properties.-1. Air is compressible, that is to say, may by pressure be made to occupy less room than in its natural state. Thus, we had formerly occasion to remark, that, in discharging a pop-gun, the rammer, in consequence of the compression of the air in the tube, is able to advance a little way before it expels the plug; and that, when a wine-glass is immersed with its mouth downwards in a vessel of water, the water will, from the same cause, ascend to a small height in the glass.2. Air, as we have said, is remarkably elastic: that is, after being compressed, it, as soon as the pressure is removed, resumes its former dimensions. Squeeze a blown bladder, and, whenever you remove your hand, it immediately regains its original bulk. Throw it on the ground, and the elasticity of the air pent up in it will display itself, by the force with which it rebounds. It is obviously by means of this quality, that it makes so excellent a football.-3. Air, like every other fluid, is heavy, and presses equally in all directions. From the equality of the pressure upon all bodies, and upon all parts of the same body, it is not easily felt or perceived; but, whenever it is partially removed from a body (which is easily done by an instrument called an airpump), then its effects upon the other parts of the body are clearly discerned. Thus, if the air be exhausted from the inside of a glass vessel called a receiver, the vessel will be held quite fast by the pressure of the external air. So, also, if you completely fill a wine-glass with water, and cover it with a piece of paper, then place the palm of your hand over it, so as to hold it quite tight and accurately even; you may afterwards turn it upside down, and remove your hand from the paper, without spilling a single drop of water. For the same reason, a small phial with a narrow neck retains a liquid within it, even when its mouth is turned downwards. Upon the same principle, take a common teacup, and burn some paper in it, by which the air within it will be made to expand; invert the cup saucer containing water; when the air cools it will return to its former density, and thus leave within the cup what philosophers call a vacuum, that is to say, an empty space containing no air; in consequence of which

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the pressure of the external air from below will force up a great portion of the water into the cup to fill up the void. These instances will be sufficient to illustrate the upward pressure of the air; its lateral pressure (that is to say, its pressure sideways) is no less easily shown by means of the air-pump. If we place a bladder, containing very little air, under the receiver of the pump, having previously tied the neck of the bladder quite close; so soon as the external air is exhausted, the air within the bladder will immediately distend itself in a surprising manner; but, on the re-admission of the external air, will resume its former state. You will find an amusing experiment of this kind mentioned in the "Conversations on Natural Philosophy," by which a shrivelled apple was, under the receiver, rendered quite plump and apparently fresh. It is in consequence of the downward pressure of the atmosphere (as was formerly explained to you), that its lower strata are so much denser than the higher. The difference in this respect is very considerable between the atmosphere at the top of a high mountain, and in the valley below. The density of the atmosphere at its different heights, and in its different conditions, is determined by an instrument called a BAROMETER, of a very simple construction. For this purpose, a glass tube of about three feet in length, and open only at one end, is filled with mercury; then the open end, being in the mean time stopt with the finger, is immersed in a cup containing some mercury part of the mercury in the tube falls into the cup, but only a part, the remainder being supported by the pressure of the air from below; because, as you will remember, there is no counteracting pressure from above, the tube being completely shut out at the top from all communication with the atmosphere. The denser the atmosphere is, the more strongly it upon the mercury, which accordingly rises higher in the tube. When the atmosphere is dense, it generally carries up the clouds to a greater height, and we are then led to expect dry weather; when it is rare, the clouds descend, and rain or snow naturally follows. Hence the barometer acts as a weather-glass, and very often receives that name. In consequence of the

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greater density of the atmosphere in the lower regions, than in the higher, it is far better adapted to the condition of man. The rarity of the air above has been found productive of great inconvenience to those, who ascend high mountains or in air-balloons, by producing great difficulty of breathing, bleeding at the nose, &c. -The atmospheric air is 800 or 1000 times lighter than water; but, from its great height (being supposed to extend at least forty-five miles from the earth), its pressure upon the earth's surface, you may believe, is extremely great, and is computed at fifteen pounds upon every square inch. A man of ordinary size, therefore, sustains a weight of no less than fourteen tons. You may, perhaps, be surprised to think how little so heavy a pressure is felt by us, or impedes our movements. This is owing to its equal influence upon all parts of the body, external and internal, at the same time. Such is the internal pressure, that, were the external pressure removed, the body would in a moment be blown up. When the pressure is removed from any part of a body, its pressure upon the other parts then becomes quite perceptible. It is from this cause that, in the surgical operation of cupping, the swelling of the part under the cup is produced.

ON THE GENERAL EFFECTS OF HEAT.

THE Word HEAT is used in two significations, being sometimes employed to denote a well-known sensation, and sometimes the cause of that sensation. Thus we speak both of the heat of a fire (to which some chemists, by way of distinction, give the name of caloric), and also of the heat which we feel on approaching a fire. It is in the former sense, as expressive of the cause of the sensation, we mean at present to employ the term. This HEAT (regarding the nature of which philosophers themselves are not agreed) is produced in bodies, not only by direct communication from others which had previously been heated, as when you apply a lighted paper to a candle, but also by percussion, as when you hammer a bar of iron; by friction, as when you

rub two pieces of wood against each other; and by attrition, as when you strike a flint with steel. Its general effects, besides the sensation already alluded to, which it produces on animated beings, are EXPANSION, LIQUEFACTION, VAPORIZATION, and INCANDescence. -I. EXPANSION, or enlargement of bulk, is produced by the addition of heat in almost all bodies, whether solid, liquid, or aëriform. Throw a bar of iron into the fire, and you will find it increase, not in one of its dimensions only, but in all: it will become longer, broader, and deeper. Hold a tube containing spirit of wineover a fire, and the liquor will rise in the tube, in consequence of its expansion. In like manner, hold over a fire a bladder nearly filled with air, and the bladder will, from the expansion of the air, be distended. In none of these cases does any increase of gravity accompany the increase of bulk; and, in all of them, so soon as the heat is removed, the body returns to its former dimensions. Some bodies expand by heat much more than others. Thus, if we put water into one tube, and spirit of wine into another of precisely the same description, and apply equal heat to both, the spirit of wine will rise much higher than the water.-The expansion of bodies by heat led to the invention of THERMOMETERS, which enable us to ascertain the state of temperature, by the rise or fall of a fluid contained in a very slender tube. The fluids found most convenient for this purpose are spirit of wine and mercury, but particularly the latter. The precise extent of its rise or fall is marked out by a scale divided into degrees. In order to make all thermometers correspond with each other, it was obviously necessary that their scales should be rendered uniform. For this purpose it was of importance to discover some fixed points, which should indicate certain conditions of temperature. It was found, that, if a thermometer be immersed in melting snow, the fluid contained in it always stands at precisely the same point. It was also found, that, if it be immersed in boiling water (when the pressure of the atmosphere is in its ordinary state), the fluid always stands at the same point. These two points, the freezing or melting point, and the boiling point, were quite sufficient for

the purpose in view. The interval between them could then be easily divided into any number of equal parts, called degrees. Fahrenheit (whose thermometer we generally use in this country) divided this interval into 180 degrees. He did not, however, commence his scale at the freezing point. He selected a third point for this purpose. Erroneously thinking that there could be no greater degree of cold, than that which was indicated by the thermometer when immersed in a mixture of snow and salt, which is 32 degrees below the freezing point, he fixed upon this as the commencement of his scale. His freezing point accordingly is 32, and his boiling point 212. You must not imagine, that, whenever you see a body shrink in its dimensions by the addition of heat, this is to be considered as an exception to the general law of expansion. This effect is in most cases occasioned by the loss of moisture by evaporation; whereby the body is deprived of a part of its substance, and consequently of its weight. There is however a peculiarity with regard to clay, which continues to shrink even after it has ceased to lose weight by evaporation. It is remarkable, also, that water and some other substances, such as iron, sulphur, &c. at the moment of freezing, in place of contracting by the diminution of heat, expand in consequence, as is now supposed, of the new arrangement of the particles, in passing from a liquid to a solid form. The force, with which water, in this situation expands, is amazing. A cannon, on one occasion, having been completely filled with water, and closed at the mouth, was exposed to a cold temperature; when the water began to congeal, it expanded with such force as to burst the cannon. is in consequence of this expansion, that large masses of rock are, intime of frost, so often separated from the cliff. It also accelerates the decay of old buildings. Water expands, not only when actually co gealing, but at a temperature several degrees above the freezing point. Thus, if water be cooled in a tube well : dapted to exhibit the change of bulk, it gradually conti acts according to the general law, until the temperatu e has fallen to the fortieth degree of our thermomete; if cooled farther, it continues to expand until it freezes.-II. We have

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