Other binding materials, such as vellum, parchment, canvas and patent leathers of various kinds, are seldom required in libraries, and need not be considered further. Preparations for spreading on books to protect them may also be passed over, and also the continental and American habit of covering all books in manilla or other paper covers of uniform colour. Most of the so-called leather “preservatives” are hurtful rather than helpful, but it may be observed that some leather bindings which get dry and worn will improve if treated with ordinary vaseline. It should be rubbed well and plentifully into the texture of the leather with the fingers, and when it has soaked in, should be wiped with a soft cloth. Vaseline is as good as any patent or other preservative for reviving decaying and shabby leathers of all kinds, although it dries quickly, and furniture polish has also been recommended. As regards covers, the time has not yet come when the individuality of a book, as issued by its publisher, or given by its appropriate library binding, requires to be hidden under a paper mask. =338. Class Colours.=--In systematically classified libraries there is a certain amount of advantage to be gained by re-binding each class of books, as required, in some appropriate colour. When open access to the shelves is also granted, there is a very considerable aid to the maintenance of order given by the use of distinctive class colours. Thus, Science may be light brown or fawn colour; Fine Arts, orange; Social Science, light green; Theology, etc., black; History and Travel, dark green; Biography, maroon; Philology, light blue; Poetry, red; Fiction, dark brown; and Juvenile, yellow; and so on to any degree. A variation of this plan is to vary the colours for authors, binding each distinctively; this gives a not unpleasing variety to the shelves and has a certain arranging value.