Persistent Substances.--Two main classes exist from the tactical
Point of view. There are those "persistent" substances which
remain for a long time on the soil or on the object on which they
are sprayed by shell, while retaining their dangerous effect.
Mustard gas was the chief example, but some of the lachrymators
were just as persistent. By their use it is possible to render
ground uninhabitable or ineffective for military movement.
The combination of the vesicant and persistent properties of mustard
gas rendered it a powerful military factor.

Non-Persistent Substances.--On the other hand, there are the relatively
volatile substances, such as phosgene, which can be used immediately
before an attack. The chief sternutatory compound, diphenylchlorarsine,
although not volatile, could also be used in this way, for, being a solid
and in a very finely pulverised state, its presence on the ground was
not a distinct danger, and it invited chemical decomposition.

Penetrants.--The Germans introduced an additional tactical group.
This comprised pulverised substances able to penetrate the mask
on account of their existence as minute particles. The Germans
expressed these tactical conceptions by their shell markings.
The familiar Green Cross represented the slightly persistent,
volatile, lethal compounds, such as phosgene and diphosgene.
The German gunner had no need to know the content of his gas
shell so long as he could identify the cross. Yellow Cross,
representing mustard gas, was the most highly persistent type.
It is interesting to speculate whether a new persistent compound,
whose military value was due to some other property than the blistering,
would have been grouped under Yellow Cross. Logically, this should
have been done. Blue Cross covered the arsenic group of compounds,