"It is a marvelous discovery, Doctor. I can foresee great uses for it
in medical science if a way can be found to produce it."

"Just now we are much more interested in stopping its production than
in producing it. Carry on with the line of treatment I have prescribed
and be ready for a busy time to-night."

* * * * *

From the hospital, Dr. Bird made his way to the headquarters building
where he conferred with Colonel Wesley on the measures being taken to
clear the proving ground of all persons not strictly necessary for its
guarding. The commanding officer, when he learned Dr. Bird's plans,
wished to send guards with him, but the doctor promptly vetoed the
scheme.

"My assistant, Mr. Davis, won't be able to fix up more than two masks
before dark, Colonel," he said, "and you would just be condemning men
to death to send them with me into that fog without proper protection.
I can see that you are anxious to know what is causing it, but I'm not
ready to tell just yet. I had given your medical officer enough
information to enable him to treat the hospital cases scientifically,
and to-morrow or the next day I hope to be able to tell you all about
it. Now, if you'll pardon me, I'm going to the laboratory to see how
Mr. Davis is getting along. It will be dark in three-quarters of an
hour and I hope that everyone will stay under cover as much as
possible."

Davis looked up as Dr. Bird entered the laboratory.

"I'll have the masks completed in an hour, Doctor," he said, "but I
don't know how much value they will be. If the oxygen polymerizes
before it enters the body, these masks ought to stop it, but if it
polymerizes under the influence of heat and moisture in the lungs,
they will be useless."

"I'll have to take a chance on that, Davis. From the description of
the fog, I strongly suspect that the process takes place outside the
body. Have you had your supper?"

"No, Doctor."

"Neither have I. I'll go over to the officers' mess and get a bite to
eat. As soon as you have those masks done, get your supper and then
telephone me at the club. If Carnes isn't back, I may have to ask you
to drive me down toward Michaelville."

"I'll be very glad to, Doctor."

* * * * *

Carnes had not returned when Davis called Dr. Bird at the officers'
club two hours later. Night had fallen and everyone on the proving
ground sat behind tightly closed windows with light