Throughout the history of warfare the art of defense has kept pace with the art of offense and the courage of man has never failed, no matter to what new danger he was exposed.  As each new gas employed by the enemy was detected it became the business of our chemists to discover some method of absorbing or neutralizing it.  Porous charcoal, best made from such dense wood as coconut shells, was packed in the respirator box together with layers of such chemicals as will catch the gases to be expected.  Charcoal absorbs large quantities of any gas.  Soda lime and potassium permanganate and nickel salts were among the neutralizers used.

The mask is fitted tightly about the face or over the head with rubber.  The nostrils are kept closed with a clip so breathing must be done through the mouth and no air can be inhaled except that passing through the absorbent cylinder.  Men within five miles of the front were required to wear the masks slung on their chests so they could be put on within six seconds.  A well-made mask with a fresh box afforded almost complete immunity for a time and the soldiers learned within a few days to handle their masks adroitly.  So the problem of defense against this new offensive was solved satisfactorily, while no such adequate protection against the older weapons of bayonet and shrapnel has yet been devised.

Then the problem of the offense was to catch the opponent with his mask off or to make him take it off.  Here the lachrymators and the sternutators, the tear gases and the sneeze gases, came into play.  Phenylcarbylamine chloride would make the bravest soldier weep on the battlefield with the abandonment of a Greek hero.  Di-phenyl-chloro-arsine would set him sneezing.  The Germans alternated these with diabolical ingenuity so as to catch us unawares.  Some shells gave off voluminous smoke or a vile stench without doing much harm, but by the time our men got used to these and grew careless about their masks a few shells of some extremely poisonous gas were mixed with them.

The ideal gas for belligerent purposes would be odorless, colorless and invisible, toxic even when diluted by a million parts of air, not set on fire or exploded by the detonator of the shell, not decomposed by water, not readily absorbed, stable enough to stand storage for six months and capable of being manufactured by the thousands of tons.  No one gas will serve all aims.  For instance, phosgene being very volatile and quickly dissipated is thrown into trenches that are soon to be taken while mustard gas being very tenacious could not be employed in such a case for the trenches could not be occupied if they were captured.