Phosgene was remarkable for its peculiar "delayed" effect.
Relatively small quantities, inhaled and followed by vigorous
or even normal exercise, led to sudden collapse and fatal
effects sometimes more than twenty-four hours after the attack.
The case of a German prisoner in a First Army raid after
a British gas attack was often quoted on the front.
He passed through the various Intelligence headquarters as far
as the Army, explaining the feeble effect of the British gas
and his own complete recovery. But he died from delayed
action within twenty-four hours of his last interrogation.
This effect imposed strict conditions of discipline, and men
merely suspected of exposure to phosgene were compelled
to report as serious casualties and carried as such even from
the front line.

The successful development of the phosgene cloud probably
arrived too late for the Ypres attacks, and a variety of reasons
must have led to the postponement of its use until such time
as it might once again give Germany the real initiative.
Accordingly, on December 19, 1915, a formidable cloud gas attack
was made on the north-east of the Ypres salient, using a mixture
of phosgene and chlorine in a very high concentration.
Fortunately, by this time we had established an anti-gas
organisation, which had forestalled the production of cloud
phosgene by special modifications in the British respirator.
The conditions were similar to those of April 22nd, 1915.
Instead of the first use of cloud gas, we had the first
use of the new gas in highly concentrated cloud.
In both cases the Germans reckoned on our lack of protection,
correctly in the first case, but incorrectly in the second.