wall. He drew his hand over the surface of the panelling and uttered an exclamation as he felt an unmistakable doorknob.
"Hullo! I never noticed a door here before. I wonder where it leads to?"
"If it leads to the wine-cellar I'll give an unsolicited testimonial to your detective abilities right now!" laughed Ronnie. "Come on, let's see what sort of a tap the old boy kept."
"It's locked," said Hugh, tugging in vain at the handle.
"Try some of the keys that Shale gave you," suggested his friend. "If they fail we'll have to try a little gentle persuasion with the kitchen poker."
But there was no need for the burglarious proposals to be put into operation, for the lock clicked smoothly back when Hugh inserted the third key on the bunch.
"Ah-ha! the mystery deepens!" Ronnie exclaimed dramatically, as he peered through the open doorway. "Who would expect to find an up-to-date chemical laboratory in the wilds of Exmoor?"
Hugh nodded in silent agreement. The room in which they found themselves could have been used for no other purpose. The whole of one wall was covered with glass-fronted cupboards, and inside could be seen row upon row of jars, bottles and phials. Standing against another wall was a long, breast-high bench bearing an orderly array of retorts, test-tubes, scales and recording-instruments. A powerful electric battery stood in one corner, flanked, in the opposite angle of the room, by a large and very modern-looking safe. A roll-top desk and a filing-cabinet occupied the center of the room, and toward these Ronnie gave an expressive nod.
"There ought to be plenty of data for your investigations here," he observed with a smile. "There seem to be enough papers and memoranda to clear up a thousand mysteries. And the desk is not even locked—or the cabinet, either. See here!"
He thrust back the cover of the desk and began to rummage among the papers, only to give vent to a grunt of disappointment.
"Nothing that is likely to help us here," he declared. "Bills, invoices for chemicals and apparatus supplied—the old boy seems to have been a whale for experimental chemistry. Stop a moment, though!" he added suddenly as he opened the lowest drawer. "Here's something that may shed a little light on our darkness. Just run your eagle eye over these——"
Glancing at the official-looking documents which Ronnie spread on the desk, Hugh saw that one was a printed form bearing the royal arms at its head. It was an official certificate of discharge, and the words which had been filled in by hand intimated that MARLE, Silas James, had been employed in the INVESTIGATION BRANCH of THE RESEARCH LABORATORIES of the ROYAL ARSENAL, WOOLWICH, from April the 23rd, 1915, to October the 11th, 1918, being discharged therefrom at his own request. Another was a well-worn pass, enclosed in a leather case, authorizing the same MARLE, Silas James, to enter the area of the "Danger Buildings" at the royal arsenal.
"Evidently our friend was a retired expert in explosives," Ronnie remarked. "I don't think there's much to be gathered from these papers beyond that not very interesting fact."
Trenchard did not answer immediately. He was staring at the blue-gray papers, his mind working rapidly. At length he turned to Brewster with an unexpected question.
