Great Expectations: Question about a Passage

My question has to do with a quote in Chapter 33 on page 270. Pip says, "I should have been chary of discussing my gaurdian too freely even with her, but I should have gone on with the subject so far as to describe the dinner in Gerrard Street, if she had not then come into a sudden glare of gas. It seemed, while it lasted, to be all alight and alive with that inexplicable feeling I had had before; and when we were out of it, I was as much dazed for a few moments as if I had been in lightning." What is this gas and why is Pip so startled by it that it brings back his past feelings?