This chemical change is necessary for life: the air is changed in its qualities, and the blood is also changed in its qualities. The air is changed by having one of its constituent elements (oxygen) abstracted from it: and the blood is changed by its being impregnated with this gas, and relieved of another kind called carbonic acid. If from any circumstance this process is interfered with, the individual dies of suffocation. A person may be suffocated for want of air, or for want of pure air. In the former case his death is caused in this manner:--The wind-pipe being closed, either by pressure, as in the case of criminals who die by hanging, or by something entering and obstructing it, it happens that although the muscles of the chest enlarge its internal area, as before mentioned, the air cannot descend into it. This does not, however, interfere with the action of the heart, which forces the dark blood into the minute blood-vessels of the chest, as usual; the blood passes onward unchanged; it receives no oxygen, nor is its bright red colour restored. In this state it reaches the chamber of the heart, from whence it is to be distributed to the head and body; a portion of it is forced up the vessels which convey it to the brain, and the moment it reaches this organ, it produces violent convulsions, insensibility, and in a few moments death. A similar result takes place from breathing foul air. In this case, although air may descend into the air-vessels of the lungs, yet, as the grand element, the oxygen, is not present, no change is produced in the blood; it pursues the same course as that just pointed out, unchanged in its quality, and the same fatal result is the inevitable consequence. The atmosphere in a state of purity is composed of two gases mixed together; the one termed _oxygen_, the other _nitrogen_. After escaping from the lungs, the air is found to have undergone a remarkable change; the oxygen has disappeared, and its place is supplied with an equal volume of another gas called _carbonic acid_; while at the same time the air is altogether altered in many of its more important qualities; it is no longer fit for the purposes of life, nor will a light burn in it. A person shut up in a confined place without a supply of fresh air, very soon expires: and a candle placed under a glass vessel filled with air that has been breathed, immediately goes out. In short, respiration and combustion are similar processes, and the same result is produced by both, namely, carbonic acid gas.