What is "lapis infernalis"

Infernal \In*fer"nal\, a. [F. infernal, L. infernalis, fr. infernus that which lies beneath, the lower. See Inferior.]

1. Of or pertaining to or suitable for the lower regions, inhabited, according to the ancients, by the dead; pertaining to Pluto's realm of the dead, the Tartarus of the ancients.

   The Elysian fields, the infernal monarchy.
   --Garth.

2. Of or pertaining to, resembling, or inhabiting, hell; suitable for hell, or to the character of the inhabitants of hell; hellish; diabolical; as, infernal spirits, or conduct.

   The instruments or abettors in such infernal dealings.
   --Addison.

   Infernal machine, a machine or apparatus maliciously designed to explode, and destroy life or property.

   Infernal stone ( lapis infernalis), lunar caustic; formerly so called. The name was also applied to caustic potash.

   Syn: Tartarean; Stygian; hellish; devilish; diabolical; satanic; fiendish; malicious.

n. Fused silver nitrate; lunar caustic.