the physiological side. Let us use their classification as a basis
for a review of the chief chemicals concerned.

The French Physiological Classification;--Asphyxiating Substances;--
Toxic Substances;--Chemicals or poison gases were either asphyxiating,
toxic, lachrymatory, vesicant, or sternutatory. It is perfectly true
that the asphyxiating and toxic substances, used during the war,
produced a higher percentage of deaths than the other three classes,
but the latter were responsible for many more casualties.
The so-called asphyxiating gases produced their effect by producing lesions
and congestion in the pulmonary system, causing death by suffocation.
The best known substances of this type was chlorine, employed in the liquid
state in cylinders on the occasion of the first German gas attack,
but the most formidable were phosgene (an important substance required
in the manufacture of dyes), diphosgene, chlor-picrin, made from bleaching
powder and picric acid, brom-acetone, which was also a powerful lachrymator,
and diphenylchlorarsine, known as sneezing gas, the first sternutatory
or sneezing compound to appear on the front in large quantities.
The toxic compounds were so called because of their specific effect upon
particular parts of the organism such as, for example, the nervous system.
The chief example, with regard to the military value of which there
has been much dispute, was prussic, or hydrocyanic, acid. The French
had definite evidence of the mortal effect of this compound upon
German gunners, but it was doubted by other Allies whether French gas
shell produced a sufficient concentration of gas to be of military value.
It was a kill or cure compound, for recovery was rapid from any
concentration which did not produce death.

A prominent Cambridge physiologist, in the heat of the controversy
on this matter, made a very brave and self-sacrificing experiment.