The mildew referred to is that which shows itself in the form of
roundish or irregular brown spots.

It is usually most abundant in those parts which are most exposed to the
air.

In making a microscopic examination of the spots, I ascertained that
there was no new structure present; but in manipulating I found that
these spots absorbed water more rapidly than the rest of the paper.

On applying litmus, these spots were found to have a powerful acid
reaction.

On submitting the matter to a chemical friend, he ascertained that the
acid in question was the sulphuric, or oil of vitriol. Experiments were
then made with a dilute solution of this acid on {237} clean paper, and
spots were produced similar to those of mildew.

The acid does not naturally exist in paper, and its presence can only be
accounted for by supposing that the paper has been bleached by the fumes
of sulphur. This produces sulphurous acid, which, by the influence of
atmospheric air and moisture, is slowly converted into sulphuric, and
then produces the mildew. As this may be shown to be an absolute
_charring_ of the fibres of which the paper is composed, it is to be
feared that it cannot be cured. After the process has once commenced, it
can only be checked by the utmost attention to dryness, moisture being
indispensable to its extension, and vice versâ.

I do not know whether these facts are generally known, but they would