Under the microscope a mould-spot is seen to be a miniature forest of lovely trees, covered with a beautiful white foliage, upas trees whose roots are embedded in the leather and destroy its texture.
Inside the book, damp encourages the growth of those ugly brown spots which so often disfigure prints and "livres de luxe." Especially it attacks books printed in the early part of this century, when paper-makers had just discovered that they could bleach their rags, and perfectly white paper, well pressed after printing, had become the fashion.

This paper from the inefficient means used to neutralise the bleach, carried the seeds of decay in itself, and when exposed to any damp soon became discoloured with brown stains.

Dr.Dibdin's extravagant bibliographical works are mostly so injured; and although the Doctor's bibliography is very incorrect, and his spun-out inanities and wearisome affectations often annoy one, yet his books are so beautifully illustrated, and he is so full of personal anecdote and chit chat, that it grieves the heart to see "foxey" stains common in his most superb works.
In a perfectly dry and warm library these spots would probably remain undeveloped, but many endowed as well as private libraries are not in daily use, and are often injured from a false idea that a hard frost and prolonged cold do no injury to a library so long as the weather is dry.
The fact is that books should never be allowed to get really cold, for when a thaw comes and the weather sets in warm, the air, laden with damp, penetrates the inmost recesses, and working its way between the volumes and even between the leaves, deposits upon their cold surface its moisture.

The best preventative of this is a warm atmosphere during the frost, sudden heating when the frost has gone being useless.
Our worst enemies are sometimes our real friends, and perhaps the best way of keeping libraries entirely free from damp is to circulate our enemy in the shape of hot water through pipes laid under the floor.

The facilities now offered for heating such pipes from the outside are so great, the expense comparatively so small, and the direct gain in the expulsion of damp so decided, that where it can be accomplished without much trouble it is well worth the doing.
At the same time no system of heating should be allowed to supersede the open grate, which supplies a ventilation to the room as useful to the health of the books as to the health of the occupier.