dangerous area, which is in fact as unoccupiable by his foe as by himself.
Fortunately the difficulty of maintaining a concentration of toxic substances in the air which is dangerous to life or health is very great. But the possibility of doing so is sufficient to make one take the gloomiest views of the work of Science in connection with the future of this type of warfare. The substances more recently favoured by the belligerents in this war are specially distinguished by their physiological effect. Mustard gas is an example. It cannot be said to be chemically inert but it is not specially active while its physiological action is terrible. A heavy liquid with a high boiling point and giving off a slight amount of very tenuous vapour it can yet render a position untenable for days and produces horrible mischief both in the breathing tract and on the skin for which no specific remedy is known. The many arsenical compounds that have been tried or proposed on the one side or the other have also the characteristic of special 48
