lettre de M. Matthiessen ?? M. Arago, in the 'Comptes Rendus', t.
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