The third group of poisonous smokes, mostly arsenic compounds, were little developed during the war. They are, however, weapons of very great efficiency, and it is well known that they would have been used by the British at any rate on a very extensive scale in 1919.[A] In small amounts, these smokes merely make one sneeze. In somewhat larger amounts they cause pain of the most terrific character in the head and chest. The pain in the head is described as like that caused when fresh water gets into the nose when bathing, but infinitely more severe. These symptoms are accompanied by the most appalling mental distress and misery. Some soldiers poisoned by these substances had to be prevented from committing suicide; others temporarily went raving mad, and tried to burrow into the ground to escape from imaginary pursuers. And yet within forty-eight hours the large majority had recovered, and practically none became permanent invalids. These substances, when in the form of smoke, will penetrate any of the respirators used in the late war, though the British box-respirator would stop all but a little of them in the concentrations then used. In future they will probably be used in much larger concentrations and in finer particles than those formed by the German smoke-shells. It is extraordinarily difficult to produce a respirator which will completely stop very fine smoke, for the following reason. In a gas the molecules (or ultimate particles) are moving very rapidly, with speeds of several hundred yards per second, continually colliding and rebounding. A gas molecule, therefore, will probably hit the sides of a fairly narrow passage through which it is drawn. But a smoke particle is moving at a speed measured in inches per second, and is far less likely to hit the wall of the respirator, and be held by its absorbent surface. If we try to make the passages through which air is drawn very narrow, as by sucking in our air through cotton-wool (which will stop most smokes), we find that we have created an appalling resistance to breathing. There is an electrical method of removing smoke-particles completely, but it would probably more than double the weight of respirators, and does not appear to be either water-proof or fool-proof. [A] The American “Lewisite,” of which so much was heard in 1918 and 1919, is a substance of this class.