He entered a chamber of prussic acid which was sufficiently
concentrated to cause the death of other animals which were present.
They were removed in time, and he escaped because the concentration
was not a mortal one for man. This was, in a sense, an _experimentum
crucis_ and, although it did not disprove the extreme danger
of prussic acid, if employed in high concentrations, it showed,
on the other hand, that it was difficult to gauge the military
value by field experiments; battle results were necessary.
The Germans' disappointment with the use of arsenic compounds
confirms this need for battle evidence.

Lachrymators.--There is hardly need to dwell on the next class,
the lachrymator. These compounds were employed on a large scale
to produce temporary blindness by lachrymation, or weeping.
We give later some interesting examples of their use on the front.
It is an arresting thought that even as early as 1887
Professor Baeyer, the renowned organic chemist of Munich,
in his lectures to advanced students, included a reference
to the military value of these compounds.

Vesicant or Blistering Compounds.--It was the introduction of
the fourth, the vesicant class, which revealed, more than any other
enemy move, the great possibilities inherent in chemical warfare.
These compounds, the chief of which was mustard gas, produced vesicant,
or skin burning, effects, which, although rarely mortal,
were sufficient to put a man out of action for a number of months.
Mustard gas resulted from pure scientific investigation as early as 1860.
Victor Meyer, the famous German chemist, described the substance in 1884,
indicating its skin-blistering effects. There is evidence of further
investigation in German laboratories a year before the outbreak of war,