The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
Nouns wanting in the Plural.
99. Some nouns are ordinarily found in the Singular
number only (singulária tantum). These are -
1. Most proper names: as, Caesar, Caesar; Gallia, Gaul.
2. Names of things not counted, but reckoned in mass: as, aurum, [QUERY] áér, air; tríticum, wheat.
3. Abstract nouns: as, ambitió, ambition; fortitúdó, courage;