sensible phenomena of condensation would be totally different from what we observe. The force necessary for a quadruple condensation would be eight times greater. 22. If we could suppose that the particles of air repelled each other with invariable forces at all distances within some small or insensible limit, this would produce a compressibility and elasticity similar to what we observe. For if we consider a row of particles, within this limit, as compressed by an external force applied to the two extremities, the action of the whole row of the extreme points would be proportional to the number of particles, that is, to their distance inversely, and to their density: and a number of such parcels, ranged in a straight line, would constitute a row of any sensible magnitude, having the same law of compression. But this law of corpuscular force is unlike every thing we observe in nature, and to the last degree improbable. 23. We must therefore continue the limitation of this mutual repulsion of the particles of air, and be contented for the present with having established it as an experimental fact, that the adjoining particles of air are kept asunder by forces inversely proportional to their distances; or perhaps it is better to abide by the sensible law that the density of air is proportional to the compressing force. This law is abundantly sufficient for explaining all the subordinate phenomena, and for giving us a complete knowledge of the mechanical constitution of our atmosphere. 24. The air-pump, with some of its most important experimental illustrations, must now be adverted to. 25. We have seen that the common AIR-PUMP, as described under that article, is materially defective in the principle on which its valves are constructed, so that it ceases to operate long ere a perfect vacuum is formed in the receiver. It may now, however, be advisable to examine an instrument contrived by Mr. Stiles, the ingenious mathematical instrument maker to the London Institution; which unites in a very eminent degree all the advantages of those that have hitherto been constructed, with the very important desideratum of performing exactly twice the work of a common double-barrel pump. 26. Fig 1, plate I., PNEUMATICs, represents a section of the principal parts of the pump, from which it will be seen that it is worked in the usual way, by means of a winch with a wheel and racks; this part, therefore, requires no explanation. But to the end of each rack is firmly attached, by means of the connecting pieces of brass marked a, a, the cylindrical rods b, b, passing through the collars of leather, c, c, which have reservoirs of oil in the cups above them, for the purpose of more effectually rendering them air-tight. The pistons, d, d, are solid, having no valve in them, and consist of discs of leather steeped in oil and tallow, and screwed up fast between their shoulders; they are then turned to fit the bore of the barrels. 27. The positions assumed by them as shown in the section must next be attended to. The one in barrel A is shown nearly at the end of its as cending stroke, while the piston in barrel B is equidistant from the bottom in its descending motion; the piece C is fitted in between the caps which contain the collars of leathers, and is screwed firmly to them. The barrel B is withdrawn from its cap E, in order to explain the mode of connexion between the cap and the barrel, as each cap, D and E, is similarly fastened by screws, placed at convenient distances, to the flanges of the respective barrels A and B. The angular perforated passages, e, e, as seen in the piece C, communicate with the main inlet pipe or passage from the receiver; the one leading into barrel B is seen open, and allows a free and unobstructed way for the air to enter above the piston d, in its descending stroke, as marked by the darts pointing downwards; while the air is also passing down the pipe F, whose connexion with the piece C is more clearly shown in the perspective view of the instrument, and through the horizontal way or channel communicating with barrel A, as shown by the letters ƒ, ƒ. Here the air passes through an oiled silk valve, which consists of a brass valve piece, having a hole perforated through its centre, and a small groove or nick cut in the upper part; a piece of oiled silk is strained over its surface, and secured by silk thread twisted round in the groove; this piece, with the valve, is shown in the bottom of barrel A, opening upwards, permitting the air to enter beneath the piston in its ascending motion, as shown by the darts pointing in that direction. Having thus traced the inlet ways to the top of barrel B, and bottom of barrel A, we may now describe the mechanism by means of which the top inlet valves are connected with their respective barrels. The valves we are now about to describe consist of the two metallic cylinders F and G, the first being closed, and G, which is shown open; the rods or cylinders pass through the small collars of leather, g, g, with an oil cup to each cap, as shown by the curved lines above them; which caps may be screwed up when requisite in order to press the collars o. leather closer, and render them air-tight; the cylindrical valves or rods are kept in the vertical position by passing through a piece of brass, which is attached by means of screws to the under side of the head of the pump marked H; to this piece are attached two levers I and K, revolving upon the steel pins of the milled headnuts h, h. The levers work in a mortice cut to receive them in that part of the piece H shown by the letters i, i. Attached to one end of each of those levers are seen the small steel screws k, k. Two small plates of brass 1, (the front plate of each being only shown in the section), whose extremities are again attached by the screws m, m, to the pieces n, n, answer the purpose of sling rods for connecting the motion here requisite for raising and depressing the cylinders or valves according to the alternate motion of the levers I and K. The pieces n, n, are perforated, and slide freely on the valves F and G. The way in which this alternate motion takes place may easily be explained. On the back part, or opposite edge, of each toothed rack, as seen in fig. 2, is placed a plate of steel (fastened by small screws), the length of which is limited by the working stroke of each piston, and projects on that side of each rack on which the levers are represented. The lever K is shown in the position with the valve G open, for permitting the air from the receiver to enter the top of barrel B, and the bottom of barrel A, as before described; while the lever I, with the valve F, is seen as thrown down, closing the top inlet of barrel A. We shall now suppose the piston of barrel B to conclude its descent to the bottom, having expelled the air beneath, through the outlet valve s, and the piston of barrel A, its ascent to the top of the barrel; the rods and racks will also pass through the same space, and, the moment the pistons reach their respective limits, the levers I and K are relieved from the opposite ends of the plates or fillets of steel; the lever I, by the action of the spiral spring o, which is coiled round the cylindrical valve F, and pressing between the turned shoulder p, and the under side of the perforated or sliding piece n, is then returned to an horizontal position. The lever K, by a like action, produced by the spring 7, which is also coiled round the cylindrical valve G, and pressing between the piece of brass H, and shoulder piece N, closes the cylindrical valve G by its pressure, and its lever K, of course, takes an horizontal position. By reversing the motion of the winch, for the next stroke of the pistons, the positions of those levers are again changed by the ends of the fillets of steel, placed on the back edge of the racks, coming in contact with their extremities: The lever I is thrown up in the direction of the dotted lines, carrying with it the cylindrical valve F, which is consequently opened, and a free access for the air to enter the barrel A, above the piston d, on its downward motion, now takes place; the valve f, placed at the bottom, closes, and the air received by it is expelled through the valve s, which is similar in construction with the valve f, but in this case opens outwards, while the lever K, in consequence of the ascending motion of the rack and the fillet of steel, must come into contact with its extremity, and is thrown down on the spiral spring o, coiled round the cylindrical valve G, which still more effectually secures the valve in this position; the return of the air by the upward motion of the piston being also prevented. The lever K will now be in the position shown by the dotted lines, and the air received above the piston in its former downward stroke is thus expelled through the top outlet valve t, and passes through the side, or leadingoff pipe L, in the direction as shown by the darts pointing downwards, and which communicates with the same general outlet as the bottom discharging valves s, s. A reference to the above description will show that while one barrel is discharging its contents by the upward motion of its piston, it is at the same time filling to discharge by the downward stroke. The other barrel is discharging by the downward action of the piston, and also filling through the ways described to discharge again by its upward motion, so that it performs the work of two pumps of the same capacity of barrel, constructed on the common principle. In addition to the above advantages, the mode of working the top inlet valves mechanically, insures a much more perfect vacuum than could otherwise be obtained. Thus, if we suppose the bottom inlet valves f,f, and also the discharging valves s, s, to have become leaky, by simply turning off the cock M we cut off all communication between them and the receiver; the pump then becomes a single-acting pump, with all the advantages of the common instrument. If we now suppose the top valves to be bad, in order to cut them off, detach the centre screws h, h, from the levers I and K, permitting those parts to hang loose down by the sides of the cylindrical valves; the spiral springs 4, 9, will then press those valves close down over the top angular inlet ways, and prevent the access of air from the receiver above the piston. If we then open the cock M, which in the former case was closed, the pump may be worked from the bottom set of valves alone. 28. The air-pump offers a variety of very beautiful illustrations, tending to prove the materiality, weight, pressure, and elasticity of the atmosphere, some of which have already been detailed under the article AIR-PUMP. 29. There is a very ingenious air-pump for the use of persons suffering from suspended animation that must here be noticed, as it combines the principle of the force-pump with that of the exhausting apparatus. Fig. 3 shows the interior of two pump cylinders, a and b, joined together, so that they make one body; in each of these cylinders is placed a piston , which are both by the piston-rod d (passing through the line e) attached to the handle f, by means of the small screws g; h is a discharging, i an introduction pipe, with an opening k; l are two leather elastic tubes, with a horn band m, in which band is attached a small Indian rubber pipe n; o is an injection pipe with a moveable shield p, and the screw to fix it; ra blade. 30. As soon as the body is taken out of the water, the two elastic tubes n are dipped for a moment in warm water, bent as may be found necessary, and then placed to such a depth in the nose that the horn-bands m are half in the nostrils, these bands being necessary to prevent the circulation through, the pipes being stopped, when the nostrils are held close by the hand of the operator. The pipe o is then put into the mouth, until the shield p is close to the lips; the latter is shifted according to the size of the sufferer, and fastened by the screw q, so that the pipe may go the required depth into the mouth, with the blade r upon the tongue. 31. As soon as the breathing pump is placed in this position by the operator, who holds it in the left hand, another person, B, must hold the nose and mouth air-tight round the pipe and tubes; the handle of the piston is drawn upwards by the right hand of the operator, and immediately both pistons rise to the top of the cylinder; upon this movement the valvess, u, shut, and t, v, open; and while the cylinder a is filled, through the nose, with foul air from the lungs, the cylinder is filled with fresh or atmospheric air through the introduction pipe i, the piston being pressed downwards, the valves s, u, are opened, and t, v, shout, and, while the foul air from the cylinder a is discharged through the pipe h, the atmospheric air by which the cylinder 6 was filled, is pressed through the pipe o in the mouth, and consequently into the lungs, and breathing will be immediately restored. 32. The operator ought to make the strokes as regular as the breath is usually drawn, and proper care must be taken that the stomach and breast be pressed every time the piston rises in the cylinder, in order to assist the discharge of the foul air. 33. If, at the commencement of the operation, there is reason to believe that the lungs are too heavily charged with foul air, the elastic Indian rubber tubes ought alone to be brought into the nose, and, while keeping the mouth air-tight with the hand, a few strokes will immediately remove and discharge it. This operation will give room immediately for the fresh air to act with success by means of the pipe o, which must then be put into the mouth. Should it be wished to make an experiment to draw the foul air out by the mouth, and to introduce the fresh air through the nose, unscrew the pipe o and the tubes 7; then turn the pump so that the cylinder b is up, and a downwards; the pipes and tubes are then replaced, so that I is joined to the cylinder b, and o to a. 34. The valves are placed in such a manner between the screws at the bottom of the cylinder, that they may easily be taken out and turned another way, as is shown in the drawing, so that they have different directions, and may act in either way, as is judged necessary. If it be wished to make a trial of purified air, the apparatus containing the air must be screwed into the opening k of the introduction pipe i, the gas will immediately, by drawing the handle of the pumps upwards, float out of the apparatus into the cylinder a; and in consequence, by the reaction of the pistons downwards, be introduced into the lungs; and should the room where the operation is performed be too close, and filled by foul air, then take a long tube, and place the funnel either out of the window, or into the next room, where the air is cool and fresh. 35. It will be perceived that the air-pump employed for pneumatic experiments depends for its efficacy entirely on the elastic quality in the air, by which, while there is any portion of air in the receiver and exhausting tube, that portion, however small, will expand and diffuse itself equally through the barrel in addition to the space it before filled. It must be pretty evident, with very little consideration, that by this process a perfect vacuum can never be produced under the receiver. For some air, however small the quantity be, must remain after every depression of the piston. Let us, however, examine how nearly we may approach to a vacuum, or, more properly speaking, let us determine what degree of rarefaction may be effected, supposing the mechanical construction of the instrument we have described to be perfect, and no obstructions to arise from circumstances merely practical. Let 36. At the commencement of the process the air which fills the receiver, exhausting tube, and barrel, is of the density of the external air; let its entire quantity in this state be called one. the capacity of the air barrel bear any proposed proportion to that of the receiver and tube; suppose that it is one-third of their united magnitudes, and therefore that it contains one-fourth of the air contained within the valve in the entire apparatus. Upon the first depression of the piston this fourth part will be expelled, and threefourths of the original quantity will remain. Onefourth of this will in like manner be expelled upon the second depression of the piston, which is equivalent to three-sixteenths of the original quantity, and consequently there remains in the apparatus nine-sixteenths of the original quantity. Calculating in this way, that one-fourth of what is contained in the apparatus is expelled at every descent of the piston, the following table will be easily computed :: Air remaining in the Receiver 37. The method by which the computation might be continued is obvious. The air expelled at each stroke is found by multiplying the air expelled at the preceding stroke by 3 and dividing it by 4; and the air remaining after each stroke is also found by multiplying the air remaining after the preceding stroke by 3, and dividing it by 4. 38. It appears, by this computation, that after the fifth stroke the air remaining in the receiver is less than one-fourth of the original quantity. Less than one-fourth of this will remain after 4 X 4 X 4 X 4 X 4 the next five strokes, that is, less than one-sixteenth part of the original quantity. If we calculate that every five strokes extract three-fourths of the air contained in the apparatus, we shall then under-rate the rapidity of the exhaustion; and yet, even at this rate, after thirty strokes of the pump, the air remaining in the receiver would be only 30 th part of the original quantity. The pressure of this would amount to about the sixteenth part of an ounce upon the square inch. It is evident that, by continuing the process, any degree of rarefaction which may 3096 be desired can be obtained. For all practical purposes, therefore, a vacuum may be considered to be procured; but, in fact, we are as far from having a real vacuum in the receiver as ever, for such is the infinitely expanding power of air, that the smallest particle will as completely fill the receiver and barrel as the most dense substance could; that is to say, no part of the receiver or barrel, however small, will be found absolutely free from air, however long the process of exhaustion may be continued. 39. With regard to the real amount of rarefaction, it may be proper to state that Mr. Nairne having been perplexed by the disagreement of the pear gauge and the common barometrical gauges, in their indications of the degree of exhaustion in the receiver, was induced to undertake a series of experiments, in order to investigate the cause of the disagreement. He exhibited to the honorable Mr. Cavendish, Mr. Smeaton, and other members of the Royal Society of London, an experiment in which this disagreement amounted to some thousand times, and Mr. Cavendish immediately furnished him with a satisfactory explanation of the fact. It appeared,' he said, from some experiments of his father, lord Charles Cavendish, that water, whenever the pressure of the atmosphere on it is diminished to a certain degree, is immediately turned into vapor, and is as immediately turned back again into water on restoring the pressure. This degree of pressure is different according to the heat of the water. When the heat is 72° of Fahrenheit's scale, it turns into vapor as soon as the pressure is no greater than that of three-quarters of an inch of quicksilver, or about one-fortieth of the nsual pressure of the atmosphere; but when the heat is only 41° the pressure must be reduced to that of a quarter of an inch of quicksilver before the water turns into vapor. Hence it follows that, when the receiver is exhausted to the above-mentioned degree, the moisture adhering to the different parts of the machine will turn into vapor, and supply the place of the air, which is continually drawn away by the working of the pump, so that the fluid in the pear gauge, as well as that in the receiver, will consist in good measure of vapor. Now, letting the air into the receiver, all the vapor within the pear gauge will be reduced to water, and only the real air will remain uncondensed; consequently the pear gauge shows only how much real air is left in the receiver, and not how much the pressure or spring of the included fluid is diminished, and that equally, whether it consists of air or vapor. 40. Now this ingenious explanation, which Mr. Nairne considered as perfectly satisfactory, contains the fundamental principle of producing artificial dryness and cold in the receiver of the air-pump, viz. that the evaporation from any wet body, and consequently its dryness, and the cold arising from evaporation, increases with the degree of exhaustion; and, consequently, that artificial dryness and artificial cold may be produced under the receiver of an air-pump. 41. In following out the valuable principle of Mr. Cavendish, Mr. Nairne placed several fluids and wet substances under the receiver. Three grains of water in a watch-glass lost one grain and a half by evaporation in ten minutes; 100 grains of spirit of wine lost nine grains. Every substance which he tried sustained a certain luss by evaporation, excepting sulphuric acid, which always gained, by absorbing the vapor exhaled from the wet part of the pump. Having thus ascertained that the exhaustion of the air produced a rapid evaporation, and that the sulphuric acid absorbed the vapor thus exhaled, Mr. Nairne used this process for producing dry air in the receiver, in order to try the effect of the passage of the electric fluid through a dry and a moist atmosphere. 42. 'I now,' says he, 'put some sulphuric acid into the receiver, as a means of trying to make the remaining contents of the receiver, when exhausted, as much as possible to consist of permanent air only, unadulterated with vapor.' The consequence of this was, that the electrical phenomena were exhibited in the air which he had dried, and very imperfectly in air which he had made damp, by introducing a piece of wet leather, and removing the sulphuric acid. 43. The next step which Mr. Nairne takes is to produce artificial cold by the air-pump, and he gives an account of his experiment in the following words :- Having lately received from my friend, Dr. Lind, some ether prepared by the ingenious Mr. Wolfe, I was very desirous to try whether I could produce any considerable degree of cold by the evaporation of ether under a receiver whilst exhausting. For this purpose I put the ether into a phial, the neck of which was sufficient to admit the ball of a thermometer; this being placed on the air-pump, under a receiver which had a plate at the top, with a wire passing through a collar of leather; to this wire the thermometer was fixed, by which means I could easily dip the ball of the thermometer into the ether. 44. The pump was now worked; and, whilst the receiver was exhausting, the ball of the thermometer was often dipped into the ether; and, when the degree of exhaustion by the barometer gauge was 65° (which was the utmost in this case that the pump would exhaust to), the degree of cold indicated by the fall of the quicksilver in the thermometer was 48° below 0° on Fahrenheit's scale; so that there was a degree of cold produced 103° colder than the air in the room where the experiment was made, the thermometer in it being at 55° above 0°. The pump was kept continually working for half an hour, and the ball of the thermometer often dipped into the ether; but no greater degree of exhaustion or cold could be produced. The air being let into the receiver, the quicksilver in the thermometer rose 10°, viz. to 38° below 0°. 45. Fresh ether being put into the phial to what was remaining, the thermometer rose to 30° above 0°: the pump was then worked again constantly for half an hour; yet by the barometer-gauge, the degree of exhaustion was now not more than 16°, and the degree of cold produced did not exceed 11° below 0°, as appeared by the quicksilver in the thermometer. The air being let into the receiver, the remaining ether was examined, and there were found several pieces of ice at the bottom of the phial, some of them as big as large peas, which, when the ether became nearly of the heat of 32°, or freezing point of water, were entirely, dissolved.' 46. A very elegant hydro-pneumatic fountain may be formed by employing the air pump: the arrangement of the apparatus may be easily understood. A brass plate is furnished with a receiver made to fit air-tight, and the whole may be connected by a stop-cock placed beneath. If the receiver be now exhausted of air, the stopcock turned, and the lower extremity of the tube immersed in a vessel of water, the moment a communication is opened with the receiver a jet of water will be seen to ascend in a continuous stream. There is another mode of producing the same effect, without the intervention of the airpump. To exhaust the receiver, in this appara ratus, the lower part of the glass must, in the first instance, be filled with mercury, and a communication opened by a pipe with the cup of mercury; if the pipe be thirty inches in length, it is evident that the fluid metal must sink; and as it descends a partial_vacuum will be formed within the receiver. The air then pressing on the surface of the water will drive it up the perpendicular tube, and a jet, exactly similar to the one already described, will be the result. That the air-spring causes it to expand when the external pressure is reduced may now be rendered apparent by the air-pump. To effect this, we need only place an egg beneath the receiver, from which a piece of the shell has been broken at the small end, and it will be found, on rarefying the air, that the small bubble of air contained in the large end, will, by its expansion, drive out the whole of the contents of the shell. A withered apple, or any other fruit, placed beneath the receiver of the air-pump, will immediately expand and appear perfectly fresh; but they will return to their original bulk the moment that the air is re-admitted; and a bladder about half filled with air will expand, and in some cases burst, by a similar process, the moment, however, that we re-admit the air, it will return to its original size; thus proving that no additional air was admitted, but that the spring of that which it previously contained produced the effect. This experiment may be varied by putting the bladder in a frame, and placing weights upon it, which will be raised by the expansion of the enclosed air. 47. There is a very interesting experiment that may be easily performed with a common airpump, which is intended to illustrate the facility with which fluids boil beneath an exhausted receiver. If water, below 212° of Fahrenheit's thermometer, be placed on the pump-plate, and the air withdrawn, it will instantly boil, but on the readmission of air ebullition will cease. There is an experiment nearly analogous to this, which may be performed without the aid of an air-pump: if we half fill a Florence flask with water and place it over a lamp, letting it boil briskly for a few minutes, and then cork the mouth of the flask as expeditiously as possible, it will be found, on removing the flask from the lamp, that ebullition will still continue. The boiling may be afterwards renewed, by wrapping round the empty or upper part of the flask a cloth wetted with cold water, or by gradually pouring cold water upon its external surface; but, if hot water be applied to the flask, the boiling instantly ceases. In this manner the ebullition may be renewed and again made to cease alternately by the mere application of hot and cold water. 48. One of the most striking and self-evident proofs of the materiality of air will be found in the resistance that it offers to the motion of any body whose surface is large. The sails of a ship and a wind-mill are affected in the same way as the minutest blade of grass; and this will be equally apparent in the motion of smoke, and the devastating effects of the African or West Indian tornado. When the air is at rest in a quiescent state we can move in it with the utmost facility; but when the motion is quick, or the surface extensive, as in the fly of a clock, its resistance then becomes obvious to the senses. illustrate this fact a double fly, with vanes of unequal size, is usually placed beneath the receiver of the air-pump, and so constructed that motion may be communicated when the air is withdrawn. When this is effected it is found that the fly that exposes a large surface to the action of the atmosphere passes round as swiftly as the smaller vanes; but, on re-admitting the air, its velocity will be very much diminished if not altogether destroyed. To 49. The utility of a fly in machinery may, upon this principle, be very readily explained. A fly is an equaliser of motion, and, if we suppose that motion to be too quick, a larger surface is exposed to the resisting medium or air, and vice versa. 50. Another mode of showing the air's resistance by means of the air-pump may now be adduced; it is well known that, bulk for bulk, feathers are lighter than gold; and, in proof of this, we find that if a feather and a guinea be dropped from the hand at the same time the latter will reach the ground first. If, however, the experiment be performed in vacuo, a very different result is obtained. If we take a tall receiver furnished with a plate at the top, and provided with an apparatus for supporting and lowering a guinea and a feather at pleasure. When this is exhausted the guinea and feather may be discharged, and they will reach the bottom at the same instant of time; thus proving that all bodies would descend to the earth from equal heights in equal times were it not for the resistance of the air. That bodies float in the air by which they are surrounded may also be shown by withdrawing a portion of the air from a large receiver, and we shall find that the weight of any enclosed body will be increased. The usual mode of exhibiting this is, to suspend a bunch of feathers at one end of a beam of a delicate balance, and attach a piece of lead of equal weight at the other. Now it will be evident that the bulk of the feathers must be greater than that of the lead, and, as such, that their buoyancy must be the greatest; so that on withdrawing the air they will cease to float in equilibrio, the feathers will descend, the equilibrium will be destroyed, and we shall find that one of the lightest substances there is will apparently become the heaviest. The exceeding minuteness of the particles of |