“And the murder of the chef?” shot out Kennedy.

She looked from one to the other of us in alarm.  “Before God, I know no more of that than does Mr. Pitts.”

Was she telling the truth?  Would she stop at anything to avoid the scandal and disgrace of the charge of bigamy?  Was there not something still that she was concealing?  She took refuge in the last resort—­tears.

Encouraging as it was to have made such progress, it did not seem to me that we were much nearer, after all, to the solution of the mystery.  Kennedy, as usual, had nothing to say until he was absolutely sure of his ground.  He spent the greater part of the next day hard at work over the minute investigations of his laboratory, leaving me to arrange the details of a meeting he planned for that night.

There were present Mr. and Mrs. Pitts, the former in charge of Dr. Lord.  The valet, Edward, was also there, and in a neighbouring room was Thornton in charge of two nurses from the sanitarium.  Thornton was a sad wreck of a man now, whatever he might have been when his blackmail furnished him with an unlimited supply of his favourite drugs.

“Let us go back to the very start of the case,” began Kennedy when we had all assembled, “the murder of the chef, Sam.”

It seemed that the mere sound of his voice electrified his little audience.  I fancied a shudder passed over the slight form of Mrs. Pitts, as she must have realised that this was the point where Kennedy had left off, in his questioning her the night before.

“There is,” he went on slowly, “a blood test so delicate that one might almost say that he could identify a criminal by his very blood-crystals—­the fingerprints, so to speak, of his blood.  It was by means of these ‘hemoglobin clues,’ if I may call them so, that I was able to get on the right trail.  For the fact is that a man’s blood is not like that of any other living creature.  Blood of different men, of men and women differ.  I believe that in time we shall be able to refine this test to tell the exact individual, too.

“What is this principle?  It is that the hemoglobin or red colouring-matter of the blood forms crystals.  That has long been known, but working on this fact Dr. Reichert and Professor Brown of the University of Pennsylvania have made some wonderful discoveries.

“We could distinguish human from animal blood before, it is true.  But the discovery of these two scientists takes us much further.  By means of blood-crystals we can distinguish the blood of man from that of the animals and in addition that of white men from that of negroes and other races.  It is often the only way of differentiating between various kinds of blood.

“The variations in crystals in the blood are in part of form and in part of molecular structure, the latter being discovered only by means of the polarising microscope.  A blood-crystal is only one two-thousand-two-hundred-and-fiftieth of an inch in length and one nine-thousandth of an inch in breadth.  And yet minute as these crystals are, this discovery is of immense medico-legal importance.  Crime may now be traced by blood-crystals.”