Essentially a double-boiler, it was (and is) used in chemistry for processes that required gentle heating.
ᵻsˈɛnʃəli ɐ dˈʌbəlbˈɔɪlɚ, ɪt wʌz ænd ɪz jˈuːzd ɪn kˈɛmɪstɹi fɔːɹ pɹˈɑːsɛsᵻz ðæt ɹᵻkwˈaɪɚd dʒˈɛntəl hˈiːɾɪŋ.