A third enemy of the book is heat. Most libraries are unfortunately over-heated,--sometimes from defective means of controlling the temperature, and sometimes from carelessness or want of thought in the attendant. A high temperature is very destructive to books. It warps their covers, so that volumes unprotected by their fellows, or by a book support, tend to curl up, and stay warped until they become a nuisance. It also injures the paper of the volumes by over-heating, and weakening the tenacity of the leaves held together by the glue on the back, besides drying to an extreme the leather, till it cracks or crumbles under the heat. The upper shelves or galleries of any library are most seriously affected by over-heating, because the natural law causes the heat to rise toward the ceiling. If you put your hand on some books occupying the highest places in some library rooms, in mid-winter, when the fires are kept at their maximum, the heat of the volume will almost burn your fingers. If these books were sentient beings, and could speak, would they not say--"our sufferings are intolerable?" The remedy is of course a preventive one; never to suffer the library to become over-heated, and to have proper ventilation on every floor, communicating with the air outside. Seventy degrees Fahrenheit is a safe and proper maximum temperature for books and librarian. The mischief arising from gas exhalations is another serious source of danger to books. In many well-lighted libraries, the heat itself from the numerous gas-burners is sufficient to injure them, and there is besides a sulphuric acid escaping from the coal-gas fluid, in combustion, which is most deleterious to bindings. The only remedy appears to be, where libraries are open evenings, to furnish them with electric lights. This improved mode of illumination is now so perfected, and so widely diffused, that it may be reckoned a positive boon to public libraries, in saving their books from one of their worst and most destructive enemies.