All 4 Uses

solemn
in
Incidents in the Life of a Slave Girl
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Pious Mr. Pike brushed up his hair till it stood upright, and, in deep, solemn tones, began: "Hearken, ye servants!†
Chpt 13

solemn = very serious--possibly dignified
But first you must solemnly promise that my name shall never be mentioned.†
Chpt 18 *

solemnly = with seriousness and dignity
He laid his hand on her head, and said, solemnly, "Ellen, this is the secret you have promised grandmother never to tell.†
Chpt 27
But in a calm, solemn way, for it cost me great effort, I said, "Listen to me, Ellen; I have something to tell you!"†
Chpt 39

solemn = very serious--possibly dignified

Definitions:

(1) (solemn)
in a very serious (and often dignified) manner
(2) (meaning too rare to warrant focus)
Less commonly, solemn can mean that something was done with great or appropriate ceremony. It can also be used to describe something as dark or undecorated.