Was there anything in the gas-bomb scare? In the first place, many otherwise well-informed people have very erroneous views as to the poisonousness of gases. Gases are dangerous in the laboratory or factory if they kill without giving warning by odour and irritation; but gases of this kind, such as carbon monoxide and hydrogen arsenide, have to be present, in order to kill, in concentrations which cannot practically be produced in the open. The insidiousness of hydrogen arsenide has, however, so alarmed chemists that a tradition persists of a man having been killed by a single bubble of it, while they are so afraid of smelling carbon monoxide that it is generally stated to be inodorous. Besides errors due to this cause, there were errors of arithmetic. In one calculation which was made to show how easily London could be poisoned a decimal point went astray in one place! As the calculation was concerned with volumes of gas, the result came out as 10 metres cubed or 1,000 cubic metres, in place of one. For this reason it appeared that ten aeroplanes could do the damage which would actually have required ten thousand. However, most of the prophets of disaster from gas-bombs made no calculation at all. Let us try to make a rough one. On the nights of March 11th to March 14th, 1918, just before the great offensive of March 21st, the Germans fired 150,000 mustard gas shells into the villages and valleys of the Cambrai salient, an area of about twenty square miles, the same as that of central London. This caused 4,500 casualties, of whom only fifty died (all of them because they took off their respirators too soon). The area was not evacuated. In central London, if the population had had gas-masks, the casualties would have been perhaps ten times greater. But we have to compare this hypothetical air-raid, not with any raid that actually occurred, but with a bombardment of 150,000 high-explosive shells or their equivalent in bombs. This would hardly have left a house in central London untouched, and the dead would have been numbered not in hundreds, but in tens of thousands. Such an attack would have required the visits on repeated nights of something like 1,000 aeroplanes. Such a number is not yet a practical possibility. We are, perhaps, inclined to underestimate the potentialities of town-bombing with high explosive and incendiary bombs. In London, for example, there were never too many big fires started at any given time for the fire-brigades to deal with. An attack by ten or twenty times as many aeroplanes as ever bombed London simultaneously might well ring round a given area fairly completely with wrecked streets or burning houses, in which case most of the buildings and a good proportion of the inhabitants would perish. In one or two air-raids on other towns it seems probable that the Germans were not far from outstripping the capacities of the fire-brigades and producing very large conflagrations.