ATMOSPHERE. 



55 



Atmo- 

 sphere. 



Proportion 

 of aqueous 

 vapour. 



Unknown 

 bodies. 



Dalton's experiment. Perhaps we shall not err far 

 if we state the bulk of carbonic acid gas in the at- 

 mosphere at la ' QO th part. 



The 4-th 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, Saus- 

 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 7 ' T tn f ' ts 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 O.G 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 

 l n of the whole. Mr Dalton supposes, that 

 the medium quantity of vapour held in solution at once 

 in the atmosphere, may amount to T 'j tu f ltA bulk. 



These four bodies, oxygen, azote, carbonic acid, 

 and vapour, are the only known constituents of the 

 atmosphere. It cannot be doubted, that i/.her bo- 

 dies are 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 

 pfi -ence 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 

 , as those of the muriatic acid, nitric acid, and 

 above all, of the oxymunatic acid, have the property 

 of destroying these miasmata, or at least of preventing 



Physical 

 properties 

 of the at- 

 mosphere. 



Gradation 

 in the den- 

 sity of the 

 atmosphere 



them from producing deleterious effects on the ani- Atm- 

 mal 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. 



That atmospherical air is a heavy, compressible, 

 and elastic substance, may be proved by many simple 

 and direct experiments. A bladder filled with air is 

 heavier than when it is in its flaccid state. When 

 subjected to compression, it may be made to occupy 

 a smaller space ; and when that pressure is removed, 

 its elasticity enables it to resume its original size. 

 Since the air is heavy, the lower strata of the atmo- 

 sphere are compressed by the weight of the super- 

 incumbent mass. The lowest stratum supporting 

 the weight of almost the whole atmosphere, will be 

 more dense than the rest ; and the superior strata will 

 gradually become more rare, in proportion to the 

 weight which they sustain. The air in the higher 

 regions, therefore, will be extremely rare, on account 

 of its elasticity, which is not checked by any super- 

 incumbent pressure. If the air were perfectly elastic, 

 it is obvious that there would be no limit to its ex- 

 pansion, and that the whole atmosphere would be dis- 

 sipated through infinite space. The elasticity of the 

 atmosphere must consequently diminish in a greater 

 ratio than the weight which compresses it, and there 

 must be a certain state of rarity at which its elasticity 

 ceases. Upon the supposition that the rarity of the 

 air is reciprocally proportional to its superincumbent 

 weight, it may be demonstrated, that if the heights in 

 the atmosphere be taken in arithmetical progression, 

 the rarity of the air at these heights will be in geome- 

 trical progression, or, what is the same thing, the al- 

 titudes in the atmosphere are as the square roots of 

 the corresponding rarities. Hence we have a method 

 of measuring differences of altitude, by ascertaining 

 with the barometer the rarity of the air at two places 

 whose vertical distance is required. A full account 

 of this method will be found under the article Baro- 

 meter. 



The weight and pressure of the atmosphere maybe Weight-of 

 ascertained by very simple experiments. If we im- t,le :itm - 

 merse in water a glass tube open at both ends, the S P re " 

 water included in the tube will be on the same level 

 with the fluid which surrounds it. When we apply 

 our mouth to the upper end of the tube, and draw 

 out the air, the included water instantly ascends till 

 the weight of the elevated column added to the elas- 

 ticity of the remaining air, exactly balances the pres- 

 sure of the atmosphere on the surrounding fluid. If 

 we now take a long tube, 40 feet long for example, 

 shut at one end, and having filled it with water, plunge 

 the open end into a vessel of water, the fluid will 

 then descend in the tube till the weight of the column 

 exactly equals the pressure of the atmosphere ; for 

 the air is now excluded from the upper part of the 

 tube, and the weight of the column of fluid is the 

 only force which is left to balance the weigiit of the 

 atmospherical column. By making this experiment, 

 it will be found that the water stands at from 34 te 



