The next morning, with characteristic energy, Craig was at work early, examining the cultures he had made from the black spots on the gelatine.

By the look of perplexity on his face, I knew that he had discovered something that instead of clearing the mystery up, further deepened it.

“What do you find?” I asked anxiously.

“Walter,” he exclaimed, laying aside the last of the slides which he had been staining and looking at intently through the microscope, “that stuff on the gelatine is entirely harmless.  There was nothing in it except common mold.”

For the moment I did not comprehend.  “Mold?” I repeated.

“Yes,” he replied, “just common, ordinary mold such as grows on the top of a jar of fruit or preserves when it is exposed to the air.”

I stifled an exclamation of incredulity.  It seemed impossible that the deadly germ note should be harmless, in view of the events that had followed its receipt.

Just then the laboratory door was flung open and Reginald Blake, pale and excited, entered.  He had every mark of having been up all night.

“What’s the matter?” asked Craig.

“It’s about my mother,” he blurted out.  “She seems to be getting worse all the time.  Miss Sears is alarmed, and Betty is almost ill herself with worry.  Dr. Wilson doesn’t seem to know what it is that affects her, and neither does the new nurse.  Can you do something?”

There was a tone of appeal in his voice that was not like the self-sufficient Reginald of the day before.

“Does there seem to be any immediate danger?” asked Kennedy.

“Perhaps not—­I can’t say,” he urged.  “But she is gradually getting worse instead of better.”

Kennedy thought a moment.  “Has anything else happened?” he asked slowly.

“N-no.  That’s enough, isn’t it?”

“Indeed it is,” replied Craig, trying to be reassuring.  Then, recollecting Betty, he added, “Reginald, go back and tell your sister for me that she must positively make the greatest effort of her life to control herself.  Tell her that her mother needs her—­ needs her well and brave.  I shall be up at the house immediately.  Do the best you can.  I depend on you.”

Kennedy’s words seemed to have a bracing effect on Reginald and a few moments later he left, much calmer.

“I hope I have given him something to do which will keep him from mussing things up again,” remarked Kennedy, mindful of Reginald’s former excursion into detective work.

Meanwhile Craig plunged furiously into his study of the substances he had isolated from the saline solution in which he had “washed” the blood of the little Pekinese.

“There’s no use doing anything in the dark,” he explained.  “Until we know what it is we are fighting we can’t very well fight.”

For the moment I was overwhelmed by the impending tragedy that seemed to be hanging over Mrs. Blake.  The more I thought of it, the more inexplicable became the discovery of the mold.