"Who admitted him?" "The young lady herself." Many more questions Patterson put to the old lady, but elicited no noteworthy fact, except that two large, heavy trunks had been sent away by Parcels Delivery a couple of days before. Therefore, thanking Mrs. Luff, who, of course, was extremely curious to know why the police were taking such an active interest in her neighbour, we left and made inquiries of the people in the adjoining house on the opposite hand. It was a lodging-house and the owner, a rather surly old widow, was not at all communicative. What she told us amounted practically to what we had already learnt. She, too, had long ago set the old man and his daughter down as mysterious persons, and her two servants had never been able to find out anything regarding them. So after nearly half an hour's absence we returned to the house of mystery, watched, of course, by the persons in the houses on either side. None suspected a tragedy, but all remained at their windows expecting to see somebody arrested. In the dining-room we found Doctor Knowles, the police divisional surgeon, who had been sent for by the police. He had already examined the bodies and was on the point of returning home. "Well, doctor, what's your opinion?" asked Patterson. "I can form none until after the post-mortem," answered the prim, youngish, dark-moustached man in silk hat and frock coat, a typical Kensington practitioner, who was known to be a great favourite with his lady patients. "Are there no marks of violence?" "None," he responded. "Although there seems no doubt that there has been foul play, yet the means used to encompass their death remains an entire mystery. That laboratory, too, is a very remarkable feature." "Why?" I asked. "Because the occupant of that place has made a discovery for which scientists have for years striven in vain," the doctor replied. "What is it?" "You noticed those strange globes with the coil of tubing," he said. "Well, from what I've found, it seems that the experimenter has invented a means for the liquefaction of hydrogen in large quantities." "Is that anything very remarkable?" I asked, in my ignorance of recent science. "Remarkable!" he echoed. "I should rather say it was. The discovery will create the greatest interest in the scientific world. Other gases have all been handled as true liquids in measurable quantities, while until now hydrogen has only been seen in clouds or droplets, and never collected into a liquid mass. Upstairs, however, there is actually a glass bowl of liquid hydrogen. The experimenter, whoever he is, has determined at last the exact temperature at which it will liquefy, and thus a field for quite new researches, as also for new generalisations, has been thrown wide open." "But why is the discovery so very important?" I asked, still puzzled at the doctor's unusual enthusiasm.