She pondered the matter, drumming on the table with her impatient fingers. “But how is all this going to put us inside that eight-ton safe?” “That’s the trouble we’ve got to face!” he laughed back at her. “But haven’t you thought of anything, candidly?” “Yes, I have. I’ve been cudgeling my brains until I feel light-headed. Now, nitro-glycerine I object to, it’s so abominably crude, and so disgustingly noisy.” “And so odiously criminal!” she interpolated. “Precisely. We’re not exactly yeggmen yet. And it’s brain we’ve got to cudgel, and not safe-doors! I mean, now that we really are mixed up in this sort of thing, it’s better to do it with as clean fingers as possible. Now, once more, speaking as an expert, by lighting a small piece of sulphur, and using it as a sort of match to start and maintain combustion, I could turn on a stream of liquid oxygen and burn through that safe-steel about the same as a carpenter bores through a pine board. But the trouble is in getting the oxygen. Then, again, if it was a mere campaign of armour against the intruder, I could win out in quite a different way. I could take powdered aluminum, mixed with some metallic superoxide, such as iron-rust, and get what you’d call thermit. Then I could take this thermit, and ignite it by means of a magnesium wire, so that it would burn down through three inches of steel like a handful of live coals through three inches of ice. That is, if we wanted to be scientific and up-to-date. Or, even a couple of gallons of liquid air, say, poured on the top of the safe, ought to chill the steel so that one good blow from a sledge would crack it.” “But that, again, is only what cracksmen do, in a slightly different way!” “But, of course, by tapping an exceptionally strong power-circuit somewhere in the neighborhood, I could fuse portions of the steel with electricity, and then cut it away like putty. Yet all that, you see, is not only mechanical and coarse, and full of drawbacks, but it’s doing what we don’t want to do. It’s absolutely ruining a valuable deposit-vault, and might very well be interpreted as and called a criminal destruction of property. We have no moral and legal right to smash this gentleman’s safe. But in that safe lies a stone to which he has neither moral nor legal right, and it’s the stone, and only the stone, that we want.” “Then what are we to do?” “Use these thick heads of ours, as we ought. We must _think_, and not _pound_ our way into that vault. I mean, Frank, that we have got to get at that stone as Ottenheimer himself would!” They looked at each other for a minute of unbroken silence, the one trying to follow the other’s wider line of thought. “Well, there is where our test comes in, I suppose,” said Frances, valiantly, feeling for the first time a little qualm of doubt. Durkin, who had been plunged in thought, turned to her with a sudden change of manner. “You’re a bad lot, Frank!” he said, warmly, catching her frail-looking hands in his own.