We are sure, however, that many of our intelligent readers are ready now to start an objection to the account just given of the cause of bad air in cities. If the air of a city be injured by the large quantity of carbonic acid which is formed, a city should be the best place possible for the health of vegetables. If the air which is bad for man be good for plants, the vegetation in a confined street should surpass, in brilliancy and verdure, that of the most open and best attended gardens. It is true, unfortunately, that the only produce of our once industrious Liberty is now the grass which is growing in the seats of former bustle; but we have not even the satisfaction of knowing that that flourishes. It is pale, sickly, and stunted; for the air of the city is vitiated by causes different from that which alone has hitherto occupied us, and these causes are as injurious to plants as to man. The carbon of our fuel produces, in burning, carbonic acid, but carbon is not the only substance in ordinary fuel. Most coals contain sulphur, and in burning, this body produces sulphurous acid, also a gas, which is highly irritating and poisonous, particularly to plants, and which, mixing with the air, renders the city as injurious to the organization of a plant as the carbonic acid to the respiration of an animal. To render air fit for respiration, it is necessary to do more than keep the proper quantity of oxygen in it; the carbonic acid must be taken away. Plants, our readers have already remarked, do both, and hence the admirable fitness of external nature to the objects for which the Creator has designed it. If the carbonic acid were not taken away, all animals would be poisoned, even if the proper quantity of oxygen remained, for carbonic acid is a positive poison, which kills by acting on the brain like opium. A person can live, breathing with only one lung; in the disease of consumption, an individual may live for months with only one lung, or even only part of a lung, remaining fit for use; but if perfectly good air be breathed with one lung, and carbonic acid with the other, the person will be poisoned after a very short time; consequently, it is of great importance to prevent the accumulation of carbonic acid, even where it is not produced at the expense of the oxygen of the air.