Nothing, shows more strikingly the admirable adaptation of all things for their intended uses than the atmosphere. In it we find the source of life and health; and chemistry teaches us, most indisputably, that it is composed of certain proportions of oxygen and nitrogen gases; and experience informs us that it is on the oxygen that we are dependent for all that we enjoy. So beautifully is the atomic or molecular constitution ordered, that it is impossible to produce any change in the air without rendering it injurious to the vegetable and animal economy. It might be thought, from the well-known exhilirating character of oxygen gas, that, if a larger quantity existed in the atmosphere than that which we find there, the enjoyments of life would be of a more exciting kind; but the consequences of any increase would be exceedingly injurious; and, by quickening all the processes of life to an unnatural extent, the animal fabric would soon decay: excited into fever, it would be destroyed by its own fires. Chemistry has made us acquainted with six other compounds of oxygen and nitrogen, neither of them fitted for the purposes of vitality, of which the following are the most remarkable:-- Nitrous oxide, or the, so called, _laughing gas_, which contains two volumes of nitrogen to one of oxygen, would prove more destructive than even pure oxygen, from the delirious intoxication which it produces. Nitric oxide is composed, according to Davy, of two volumes of nitrogen and two of oxygen. It is of so irritating a nature, that the glottis contracts spasmodically when any attempt is made to breathe it; and the moment it escapes into the air it combines with more oxygen, and forms the deep red fumes of nitrous acid. Nitrous acid and the peroxide of nitrogen each contains an additional proportion of oxygen, and they are still more destructive to all organization. Nitric acid contains five volumes of oxygen united to two of nitrogen; and the well-known destructive properties of aqua fortis it is unnecessary to describe. The atmosphere, and these chemically active compounds, contain the same elements, but their mode of combining is different; and what is, in the one case, poisonous to the highest degree, is, in the other, rendered salubrious, and essential to all organized beings.