Numbers 5:17 - Matthew Poole's English Annotations on the Holy Bible

Holy water; either water out of the holy laver, Exodus 30:18, or rather the water of purification appointed for such kind of uses, Numbers 19:9. This was used, that if she were guilty, she might be afraid to add profaneness and the pollution of holy things to her other crime. In an earthen vessel; either to signify that frailty and vileness of which she stood accused, or express her sorrowful and shameful condition, or because, after this use, it was to be broken in pieces, that the remembrance of it might be blotted out as far as possible. Compare Leviticus 6:28, Leviticus 11:33 15:12. And of the dust; all emblem of vileness and misery, as appears from Job 2:12 Psalms 22:15 Lamentations 3:29; and the serpent's food, Genesis 3:14; very proper for her who had been seduced to folly by the serpent's instigation. In the floor of the tabernacle; which made it holy dust, and struck the greater terror into the woman, if she were guilty. Put it into the water, to make it more unpleasant and bitter, which was suitable to one in that sorrowful state.