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. In warm climes the respiration becomes slower, and in consequence there is less of carbon burned and less oxygen absorbed; but on the other hand the functions of the skin become vastly increased, the bilious secretions become more active, and the excess of carbon is eliminated by this channel. That we expire more carbonic acid in a warm, moist atmosphere, and less in a cold, dry climate, is shown by the exhilaration of our spirits on a fine frosty morning. No wonder that men lost their reason in this prison, for the blood no longer reddened from the imperfect arterialization, and burdened the brain with its effete matter, paralyzing and clogging up the delicate filaments and the narrow channels of thought and life. We have seen that the blood is subject to incessant variations in its precise chemical constitution; a free atmosphere, well supplied, oxygenates and destroys the numerous impurities that tend to lurk in the system and develop disease.