and
brought him and his four men back, all of them stricken with what I
take to be an extremely rapidly developing form of lobar pneumonia.
All of the men who went down were stricken with the same disease, two
of them as soon as they got back. So far we have had eight deaths
among these men and all of the rest, except Lieutenant Burroughs, are
apt to go at any moment.

"The trouble seemed to come from a cloud of some dense heavy gas which
rolled in from the marsh. On the advice of Major Martin, every door
and window in the post was kept closed until morning. The gas never
reached the upper part of the post but it reached the stables. Eleven
horses and mules are dead and all of the rest are stricken. The stable
detachment either failed to close their barracks tightly or else the
gas went in through cracks for seven out of the nine are here in the
hospital, although none of them are very seriously ill. As soon as the
sun came up, the gas seemed to disappear."

"Let me see the men who are sick."

Captain Murdock led the way into the ward. Dr. Bird went from man to
man, examining charts and asking questions of the nurses and medical
corps men on duty. When he had gone the rounds of the ward he entered
the morgue and carefully examined the bodies of the men who lay there.

"Have you performed any autopsies?" he asked.

"Not yet."

"Have you the authority?"

"On the approval of the commanding officer."

"Please secure that approval at once. Have all lights taken out of the
operating room and the windows shaded. I want to work under red light.
We must examine the lungs of these men at once. With all due respect
to your medical knowledge, Captain, I am not convinced that these men
died of pneumonia."

"Neither am I, Doctor, but that is the best guess I could make. I'll
have things fixed up for you right away."

* * * * *

Dr. Bird stepped to the telephone and called the laboratory. When, in
half an hour, Captain Murdock announced that he was ready to proceed,
Davis had arrived with an ultra-microscope and other apparatus which
the doctor had telephoned for.

"Did you arrange about the horses, Davis?" asked Dr. Bird.

"Yes, sir. They will be up here as soon as the trucks can bring them."

"Good enough. We'll start operating."

An hour later, Dr. Bird straightened up and faced the puzzled medical
officer.

"Captain," he said, "your diagnosis is faulty. With one possible
except