"the butterfly-shaped gas flames posed on the ends of the suspended T-bracket"

A 19th century gas lamp
GNU Free Documentation LicenseA 19th century gas lamp - Credit: Roby

Following the introduction of gas brackets to the Houses of Parliament in 1859, gas became a popular method for lighting the home. Burners produced fan or butterfly-shaped flames (a bit like the safe flame from a Bunsen burner).

The Verloc’s home is lit downstairs by piped gas. This is probably channelled into the burners through simple brass, copper or iron pipes and the burner may be topped with a glass tube to shield the flame. These can sometimes be very ornate.