GENERAL REMARKS

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tain 0.21 of oxigen, and the same was the case with air taken atIslington, and Highgate ; in the solitary cells in Cold-bath-fieldsprison, and on the River Thames. But the quantity of watercontained in a given bulk of air from these places differed consi-derably.

Having thus ascertained the nature and the proportion of thecomponent parts of air, it remains only to enquire in what man-ner these component parts are united. Are they merely mixedtogether mechanically, or are they combined chemically ? Is aira mechanical mixture or a chemical compound ? Philosophersseem at first to have adopted the former of these opinions, ifwe except Scheele, who always considered air as a chemicalcompound. But the supposition that air is a mechanical mixture,by no means agrees with the phenomena which it exhibits. Ifthe two gases were only mixed together, as their specific gravityis different, it is scarcely possible that they would be uniformlymixed in every part of the atmophere. Since therefore air is iuall places composed of the same ingredients, exactly in the sameproportions, it follows that its component parts are not onlymixed, but actually combined. 'When substances differing inspecific gravity combine together, the specific gravity of thecompound is usually greater than the mean. This holds good alsowith respect to air. The specific gravity, by calculation, amountsonly to 0.001lg, whereas it actually is 0.0012.

We must therefore consider air as a chemical compound.Hence the reason that it is in all cases the same, notwithstandingthe numerous decomposing processes, to which it is subjected.The breathing of animals, combustion, and a thousand otheroperations, are constantly abstracting its oxigen, and decompo-sing it. The air thus decomposed or vitiated, no doubt ascendsin the atmosphere, and is again, by some unknown processre-converted into atmospheric air. But the nature of thesechanges is at present concealed under an impenetrable veil.