As I spoke I lit the lamp again and held it just above a tiny fissure in the rock. It was at once extinguished. "What is it?" said Bainbridge, panting with excitement. "Something that nearly finished _me_," I replied. "Why, this is a natural escape of choke damp. Carbonic acid gas--the deadliest gas imaginable, because it gives no warning of its presence, and it has no smell. It must have collected here during the hours of the night when no train was passing, and gradually rising put out the signal light. The constant rushing of the trains through the cutting all day would temporarily disperse it." As I made this explanation Bainbridge stood like one electrified, while a curious expression of mingled relief and horror swept over Henderson's face. "An escape of carbonic acid gas is not an uncommon phenomenon in volcanic districts," I continued, "as I take this to be; but it is odd what should have started it. It has sometimes been known to follow earthquake shocks, when there is a profound disturbance of the deep strata." "It is strange that you should have said that," said Bainbridge, when he could find his voice. "What do you mean?" "Why, that about the earthquake. Don't you remember, Henderson," he added, turning to the Inspector, "we had felt a slight shock all over South Wales about three weeks back?" "Then that, I think, explains it," I said. "It is evident that Pritchard really did climb the rocks in a frantic attempt to escape from the gas and fell back on to these boulders. The other man was cut down at once, before he had time to fly." "But what is to happen now?" asked Bainbridge. "Will it go on for ever? How are we to stop it?" "The fissure ought to be drenched with lime water, and then filled up; but all really depends on what is the size of the supply and also the depth. It is an extremely heavy gas, and would lie at the bottom of a cutting like water. I think there is more here just now than is good for us," I added. "But how," continued Bainbridge, as we moved a few steps from the fatal spot, "do you account for the interval between the first death and the second?" "The escape must have been intermittent. If wind blew down the cutting, as probably was the case before this frost set in, it would keep the gas so diluted that its effects would not be noticed. There was enough down here this morning, before that train came through, to poison an army. Indeed, if it had not been for Henderson's promptitude, there would have been another inquest--on myself." I then related my own experience. "Well, this clears Wynne, without doubt," said Bainbridge; "but alas! for the two poor fellows who were victims. Bell, the Lytton Vale Railway Company owe you unlimited thanks; you have doubtless saved many lives, and also the Company, for the line must have been closed if you had not made your valuable discovery. But now come home with me to breakfast. We can discuss all those matters later on." IV THE EIGHT-MILE LOCK