The presence of carbonic acid in the air we expire is readily seen by blowing our breath, by means of a tube, into a bottle containing ordinary lime-water. The water soon becomes opalescent and then milky in appearance, owing to the formation of carbonate of lime or chalk; the carbonic acid of the expired air having combined with the lime previously held in solution. It is this principle which is taken advantage of in order to estimate the amount of carbonic acid in atmospheric air. Dr. Angus Smith lays down a simple practical rule whereby any one may ascertain if the air of an apartment contains carbonic acid to a dangerous amount, viz.: “Let us keep our rooms so that the air gives no precipitate when a 10½-ounce bottle full of air is shaken with half an ounce of clear lime water.” It is well known that speedily fatal results arise from overcrowding and want of fresh air. Out of the one hundred and forty-six prisoners confined in the “Black Hole of Calcutta,” one hundred and twenty three died in one night; and it is significant that many of the survivors afterward succumbed to “putrid fever.” Nor have similar instances been wanting in this country. Of the one hundred and fifty passengers that were shut up in the cabin of the Irish steamer _Londonderry_, with hatches battened down during a stormy night in 1848, seventy died before morning. In these two catastrophes suffocation was doubtless the direct cause of death, but the fact that “putrid fever” attacked many of those who were carried out alive from the “Black Hole of Calcutta,” showed that the fœtid exhalations to which they were exposed must have aided largely in destroying the lives of the immediate victims. The re-breathing of fœtid matter thrown off by the skin and lungs produces a kind of putrescence in the blood, in proportion to the amount inhaled, and to the period of exposure to its influences; and even air only moderately vitiated, if breathed for a long time day after day, produces most serious results. These results are seen in pale faces, loss of appetite, a lowering of the spirits, and a decrease of muscular strength. That air polluted by respiration is the one great cause of consumption, which may be handed down from parents to children for generations, rests upon such a mass of facts, that it is no longer controvertible. For instance, we know that increase of the disease occurs _pari passu_ with an increase in the density of population; that in manufacturing centres, where the males are the chief workers at indoor employment, the male death-rate is the highest; that in others, where females are principally required at indoor work, they suffer most; and that in agricultural districts, where the men spend nearly all their lives in the open air, and the women scarcely ever leave their cottages, the female death-rate from this disease is higher than the male. Moreover, the testimony of the most able physicians