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asphyxiated. A man can walk about upright without danger, but if he were to kneel down or stoop below the level of the gas, he would be asphyxiated in like manner."

"Very interesting from a scientific point of view. But I don't see the connection with thief-catching."

"Just this; I propose to turn your strong room into an artificial Grotta del Cane."

"Much obliged to you, I'm sure. And suffocate our chief cashier, or myself, the first time we go into the room!"

"Not at all. The gas will not be generated till the thieves are actually in the strong room. To operate upon the safes, they must of necessity work at a low level. The gas will rise by degrees, as the bottom of the cellar fills. Their only warning will be a slight difficulty in breathing, which (if they notice it) they will put down to the closeness of the vault. A little later they will find they can't breathe at all; but by that time it will be too late, and they will fall insensible."

"Good heavens! you would not kill them?"

"No; my intentions are not quite so bloodthirsty as that. We shall lug them out, and bring them to life again by one or other of the artificial respiration processes. First, however, we shall call in a policeman or two to look after them during convalescence."

"And where is the gas to come from?"

"That's very plain sailing; I shall generate it when wanted, pro re natâ, as we doctors say. In the first place, I shall cover the floor of the strong room, to a depth of two or three inches, with a mixture of sawdust and ordinary washing soda, which is a coarse form of sodium bicarbonate. I suppose you can get me half a hundredweight without any difficulty?"

"I daresay I could, but it is a queer order for a bachelor to give. My oilman will think I am going to do my own washing."

"Never mind what your oilman thinks! Then I shall want half a gallon or so of rough sulphuric acid, commonly known as oil of vitriol. Lastly, an empty beer or wine cask to hold the diluted acid, and a few yards of soft metal tubing, such as gasfitters use. This tubing, first punctured freely with holes, will be embedded in the soda and connected with the barrel. At the right moment we turn on the diluted acid, and the strong room will be half full of carbonic acid gas in ten minutes."

"But if you mix an acid and an alkali, won't there be a warning fizz?"

"Very little. The sound you hear on mixing a seidlitz powder is mainly caused by the small area within which the effervescence is confined. In an open space, like the floor of a cellar, it will be barely perceptible, and I shall further diminish it by sifting fine earth all over the soda, which will make all look ship-shape, while it won't interfere in the least with the chemical process. The sawdust mixed with the soda is to prevent the gas being generated too rapidly."

"You seem to have worked out your scheme pretty minutely."

"I have, to the smallest detail. I laid awake half the night thinking it out. The only risky element will be getting the rascal (or rascals) out of the strong room afterwards. Carbon dioxide is no respecter of persons, and will knock us over as readily as a bank burglar. However, by using due caution, and holding our breaths while we have to stoop, we may venture in far enough to slip a cord round the body of each fellow, and then we can drag him out from a safe distance."

I was carried away by Macpherson's enthusiasm, and after a little further conversation I agreed, though somewhat against my better judgment, to let him try his plan. He set to work at once, and before midnight of the same day his arrangements were completed. The sulphuric acid, diluted with water to four gallons, and contained in an old wine cask, was placed in a cupboard in my private office. The tap communicated with an indiarubber tube, and this with sundry lengths of composition pipe, perforated at intervals, which were lying, embedded in soda and sawdust, on the floor of the strong room. Above this was sprinkled a layer of fine earth, restoring the floor to its ordinary cellar-like appearance. The mysterious knocking was resumed from eight o'clock till 1 a.m., but the operator did not seem to make any perceptible advance.

Six days passed without any change of the situation, save that the sound of the excavations in the cellar became daily more audible, showing that the intervening wall was growing thinner. By careful observation of the sound we satisfied ourselves that the concealed operator was working at a space of wall some two feet square, probably intending, when this was sufficiently reduced in substance, forcibly to break away the thin remaining partition.

On the seventh day, however, I had a visit from the Count. He had that morning