Here’s the completed text with the appropriate articles inserted:

1. I remarked that if there was **a** leak in **the** gas pipe there might be **an** explosion while he was reading **the** meter.
2. He said, 'As **a** matter of **fact**, there was **an** explosion in **the** last house I visited; and Mr. Smith, **the** owner of **the** house, was burnt in **the** face.'
3. 'Mr. Smith was holding **a** lighted match at **the** time of **the** explosion.'
4. To prevent **a** possible repetition of this accident, I lent him **a** torch.
5. He switched on **the** torch, read **the** meter and wrote **the** reading down on **the** back of ... envelope.

(Note: In the original text, "back of . . .envelope" seems incomplete, but I inserted "the" before "back" as it seems to imply a specific envelope.)