deliquesce
To melt by absorbing water and disappearing.
intransitive
But I could find no saltpetre; indeed, no nitrates of any kind. Doubtless they had deliquesced ages ago.
To become liquid by absorbing water from the atmosphere and dissolving in it.
intransitive
Some salts will deliquesce when left exposed.
Where it comes from
From Latin dēliquēscō, from dē- + liquēscō (“to liquefy”).
Adapted from the Wiktionary entry deliquesce (CC BY-SA 4.0).