The density is thus, air being regarded as 1000:--
Phosphuretted hydrogen, 1240 Sulphuretted " 1170 Carburetted " 559 Carbonic acid, 1524 Cyanogen, 1806
The report of the British Parliament Commission gives the following data in this important question: "The amount of carbonic acid in the air is about 1/2000 or .0005; the amount expired is about 1/12, or .083. Respired air contains 1/10 or 1 of carbonic acid, and this must be diluted ten times to make the air safe. Thus, 1/10 ÷ 10/1 = 1/100, or .01; and this again divided by 10, or 1/100 ÷ 10/1 = 1/1000 or .001, gives the amount of ventilation needed to reduce the air to that state of purity that only 1/1000 more of carbonic acid should be added to the air, when it would be represented by .0015 instead of .0005."
Observing this rule, and taking 300 cubic feet as the air respired for the 24 hours, to dilute it ten times it must be mixed with ten times the bulk, or 3000 cubic feet--the space to be allowed for each individual; but if it is wished to keep up a pure air, it must be mixed with ten times this bulk again, or 30,000 cubic feet, which shows the ventilation needed to maintain an atmosphere nearly pure; or there must be admitted into the space of 3000 cubic feet nearly 21 cubic feet per minute of fresh air by ventilation, if the man in it is to breathe an atmosphere which shall contain only three times more of carbonic acid than the air he breathes originally contained; or again, 300 cubic feet, 3000, and 30,000, mark the requirements of one individual, in 24 hours, for respiration, space, and ventilation. On a calm day, when there were no strong breezes to change the air of the stockade, the entire quantity of air in the old stockade, allowing the palisades to be on the average 20 feet high, could be exhausted in 20 minutes by the 30,000 men respiring 300 cubic inches per minute. This is not a proper estimate to offer; but it will give a just idea of the rapid and fearful vitiation of the air that took place within the enclosure.
Vierodt shows how rapidly carbonic acid increases when foul air is breathed, and Lehman proves the rapid disengagement of the gas in moist atmospheres.
Symptoms of uneasiness manifest themselves when the air contains from 6/1000 to 7/1000 carbonic acid, and when the proportion amounts to ten parts to 100 of air, death ensues. "This effect is visible upon vegetables also, and many of them are extremely susceptible of impurities in the air, and very slight modifications in the proportion of its constituents are more or less prejudicial to their growth." But plants, like animals, vary in regard to the delicacy of their constitutions, some being much more susceptible than others.