An outlet is also afforded to carbonic acid gas, and the acquisition of a quantity of oxygen and nitrogen, which, combining with the constituent parts of the chyle, convert it into the nature and quality of nutritious blood. The temperature of the animal is supposed also to be a consequence of the decomposition of air in the respiratory process. The processes of respiration and combustion perpetually tend to the destruction of the vital air, and the substitution of another, which is a deadly poison to animal life. By means of ventilation and circulation,--causing currents of air,--such poisonous air is not allowed to accumulate, but is diffused through the surrounding space, while the vital gas rushes, by a counter-tendency, to supply the deficiency which the local consumption may have created; and thus is explicable one of the self-apparent reasons as to the imperious necessity for free ventilation. Notwithstanding our imperfect acquaintance with the manner in which water is suspended in the atmosphere, it is well known that the human body is greatly influenced by the aqueous vapour in such a state of suspension, and that the sources of poisonous emanations are active in proportion to the grade of atmospheric humidity and its temperature. An atmosphere surcharged with humidity not only prevents the cuticular discharge necessary to a healthy state, but sensibly diminishes the watery exhalations from the lungs, thereby inducing various morbid effects on the system. We observe the conversion of volatile bodies into a gaseous form exemplified in the perfume of flowers being more sensible during the fall of dew of an evening or in a morning, when the dew evaporates and is dissipated by the rays of the morning sun: in the same manner, the exhalation of deleterious matters, such as the filth of ditches and badly-drained sewers, becomes more active. Excess of moisture also, by diminishing the vital action, provides another cause of disease in conjunction with the enervating effects of deleterious gases: hence the more poisonous properties or injurious action of those gases in stagnant atmospheres, which are always more humid than where there is efficient circulation, _i. e._ ventilation.