Definition
Special gravity is a technical-scientific term that entered legal usage primarily in the context of evidentiary and regulatory disputes requiring the measurement or characterization of physical substances. It refers to the ratio of the weight of a given substance to the weight of an equal volume of a reference substance — conventionally water for liquids and solids, and air for gases. In legal proceedings, the term functions as a term of art borrowed from chemistry and physics, deployed to describe or distinguish substances by their measurable density relative to a standard.
The term appears most frequently in cases involving the identification of commercial goods, the characterization of taxable commodities, the regulation of alcohol content, or the analysis of disputed substances in customs, revenue, and trade law. In these contexts, establishing the special gravity of a substance could be legally determinative — differentiating, for example, a taxable spirit from a non-taxable liquid, or a regulated chemical from an ordinary compound.
Common Language
Modern common usage (Wiktionary): The term "special gravity" does not appear as a standard entry in modern general dictionaries. The closely related term "specific gravity" is the dominant modern scientific usage, defined as the ratio of the density of a substance to the density of a reference substance, typically water at a specified temperature.
Historical common usage (Webster's 1913): Webster's 1913 similarly does not list "special gravity" as a distinct term but defines "specific gravity" in essentially the same manner as Bouvier: the ratio of the weight of a body to the weight of an equal volume of a standard substance.
The gap worth noting: In legal sources of the nineteenth century, "special gravity" and "specific gravity" appear to have been used interchangeably, with "special" reflecting an older or less standardized rendering of the concept. Researchers encountering "special gravity" in historical legal documents should treat it as functionally synonymous with "specific gravity" in modern scientific and legal usage. Confusion between the two phrasings can cause indexing and search problems in corpus research.
Common Confusion
SPECIAL GRAVITY vs. SPECIFIC GRAVITY: These terms are effectively interchangeable in the historical legal record. "Specific gravity" is the universally accepted scientific and modern legal term. "Special gravity" is an archaic or variant rendering found in older American and English legal sources. Treating them as distinct concepts would be an error. Researchers should search both phrasings when working with pre-twentieth-century materials.
Why It Matters in Research
The primary research trap here is terminological: a researcher using only "specific gravity" as a search term will miss sources that use the "special gravity" formulation, and vice versa. This is not a trivial discrepancy — legal databases and historical digests indexed terms as written, not as intended.
The term appears in revenue, customs, and excise law contexts, particularly in disputes over the classification and taxation of alcoholic beverages, oils, chemicals, and imported goods, where density measurements were used to determine the nature and value of a substance. Courts in the late nineteenth and early twentieth centuries regularly admitted or evaluated testimony about a substance's special or specific gravity as evidence going to classification or identity.
Bouvier's citation to 6 U.S. App. 58 anchors the term in federal appellate practice, suggesting its use was not merely theoretical but appeared in actual evidentiary disputes before federal courts. Researchers in trade law, excise law, and the history of chemical evidence should be alert to this term appearing in expert witness testimony and judicial opinions from this period.
The term has essentially no living legal usage as "special gravity" — modern regulatory, customs, and evidentiary frameworks uniformly use "specific gravity" or equivalent density measurements. Any current legal source using "special gravity" is either quoting older authority or reflects a drafting anomaly.
Historical Dictionary Support
Bouvier's Law Dictionary provides the sole historical legal dictionary treatment located in the Law Mind corpus. Bouvier defines the term precisely and correctly by the scientific standards of his era: the ratio of a body's weight to the weight of an equal volume of a reference substance, with water as the standard for solids and liquids, and air for gases. The citation to 6 U.S. App. 58 is notable — Bouvier anchors the definition in judicial usage rather than treating it as purely scientific vocabulary imported into law.
What historical dictionaries do not address: Neither Bouvier nor comparable period dictionaries explain the legal consequences that flowed from special gravity determinations — that is, how courts used the measurement to resolve disputes about substance identity or commodity classification. The scientific definition is faithfully recorded; the legal function is left to the case law itself.
Jurisdictional Note
No meaningful jurisdictional variation exists for the underlying scientific concept. Variation, where it appeared, concerned evidentiary standards for admitting expert testimony about density measurements, not the meaning of the measurement itself. Federal customs and excise courts were the primary venue for disputes in which special gravity was determinative.