As this acid as is produced in great quantities by combustion, respiration, fermentation, and many other of the most common processes of nature, one would be disposed to believe, at first view, that its quantity must be constantly increasing. But this does not ap pear to be the case, it must therefore be decompo sed and separated from air as fast as it is formed. It is of so deleterious a nature, that, if it were to accumulate to any extent, it would render air inca pable of supporting life. A candle will not burn in air contaminated with one-ninth of carbonic acid gas.
The quantity of this gas in air is small. Many attempts have been made to ascertain it ; but the pro cess is so difficult, that absolute precision cannot be looked for. It was long believed that the carbo nic acid present in the atmosphere amounted to one per cent. Humboldt made many experiments on the subject, and concluded from them, that its bulk varied from one per cent. to half a per cent. But this determination is certainly excessive. According to time experiments of Mr Dalton, a quantity of air, equal in bulk to 102,400 grains of water, contains a quantity of carbonic acid just capable of saturating 125 grains of lime-water : 70 measures of carbonic acid gas would produce the same effect : Hence he concludes, that the atmosphere contains part of its bulk of carbonic acid gas, (Phil. Mag. xxiii. 354.) This quantity we consider as rather below the truth. Mr Cavendish has shewn, that lime-water is not capable of depriving air completely of carbonic acid gas : Hence a portion would still remain in Mr Dalton's experiment. Perhaps we shall not err far if we state the bulk of carbonic acid gas in the at mosphere at ,,,'„„th part.
The 4th constituent of the atmosphere is water in the state of vapour. That water forms a con stituent part of the atmosphere, has been known in all ages, and indeed is demonstrated by the rain and dew which is continually falling, and by the great quantity of moisture which sulphuric acid, potash, and other bodies, absorb when exposed to the atmo sphere. The quantity of moisture in the atmosphere has been observed to vary greatly at different times, and various instruments have been invented to mea sure that quantity. These instruments are called hygrometers. The most ingenious of them are those of Leslie, Saussure, and De Luc.
It was at first supposed, that the water in the at mosphere was still in the state of water, and that it was held in solution in air precisely as salts are dis solved in water. But it has been at last established by satisfactory experiments, that the water in the at mosphere is in the state of vapour. To De Luc, Sans
sure, and Dalton, we are chiefly indebted for these experiments.
As to the quantity of water which exists in the at mosphere, it depends upon a variety of circumstances, the investigation of which belongs to that branch of science called METEOROLOGY ; to which, therefore, we refer. Saussure found that a cubic foot of air, saturated with moisture at 66°, contains about 8 grains troy of water, or nth of its weight. Sup posing air always saturated with moisture, the quan tity always increases with the temperature, because the elasticity of aqueous vapour increases with the temperature. Hence, in cold weather, the quanti ty of vapour in air is always small ; whereas, in warm weather, it is often considerable. In the torrid zone the aqueous vapour in the atmosphere is capable of supporting from 0.6 to 1 inch of mercury. In Bri tain it is hardly ever capable of supporting 0.6 inch of mercury ; but in summer it is often capable of sup porting 0.5 inch, while in winter it often does not ex ceed 0.1 inch. From these facts it follows, that the weight of water present in the atmosphere varies from as to 1 Z-Z7 ' of the whole. Mr Dalton supposes, that the medium quantity of vapour held in solution at once in the atmosphere, may amount to -fth of its bulk.
These bodies, oxygen, azote, carbonic acid, and vapour, are the only known constituents of the atmosphere. It cannot be doubted, that c.-..her bo dies arc occasionally present in it. The dreadful effects of marshy situations upon the health of the in. habitants, and the fatal rapidity with which certain diseases are propagated, cannot well be accounted for, without supposing that certain substances which pro duce a deleterious effect on the animal economy, are occasionally present in the atmosphere. But hitherto no method has been discovered of ascertaining the presence of these bodies, and subjecting them to exa mination. They are too subtile for our apparatus, and altogether escape the cognizance of our senses. It has been ascertained, however, that certain acid fumes, as those of the muriatic acid, nitric acid, and above all, of the oxyinuriatic acid, have the property of destroying these miasmata, or at least of preventing them from producing deleterious effects on the ani economy. (c ) Having considered in the preceding paragraphs the .dilatation of atmospherical air by heat, and its chemi cal composition, we shall now proceed to give a brief and general view of its physical properties, reserving the full discussion of the subject to the article PNEU MATICS.