be pressed. The libraries that aim at supplying materials for research must necessarily view this process in a different light from the more popular collections. Thus the student of a science will, as a rule, take no account of obsolete editions, but to the historian of the science they are of a certain interest. Even the successive editions of school- books throw light on the progress of educational methods. The largest libraries, then, will only discard their duplicate copies. These may perhaps be more fairly called weeds, since, as a rule, their presence in a library is accidental. They are acquired in various ways. At one time a collection of books is offered by a dealer on condition of being bought en bloc, and a certain proportion will be in the library already. Donations of collections produce the same effect, and so do donations of single books, if the practice is adopted, as it often is, of never refusing a donation for fear of general discouragement among donors. Again, a printer's error, or a wrong alphabetical arrangement in the catalogues, will lead to the purchase of second copies. Again, unless the system of purchases is perfect, and administered with preternatural sagacity, two copies may be simultaneously ordered from different sources, and the error remain undiscovered till the Accession-catalogue reveals it. The duplicates thus acquired will sometimes be kept, the librarian persuading himself that after all he was timid in only ordering a single copy of a book likely to be largely used. The rest will be disposed of, but with certain precautions. A
