nitre

Concise Bible Dictionary:

The natron of the moderns, not what is now called nitre, which is saltpeter. As vinegar upon natron or alkali (which would effervesce and evaporate) so is the unsuitableness of singing mirthful songs to a heavy heart (Prov. 25:2020As he that taketh away a garment in cold weather, and as vinegar upon nitre, so is he that singeth songs to an heavy heart. (Proverbs 25:20)). It is a mineral alkali, and with oil is made into soap (Jer. 2:2222For though thou wash thee with nitre, and take thee much soap, yet thine iniquity is marked before me, saith the Lord God. (Jeremiah 2:22)).

Strong’s Dictionary of Hebrew Words:

Transliteration:
nether
Phonic:
neh’-ther
Meaning:
from 5425; mineral potash (so called from effervescing with acid)
KJV Usage:
nitre