Ex. Poisons and Toxicology; Antiquities and Archæology; Insects and Entomology; Warming and Heating; Pacific Ocean and South Sea. There are some cases in which separate headings (Hydraulics and Mechanics of Fluids), which can not be combined, cover books almost identical in character, so that the inquirer must look under both. This is an evil; but there is no reason for increasing the evil by separating headings that are really synonymous, certainly not for dividing a subject in this way for verbal causes and giving no hint that it has been divided.
It sometimes happens that a different name is given to the same subject at different periods of its history. When the method of study of the subject, or its objects, or the ideas connected with it, are very different at those two periods (as in the case of Alchemy and Chemistry), of course there must be two headings. There is not so much reason for separating Fluxions and Differential calculus, which differ only in notation. And there is no reason at all for separating Natural Philosophy and {50} Physics. I am told that medical nomenclature has changed largely three times within the present century. How is the cataloguer, unless he happens to be a medical man, to escape occasionally putting works on one disease under three different heads?
To arrive at a decision in any case one must balance the advantages on the one hand of having all that relates to a subject together, and on the other of making that economical conjunction of title-entry and of subject-entry which comes from following the titles of the books in selecting names for their subjects.
In choosing between synonymous headings prefer the one that—
(a) is most familiar to that class of people who consult the library; a natural history society will of course use the scientific name, a town library would equally of course use the popular name—Butterflies rather than Lepidoptera, Horse rather than Equus caballus. But the scientific may be preferable when the common name is ambiguous or of ill-defined extent.
(b) is most used in other catalogues.
(c) has fewest meanings other than the sense in which it is to be employed.
(d) comes first in the alphabet, so that the reference from the other can be made to the exact page of the catalogue.
(e) brings the subject into the neighborhood of other related subjects. It is, for instance, often an advantage to have near any art or science the lives of those who have been famous in it; as, Art, Artists; Painters, Painting; Historians, History. If one were hesitating between Conjuring, Juggling, Legerdemain, Prestidigitation, and Sleight of hand, it would be in favor of Conjuring or Prestidigitation that one could enter by their side Conjurors or Prestidigitators.
Sometimes one and sometimes another of these reasons must prevail. Each case is to be decided on its own merits.