"Briefly, because by it physicists and chemists can henceforward obtain temperatures lying within thirty-five degrees from the so-called absolute zero of temperature--minus 459 degrees Fahrenheit. A possibility is thus given to study physical bodies in the vicinity of that point, which represents, so to say, the death of matter--that is, absence of the molecular vibrations which we describe as heat." This explanation, technical though it was, interested me. I knew Doctor Lees Knowles to be a rising man, and when reporting lectures at the Royal Institution had often noticed him among the audiences. There was no doubt that he was highly excited over the discovery, for, like myself, he had seen the liquid hydrogen boiling without any visible heat. In the papers there had been lots about Professor Dewar's experiments in the liquefaction of oxygen, fluorine and the newly-discovered helium, and I remembered how all his efforts to bring hydrogen to a liquid state had failed. Now, however, the mysterious occupier of that house had succeeded, and every known gas could now be liquefied. "But the murder," observed Patterson, his thoughts reverting to the crime, for to him the most wonderful scientific discovery was as naught. "Can you form absolutely no opinion as to how it was accomplished?" The doctor shook his head. "There is nothing whatever to account for their sudden death, as far as I can observe," he answered. "To the woman, however, death must have come instantly, while the man must have fallen and expired a few seconds later. There seem many mysterious features in the affair." "The discoverer of this latest scientific fact is undoubtedly the old man who is absent, the father of the dead girl. From him we may learn something to lead us to form conclusions," I suggested. "An old man!" echoed Dr. Knowles. "Tell me about him." Briefly Patterson related all that had been told us by the neighbours, and when he had finished the doctor exclaimed-- "Then I can tell you one thing which is proved undoubtedly. The old man seen to go in and out was in reality a young one, for while looking over the laboratory I came across a white wig and a make-up box, such as is used by actors. Go upstairs and you'll find a complete disguise there-- broadcloth coat, pepper-and-salt trousers baggy at the knees, old-fashioned white vest, and collars of antique pattern." "Surely that can't be true!" Patterson exclaimed in amazement. "It certainly is," the doctor asserted. "Depend upon it that the man lying upstairs dead was the man who has been making these successful experiments, and who for some unknown reason desired to conceal his identity. Recollect that they had few friends, if any, and that their man-servant was a most discreet foreigner, who never gossiped." "Then you think that to the world they assumed the position of father and daughter, while in reality they were husband and wife?" I said.