]8l AIR. 



very different from the former one, and which has the name of chemical 

 <; mbination. Nine grains of common water are formed from the eight 

 of oxygen and one of hydrogen. The oxygen will not now easily be 

 .separated from the hydrogen by the application of a third substance, 

 and there are few for which the oxygen has a greater affinity than for 

 the hydrogen. The general characters which distinguish chemical 

 combination from simple mixture, are, that in the former there is 

 usually an alteration in the SPECIFIC GRAVITY, REFRACTIVE POWER, law 

 of DILATATION from heat, &c. ; while there is, generally, at the same 

 time such a change of properties as no d priori reasoning could predict. 

 We have seen in the experiments already cited, that two elements 

 which, when properly combined, produce a substance very different 

 from either, may be placed in juxtaposition (such as is produced by 

 mere mixture) without any such consequence following. If nitrogen 

 and oxygen formed no other compound except atmospheric air, we 

 might, perhaps, call the latter a chemical compound ; but we should 

 then be obliged to say, that the affinity of nitrogen for oxygen was 

 : u ly small. But the contrary of this is the fact. One equivalent 

 of nitrogen may unite with one, two, three, four, or five eqmvalents of 

 oxygen, forming the nitrons and nitric ofides, and the nitroun, /(;//> 

 uiirir. and nitric ac'uls ; all of which have every character of chemical 

 combinations. 



The composition of air may be ascertained either synthetically or 

 iiiiii/i/ticaily. Synthetically, by mixing the proportions already noticed 

 of oxygen and nitrogen ; in which case it is found, that the mixture 

 differs in no respect from common air : analytically, by an experiment 

 similar to the one already cited ; in which, however, it is presumed 

 that we know the composition of water. If hydrogen be added to or 

 mixed with a portion of common air, and an electric spark be passed 

 through the mixture, it will be found that the hydrogen has combined 

 with eight times its weight of oxygen (if there be so much), and has 

 produced nine times its weight of water. In this way, by trial, the 

 quantity of hydrogen may be found which will combine with all the 

 oxygen in the mixture, and the remainder is then found to be simply 

 nitrogen. A more accurate method, however, is to allow air to stream 

 slowly over a weighed quantity of heated copper, whereby the oxygen 

 is absorbed, and the nitrogen is received into an exhausted flask, 

 weighed before the experiment was begun and after it was finished ; 

 the quantity of oxygen is found by the gain in weight experienced by 

 the tube containing the copper. 



Such are the principal chemical properties of air. For its effects 

 upon animal life, see RESPIRATION. 



\\"K have already observed that the air, in common with all other 

 bodies, has weight. This is proved by weighing a bottle which contains 

 air in a very delicate Italance, and then by repeating the process after 

 the air has been exhausted from the bottle by the air-pump. From 

 this we are immediately led to conclude that, like all other heavy fluids, 

 it exercises pressure upon all substances which are in contact with it. 

 But this was not the order of discover}'. The pressure was ascertained 

 long liefore there was any other reason except analogy for inferring the 

 weight, and the latter discovery was a consequence of the former. 

 It is tme, that Aristotle (Stanley's ' History of Philosophy, Aristotle,' 

 lrt 2, chap. vii.> expressly mentions that air has weight, and even cites 

 the experiment of a bladder, which he asserts weighs more when filled 

 with air than when empty : but his followers of the middle ages 

 entirely alandoned the doctrine. We shall speak more at length of the 

 discovery, under the heads BAROMETER and ATMOSPHERE. It is here 

 .sufficient to observe, that the density of the air depends upon, and is a 

 1 1 lence of, the pressure of the superincumbent atmosphere. For 

 tin- air is an elastic fluid, that is, its bulk increases, and its density 

 diminishes, whenever the exterior pressure is wholly or partially 

 removed. Let a loose bladder, tied at the mouth, and not so full of 

 air as to be distended, be placed under the receiver of an air-pump, so 

 that the air which presses the outeide of the bladder can be exhausted. 

 The interior air will expand so soon as the exhaustion begins, will 

 presently distend the bladder to its fullest dimensions, and will even 

 burst it. On the re-admission of the air into the space surrounding 

 t],. bliulder, the latter will gradually resume its former dimensions, 

 and its withered or flaccid appearance. 



An we ascend the atmosphere, the superincumbent column of air 

 becomes of less weight, and the density becomes less ; 

 that is, a cubic foot at the height of 1000 feet 

 above the ground is not so heavy, or does not contain 

 so much air, as a cubic foot at the surface of the 

 earth : which is thus explained. The air having in 

 itself a force which tends to separate the particles from 

 one another, or to expand the whole bulk, but which 

 force grows less and less as the particles are more and 

 more separated, that is, as the bulk increases, the 

 state of rest will always be that in which the elastic 

 force \ipon a square inch of the surface of air, arising 

 from ita own constitution, just balances the external 

 pressure upon that square inch. To illustrate this, 

 ftuppose a vertical tube, A B c D, open at both ends, at 



first, and filled with air, which communicates with the 

 \t> iior atmosphere. Place a slight membrane, E F, 

 it, which can 1* moved up .'Hid down the tube, so that, except for 

 friction, it would be displaced if the pressure* of the air above and 

 ARTS ASD SCI. I>IV. VOI r. 



AIR. 



Iflj 



Lta, ' 



below it were in the least degree unequal. At present there are two 

 equal and contrary pressures on the two sides of E F, arising from the 

 weight of the column of air above E F. For if the pressure from 

 underneath were less than that from above, E F would move down- 

 wards, and rice m-sd. Now cover the end B D of the tube, so that the 

 air in E F D B shall have no communication with the exterior air. The 

 membrane E F still remains at rest ; that is, the air E F D B, without 

 being pressed by the exterior atmosphere through the section B D, 

 exerts the same force upon E F from below as the exterior atmosphere 

 does from above. This is what we mean by the elastic force of the 

 atmosphere, as distinguished from the weight of the superincumbent 

 column of air. The two being always equal, may easily be con- 

 founded ; \ve only wish to impress upon the reader, that this repulsive 

 force of the particles of air, of which we know nothing but its effects, 

 is a counterbalancing force from within, so to speak, to the pressure 

 from without, and is greater or less according to the less or greater 

 nearness of the particles, as we shall proceed to exemplify. 



To get a more distinct idea of the superincumbent pressure on E F, 

 suppose the air to be entirely removed from above E F, so that the 

 membrane must be held down in order to prevent the imcouuter- 

 balauced force beneath from driving it up, and exhibiting the pheno- 

 mena of the air-gun. Let a liquid, mercury for example, be poured 

 into the tube, until there is no longer any occasion to hold down E F, 

 or until the weight of the mercury will just counterbalance the pressure 

 of the air from below. In the average state of the atmosphere, this 

 will require about 30 inches of the tube above E F to be filled with 

 mercury. Now, let half the mercury be removed ; that is, let it 

 only stand 1 5 inches above K F. This is not sufficient to counterbalance 

 the pressure from beneath, and the membrane will rise to twice its 

 height above B D ; that is, the air will now occupy twice the space 

 which it did before. But this will not happen immediately, for it will 

 settle at first at something less than the height we have mentioned, and 

 attain that height by degrees. The reason would be manifest if .1 

 thermometer were placed in the space K F B D ; for it would be found 

 that the thermometer would fall when the expansion began, and would 

 gradually regain its original height as the membrane acquired its full 

 distance from B D. Similarly, if the quantity of mercury were doubled 

 and made to stand at 60 inches above K F, the pressure on E F would be 

 greater than that from beneath ; the membrane would descend, the 

 tin-] monieter risini/ at the same time ; and by the time the thermometer 

 again indicated the same temperature as at first, the membrane E F 

 would stand at half its original distance from B D. If any other 

 quantities of mercury were added or taken away similar results would 

 be found, so soon as the alteration of temperature was balanced by the 

 surrounding atmosphere, which, in the first case, imparts heat to the 

 apparatus, and, in the second, receives heat from it. Thus, if only 

 one-third of the mercury were left, the air would overbalance it until 

 it had expanded into three times its dimensions. If the mercury were 

 increased five-fold, the air would never furnish a counterpoise until 

 it was reduced to one-fifth of its former dimensions. This remarkable 

 law, which holds for all gases as well as air, may be expressed as 

 follows : at the same temperature, the elastic forces of two portions of air 

 (or, which is the same thing, the weights of mercury they will balance) 

 are in direct proportion to the densities, or in inverse proportion to the 

 spaces occupied by these portions. In the apparatus above described, 

 we do not pretend to show a good practical method of actually per- 

 forming the experiment. For this purpose we must refer to AIR-PUMP. 



The very great pressure of the atmosphere is illustrated by the 

 following experiment. Two hollow hemispheres are loosely placed one 

 upon the other as in the figure : the lower communicates by a tube (in 

 which is a stop-cock, open for the present) with the 

 exhausting apparatus of an air-pump. At present 

 there is no impediment to lifting the upper from 

 the lower hemisphere except its weight ; the pres- 

 sure of the air from within counterbalancing that 

 from without. But if the air be withdrawn from 

 the interior, and the stop-cock closed so that the 

 apparatus can be unscrewed from the air-pump 

 without allowing the air to enter, it will require an 

 enormous force to Separate the two hemispheres. 

 Thus, if all the air be removed from the interior, 

 there will be a pressure of 15 Ibs. on each square 

 inch of the section of the hemispheres ; hence, 

 supposing the diameter to be 4 inches, the area of 

 the section will be about 12' square inches, and 

 the force required to separate them will be 12J X 15 = 187Jlbs. This 

 experiment was first performed by Otto Guericke, at Magdeburg, in 

 1654. Such being the external pressure, it may appear extraordinary 

 that the human body is capable of supporting it without being crushed 

 to atoms. The pressure on the body is computed at several tons. But 

 the cause of wonder is purely imaginary. In the words of Dr. Robison 

 " the human body is a bundle of solids, filled or mixed with fluids, and 

 there are few or no parts of it which are empty. All communicate 

 either by vessels or pores, and the whole surface is a siere, through 

 which the insensible perspiration is performed. The whole extended 

 surface of the 1 imgs is open to the pressure of the atmosphere ; every- 

 thing is therefore in eqnilibrio ; and if free or speedy access be given to 

 every part, the body will not be damaged by the pressure, however 



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