The properties of atmospheric air ap pear to be merely the aggregated proper ties of the gases of which it consists. It is invisible, inodorous, insipid, compressi ble, and permanently elastic. It supports combustion, aad as it does so from the oxygen it contains, the combustion is less rapid and vivid, and continues for a short er time. By the same agency rt supports animal life ; a portion of its oxygen is consumed in respiration, and from some experiments of Mr. Davy, there appears to be a consumption of a very small por tion of its nitrogen. Atmospheric air is very sparingly absorbed by water ; and the aborption is unequal, more of the oxygen being combined with the water than of the nitrogen. It is difficult, even by long boiling, to expel from water the whole of the oxygen which it holds dis solved ; and if exposed again to the at mosphere, it very quickly imbibes it.
Atmospheric air is an important agent in many of the operations of nature. Be sides serving as the vehicle of the distri bution of water, it is, by its mobility, the great agent by which temperature is in some measure equalized, or at least its extremes moderated. Animals, as we have seen, are dependent on it for life. It is essential to respiration ; in the more perfect animals, its deprivation cannot be sustained for a few moments ; and even in the less perfect, the abstraction of it is followed, though not so immediately, by death. Its agency depends chiefly on its oxygen, a quantity of which is spent in every inspiration in produing chemical changes in the blood. A part of its ni trogen also is consumed, while a portion of carbonic acid gas is formed and ex pired. Vegetable life is also in part de pendent on it ; it conveys water and per haps carbonic acid gas, and other princi ples, to the leaves of plants, and is thus subservient to their nutrition andgrowth.
ATOMICALphdosophydenotes the doc trine of atoms, or a method of accounting for the origin and formation of all things from the supposition of atoms endued with gravity and motion. The atomic physiology,accordi ng to the account given of it by Dr. Cudworth, supposes that bo dy is nothing else but an extended bulk; and resolves, therefore, that nothing is to be attributed to it but what is included in the nature and idea of it, viz. more or less magnitude, with divisibility into parts, figure, and position, together with mo tion or rest ; but so as that no part of body can ever move itself, but is always moved by something else. And conse
quently it supposes that there is no need of any thing else besides the simple ele ments of magnitude, figure, site and mo tion, which are all clearly intelligible, as different modes of extending substance to solve the corporeal phenomena by ; and, therefore, not of any substantial forms distinct from the matter, nor of any other qualities really existing in the bodies v:ithout, besides the results or aggre gates of those simple elements, and the disposition of the insensible parts of bo dies, in respect to figure, site, and mo tion, nor of any intentional species or shows, propagated from the object to our senses ; nor, lastly, of any other kind of motion or action really distinct from local motion, such as generation and alteration, they being neither intelligible as modes of extended substance, nor any ways ne cessary. Forasmuch as the forms and qua lities of bodies may well be conceived to be nothing but the result of those simple elements of magnitude, figure, site, and motion, variously combined together, in the same manner as syllables and words, in great variety, result from the different combinations and conjunctions of a few letters, or the simple elements of speech ; and the corporeal part of sensation, par ticularly that of vision, may be solved on ly by local motion of bodies, that is, either by corporeal effluvia streaming continu ally from the surface of the objects, or rather, as the latter and more refin ed atom ists conceived, by pressure made from the object to the eye, by means of light in the medium. So that the sense taking cognizance of the object by the subtle interposed medium, that is tense and stretched, (thrusting every way from it upon the optic nerves) doth by that, as it were by a staff, touch it. Again, gene ration and corruption may be sufficiently explained by concretion and secretion, or local motion, without substantial forms and qualities. And, lastly, those sensible ideas of light and colours, heat and cold, sweet and bitter, as they are distinct things from the figure, site, and motion of the insensible parts of bodies, seem plain ly to be nothing else but our own fancies, passions, and sensations, however they be vulgarly' mistaken for qualities in the bodies without us