A sand bath, as most of my readers know, is a bowl-shaped vessel filled with sand in which fragile glass flasks are placed in order to insure an even heat. A bunsen burner under the sand bath heats the sand, and, through the sand, the flask and its contents. Redfield had just lighted his burner and was busy weighing out his chemicals. Dunn passed behind him, and directing his attention to something across the room, tucked a match-end into the sand in Redfield’s bath and went on to his own table. Scarcely three minutes had elapsed when the half-dozen lads who had been watching furtively over their work heard a slight explosion, followed immediately by an exclamation from Redfield, who went crashing back on the row of tables behind. At the same time they beheld a small geyser of popping sand spurt into the air and descend in a shower about the burner. Mr. Cary rushed to the spot, likewise all the boys, both those who were in the secret and those who were not. “Go back to your work!” ordered the teacher, and the boys slunk away, though not beyond earshot. “What’s this, Redfield?” he asked sharply. The victim of the explosion, having recovered from his fright, stood giggling with nervousness. “My sand blew up, sir,” he said. “Do you know what made it do so?” demanded Mr. Cary, sternly. “No, sir. I was standing right here waiting for the thing to heat. It went off all of a sudden, right up in the air, and kept snappin’ all the way up.” “And you know absolutely nothing more about it?” “Not a thing!” answered Redfield, with evident honesty. “I wouldn’t blow myself up if I could help it.” There seemed no reason to doubt the truth of Redfield’s statements; he was not only incapable of skilful dissembling, but also, as was generally known, a favorite target for heartless schoolboy pleasantry. Mr. Cary, therefore, asked no further questions, but turned off the gas from the burner, and dumping out the smoking sand poked it over in search of clews to the explosion—to the great delight of the half-dozen unworthies who were in the secret. Finding nothing, he bade Redfield start again with fresh sand, and returned to his desk. A half-hour later Fluffy Dobbs’s mess blew up in the same way. This time the instructor, being hardly a dozen feet away, caught the full effect. He came directly to the smoking bath, but though his face blazed with indignation, he was too wise to embark on an interrogation which was unlikely to yield positive results. “Don’t you think something is the matter with the sand, sir?” asked Wilmot, innocently. “Perhaps there’s nitre in it.” “It isn’t likely.”