All 5 Uses of

solemn
in
Utopia, by Thomas More

To these things I would add that law among the Macarians—a people that live not far from Utopia—by which their king, on the day on which he began to reign, is tied by an oath, confirmed by solemn sacrifices, never to have at once above a thousand pounds of gold in his treasures, or so much silver as is equal to that in value.†
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solemn = very serious--possibly dignified

The jeweller is then made to give good security, and required solemnly to swear that the stone is true, that, by such an exact caution, a false one might not be bought instead of a true; though, if you were to examine it, your eye could find no difference between the counterfeit and that which is true; so that they are all one to you, as much as if you were blind.†

solemnly = with seriousness and dignity

only he made a solemn and severe law against such as should so far degenerate from the dignity of human nature, as to think that our souls died with our bodies, or that the world was governed by chance, without a wise overruling Providence: for they all formerly believed that there was a state of rewards and punishments to the good and bad after this life;†

solemn = very serious--possibly dignified

They despise and laugh at auguries, and the other vain and superstitious ways of divination, so much observed among other nations; but have great reverence for such miracles as cannot flow from any of the powers of nature, and look on them as effects and indications of the presence of the Supreme Being, of which they say many instances have occurred among them; and that sometimes their public prayers, which upon great and dangerous occasions they have solemnly put up to God, with assured confidence of being heard, have been answered in a miraculous manner.†

solemnly = with seriousness and dignity

When this is done, both priests and people offer up very solemn prayers to God in a set form of words; and these are so composed, that whatsoever is pronounced by the whole assembly may be likewise applied by every man in particular to his own condition.†

solemn = very serious--possibly dignified

Definitions:

(1) (solemn)
in a very serious (and often dignified) manner
(2) (meaning too rare to warrant focus)
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.