Definition
A Latin term appearing in two distinct legal contexts with unrelated meanings:
1. (Civil law) A lake; a permanent body of standing water that does not seasonally dry up. Roman law distinguished lacus from other water bodies by its permanence, treating it as a receptacle of water that is never dry. This classification carried practical consequences for questions of water rights, public access, and riparian ownership under Roman and civil law traditions.
2. (Old English law) An alloy of silver mixed with base metal; the dilution or debasement of silver coinage or silver goods by admixture with inferior metals. In this sense, lacus is a Latinized form of the vernacular term "alloy" or "allay," appearing in medieval legal texts governing the purity of silver and the regulation of metalworking.
---
Common Language
Modern common usage (Wiktionary): Lacus is a Latin word meaning "lake" or "pool," used in modern scientific nomenclature — particularly in astronomy and planetary science — to designate lake-like features on planetary bodies.
Historical common usage (Webster's 1913): Not separately defined; the term is treated as a Latin root underlying English derivatives such as "lake" and "lagoon."
The common meaning tracks only the first legal definition. The metallurgical sense — silver alloyed with base metal — has no counterpart in common or scientific usage and is entirely a product of medieval English legal and administrative vocabulary. A researcher encountering lacus in a text about coinage, assay, or silversmithing should not assume the hydrological meaning.
---
Common Confusion
The two definitions share no conceptual connection. A reader moving between civil law texts (where lacus means a lake) and medieval English legal texts (where lacus means debased silver) may not recognize that the same word is doing completely different work. Burrill explicitly flags that this metallurgical usage is not found in Spelman or Cowell, the standard glossaries of old English law — meaning the term can be genuinely difficult to trace through historical reference tools. Black's first edition compounds the confusion by including a stray passage about cargo shipping in the lacus entry, apparently a typographical or editorial error unconnected to either definition.
---
Why It Matters in Research
Researchers working in the Law Mind corpus should approach lacus as a disambiguation problem before proceeding. Context — civil law versus old English law — is the primary sorting mechanism.
In civil law materials, lacus functions within Roman water law classifications. The Digest citation (43.14.1.3) places it in the title governing rivers, streams, and related waters, which concerned public versus private rights and the praetorian interdicts protecting navigation and water use. A researcher tracing water rights, riparian doctrine, or natural resource law in civilian jurisdictions should understand that lacus represents the permanent-lake category, distinct from stagnant pools or seasonal waters.
In old English legal materials, lacus as alloy appears in the context of assay law, coinage regulation, and the control of silver purity — matters relevant to monetary history, mint regulation, and commercial law governing precious metals. Fleta (lib. 1, c. 22, § 6) is the primary anchor for this usage. Because Burrill notes that neither Spelman nor Cowell records this metallurgical sense, researchers relying on standard old English legal glossaries may reach a dead end; Fleta itself must be consulted directly.
The garbled passage in Black's first edition — apparently referencing bulk cargo shipping — should be disregarded as an editorial artifact and not treated as a third definition. Black's second edition corrects this.
---
Historical Dictionary Support
The four source dictionaries substantially agree on both definitions, though with minor variation in the Digest citation: Black's second edition and Burrill cite Dig. 43.14.1.3, while Bouvier cites Dig. 48.14.1.8 — a discrepancy that likely reflects a transcription error in one tradition, as the relevant Digest title on water and river interdicts is Title 43.14, not 43 [or 48]. Researchers should treat the Bouvier citation with caution and verify against the Digest directly.
All sources that address the metallurgical sense anchor it to Fleta, lib. 1, c. 22, § 6, and Burrill adds the useful research note that the term in this sense escapes both Spelman and Cowell — a rare instance of a historical dictionary providing explicit negative guidance about the limits of other reference tools. This kind of meta-commentary is valuable and should not be overlooked.
Black's first edition is the outlier, interpolating what appears to be text about loose-cargo shipping that belongs to a different entry entirely. This is most likely a printing or binding error in that edition. No other source supports a third, cargo-related meaning for lacus.
---
Jurisdictional Note
The civil law definition of lacus is most relevant to research in jurisdictions with civilian or mixed legal traditions — Louisiana, Quebec, Scotland, South Africa, and Latin American systems — where Roman water law classifications retained influence. The old English metallurgical definition is primarily of historical interest to English legal history and is unlikely to appear in operative modern law of any jurisdiction.
---