Also we know that every breathing and every burning object expels a hurtful gas – carbonic acid (carbon dioxide). A very small proportion of this gas is present in the purest atmospheric air, and in that small proportion is healthful; but increase that quantity by the action of furnaces, fires, living beings, gas-lamps, and the air is rendered unwholesome, just in proportion to the quantity of superfluous carbonic-acid gas it contains. If the quantity be excessive – as when many people are huddled together in a small unventilated room – speedy death be suffocation is the result.