Etymology
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plumbous (adj.)

1680s, "leaden;" 1854 in the chemistry sense of "containing lead" (especially in a low valence), from Latin plumbosus "full of lead," from plumbum (see plumb (n.)).

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ferrous (adj.)
"pertaining to or containing iron," 1865, from Latin ferreus "made of iron," from ferrum "iron" (see ferro-). In chemistry, "containing iron," especially with a valence of two. Contrasted with ferric.
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-ous 
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."
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nitrous (adj.)

c. 1600, "of nitre, pertaining to nitre," from Latin nitrosus, from nitrum (see nitre). The more precise use in chemistry (designating a compound in which the nitrogen has a lower valence than the corresponding nitric compound) is from 1780s. Middle English had nitrose "nitrous in quality; bitter, sour" (early 15c.). Nitrous oxide "laughing gas" is attested from 1800.

When inhaled it produces unconsciousness and insensibility to pain; hence it is used as an anesthetic during short surgical operations. When it is breathed diluted with air an exhilarating or intoxicating effect is produced under the influence of which the inhaler is irresistibly impelled to do all kinds of silly and extravagant acts; hence the old name of laughing-gas. [Century Dictionary, 1895]
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-ic 

Middle English -ik, -ick, word-forming element making adjectives, "having to do with, having the nature of, being, made of, caused by, similar to," from French -ique and directly from Latin -icus or from cognate Greek -ikos "in the manner of; pertaining to." From PIE adjective suffix *-(i)ko, which also yielded Slavic -isku, adjectival suffix indicating origin, the source of the -sky (Russian -skii) in many surnames. In chemistry, indicating a higher valence than names in -ous (first in benzoic, 1791).

In Middle English and after often spelled -ick, -ike, -ique. Variant forms in -ick (critick, ethick) were common in early Modern English and survived in English dictionaries into early 19c. This spelling was supported by Johnson but opposed by Webster, who prevailed.

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