"Something of equivalent force, effect, or weight on the opposite side, that which serves as a counterbalance or set off" (OED).

"A piece of fragment of a brick... It is the typical ready missile, where stones are scarce" (OED).

"Any of various lightweight cotton fabrics in plain weave. Also: a piece of such fabric; a dress or other article of clothing made of muslin" (OED).

From the 17th century to the late 18th century, muslin fabric was mostly imported from places like India. The fabric was used for dresses and curtains and was notably well liked for its simplicity; its ability to drape beautifully; and for the fabric's ability to take paint, dyes, and embroidery very well. Muslins were mostly worn by gentility in the color white. The color white was used to signify the gentility's wealthy lifestyle because white garments were harder to keep clean and were very expensive to constantly have laundered to maintain the pure white color. (Jane Austen’s World)

Here is an example of a sprigged muslin which is named for the muslin's unique design which resembles sprigs of leaves or flowers all over the dress:

The names Emily and Valancourt are reference to characters from the gothic romance novel Mysteries of Udolpho by Mrs. Ann Radcliffe, which was published in 1794. Emily is the heroine of the novel who goes through misfortunes after the death of her father. Valancourt is a traveller who Emily falls in love with while traveling with her father. After her father's death, Emily is under the guardianship of her aunt Madame Cheron who tries to keep Valancourt and Emily from being together and eventually marrying each other. Madame Cheron goes as far as to take Emily away with her to Italy to be rid of Valancourt. Valancourt asks Emily to marry him in secret, but Emily refuses and leaves him to go with her aunt to Italy. (Regency History)

Here is a novel cover of Anne Radcliffe's Mysteries of Udolpho:

"A term of reproach to women" (Regency Assembly Press).