Gas

These are the more patent facts which are to be deduced from his hat. Also, by the way, that it is extremely improbable that he has gas laid on in his house."

(Adventure VII, pg. 90)

All the energy used to light the streets and houses came from gas.

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Chronology of the UK gas industry

Oil lamp

Ordering my cab to wait, I passed down the steps, worn hollow in the centre by the ceaseless tread of drunken feet; and by the light of a flickering oil-lamp above the door I found the latch and made my way into a long, low room, thick and heavy with the brown opium smoke, and terraced with wooden berths, like the forecastle of an emigrant ship.

(Adventure VI, pg. 73)

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History of oil lamps

Lantern

This also was opened, and led down a flight of winding stone steps, which terminated at another formidable gate. Mr. Merryweather stopped to light a lantern, and then conducted us down a dark, [...]

(Adventure II, pg. 30)

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- History of lighting

Lamps

"It was a quarter past six when we left Baker Street, and it still wanted ten minutes to the hour when we found ourselves in Serpentine Avenue. It was already dusk, and the lamps were just being lighted as we paced up and down in front of Briony Lodge, waiting for the coming of its occupant."

(Aventure I, A SCANDAL IN BOHEMIA pg. 13)

The first public street lighting with gas was demonstrated in Pall Mall, London on January 28, 1807 by Frederick Albert Winsor, and it's mechanicsm was used for building all sorts of lamps throughout the entire Victorian Era. The street gas lights worked the same way as the house gas lights where the flame from the lighted gas heated up a mantle which became incandescent and gave out light.