As the number of books in libraries have steadily increased since their inception, the need for compact storage and access with adequate lighting has grown. The "stack system" involves keeping a library's collection of books in a space separate from the reading room. This arrangement arose in the 19th century. Book stacks quickly evolved into a fairly standard form in which the cast iron and steel frameworks supporting the bookshelves also supported the floors, which often were built of translucent blocks to permit the passage of light (but were not transparent, for reasons of modesty). The introduction of electrical lighting had a huge impact on how the library operated. The use of glass floors was largely discontinued, though floors were still often composed of metal grating to allow air to circulate in multi-story stacks. As more space was needed, a method of moving shelves on tracks (compact shelving) was introduced to cut down on otherwise wasted aisle space.