I could gather from Kennedy's words that it was Helen Grey.

"I shall be over immediately," he promised, as he hung up the
receiver and turned to me. "Mansfield is much worse. While I get
together some material I must take over there, Walter, I want you
to call up Miss Hargrave and tell her to start for the city right
away--meet us at Mansfield's. Then get Mina Leitch and Lewis.
You'll find their numbers in the book--or else you'll have to get
them from Miss Grey."

While I was delivering the messages as diplomatically as possible
Kennedy had taken a vial from a medicine-chest, and then from a
cabinet a machine which seemed to consist of a number of collars
and belts fastened to black cylinders from which ran tubes. An
upright roll of ruled paper supported by a clockwork arrangement
for revolving it, and a standard bearing a recording pen,
completed the outfit.

"I should much have preferred not being hurried," he confessed, as
we dashed over in the car to Mansfield's again, bearing the
several packages. "I wanted to have a chance to interview Mina
Leitch alone. However, it has now become a matter of life or
death."

Miss Grey was pale and worn as she met us in the living-room.

"He's had a sinking-spell," she said, tremulously. "Doctor Murray
managed to bring him around, but he seems so much weaker after it.
Another might--" She broke off, unable to finish.