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beaten back, and so brings the sound with it a second time to your ear; and again, after passing you, if it met with the same sort of obstacle on the other side, it would be sent back again, and so strike your ear in passing and repassing, losing a little every time until it entirely died away. This would be called an echo; people living in a flat country have not so many opportunities of observing it as those who inhabit a craggy and mountainous one.

Water—a fluid at the common temperature of the atmosphere. Have you ever seen it solid ? In winter-in frost

it is then ice.—How high does the thermometer stand when water begins to freeze ? 32o.-Look at the thermometer in the room, how high is it? 52o.-How many degrees above the freezing point ?-Does it increase in volume when it becomes ice? Water from the temperature of about 390, expands as it grows colder, and at 32°, when it becomes ice, expands so as to crack water-bottles, water-pipes ; a piece of ice floats in water, part of it being · above the surface; if it were of equal weight with the

same volume of water, it would just sink so as to have no part above.--You should never let water stand in leaden pipes, or in vessels likely to be broken by its freezing in severe frosts. This expansion of water in becoming ice, how serviceable to the farmer, in some soils, in pulverizing and making them fit for vegetation - good for gardens, etc. • " That water contracts in reducing the temperature to about 40°, and below that again expands, is easily shown, by taking two equal thermometers, the one filled with water and the other with spirit; placing them in melting ice, the spirit one will gradually fall to the freezing point, but the other will fall to about 40°, and then begin to rise. By Act of Parliament, the temperature at which the specific gravity of spirits is determined by the excise, and at which the standard weights and measures are adjusted, is 620 of Fahrenheit.”- DANIEL's Chemical Philosophy.

Quicksilver, unlike water in this respect, contracts and becomes denser in becoming solid. It has been ascertained, by leaving it exposed to the cold in high latitudes, where it has assumed a solid form, and observing the temperature

at which it begins to thaw, that the freezing point is about 40° below zero of Fahrenheit,

Attention may be called to the way in which the roads are raised up in winter by the freezing of the moisture within them-how after a thaw a loaded cart or waggon sinks in, causing deep ruts--how rocks and stone, which have absorbed much moisture, split after frost-parts of buildings peel off, etc.

Can water be made into a vapour-something you cannot see? By heat it becomes steam, thermometer 212° at the average pressure of the atmosphere; one inch of water makes about a cubic foot, 1728 inches, if further heated it exerts a greater pressure in trying to escape, pressing on the surface of the vessel in which it is. This is the property which makes it so serviceable to us in grinding our corn, moving the machinery for spinning and weaving, of steam-boats, etc., and as a motive power on our railroads, carrying us forty or fifty miles in an hour. If cooled below 212° it immediately falls back, shrinks up into one inch, and becomes visible water again, giving out a great deal of heat ;--instance steam raising the kettle-lid.

Why does the tea kettle, just before boiling, very often force out a quantity of water from the spout? Because the air, driven from the water by heat, and the steam which is forming from the water, rise to the top, and the lid happenning to be air-tight, it cannot escape, and being lighter than water it cannot descend, so the vapour or stcam under the lid increases and expands, and, pressing upon the surface of the water, forces it out at the pipe.---Did you ever see on a frosty day, when you were going with a team, what you call the breath of the horses, or your own breath ? Yes, Sir.

Teacher. The warm air from the horses' mouths, or from your own mouth, containing vapour which you cannot see when the air has a certain degree of warmth in it, as soon as it comes in contact with the colder air gets cooled, and the steam or vapour becomes water (is what they call condensed), or perhaps watery vapour, which you can see, instead of a vapour which you could not see,

Did you ever see sugar or salt melted in water? No, Sir; but we have seen sugar in tea.—Then the teacher takes a small phial containing water, and puts in a certain quantity of salt, when entirely melted they see the fluid perfectly clear; increase the quantity beyond what the water will take up, this remains undissolved. If the temperature of the water were increased, it would take up more; in the same way the air will take up a greater quantity of vapour the warmer it is, and coming from the mouth warm, it holds more vapour than it is able to do, when it comes in contact with the cold air, and throws some of it down, so that you can see it; thus water on the inside of the window in frosty weather-dew on the outer surface of a bottle of cold water in hot weather, etc. —the quantity of watery vapour in the air in hot climates greater than in cold, hence torrents of rain when it is suddenly cooled, etc.

About London, latitude 51° 30', the average fall of rain in the year is about 23 inches; while in Rome, latitude 41° 54', it is 38 inches; at Calcutta, latitude 22° 34', it is 81 inches; and in climates like the West Indies upwards of 100 inches ; but though the quantity of rain falling in hot countries is greater than in the temperates ones, the number of wet days is greater in the latter than in the former ; there is more moisture in the air in our climate in summer than in winter ; but from the greater temperature it is held up, and is not so sensible to us. By inches of rain is meant the depth at which it would stand on every square inch of surface on which it falls, supposing none to be absorbed by the soil or to evaporate.

Two fluids in the same vessel, one lighter than the other, which would get to the bottom ? The heavier one. Give instances. Milk and cream, water and oil, quicksilver and water, water and air.

The teacher, holding up a glass : What is this glass full of? Atmospheric air.- If I pour in water, what does that do ? Drives out the air, because it is the heavier fluid ? If I pour quicksilver into a glass of water, what would take place? The quicksilver would drive out the water for the same reason.-If water upon mercury, or oil upon

water? The water or oil being the lighter fluids, would rest on the top, and the same thing would take place if carbonic acid or any gas heavier than air were poured in. - Another instance: fill a small phial with water, leaving room for a bubble of air, then cork it ; holding it in a horizontal position the bubble rests in the middle, elevate one end, the bubble rises to the top; show how this may be used as a spirit-level.

Look at that cubical vessel on the table, divided into two equal parts by a division in the middle. Suppose one division full of mercury, the other of water, and the partition suddenly withdrawn, what happens ? The mercury immediately covers the bottom of both parts, and the water rises to the top.

Take a bottle of water from a cool spring or from the pump; place it in the sun or in a room-for instance, as you see it sometimes in a bedroom. You will observe airbubbles form themselves on the surface of the glass—at the bottom and the sides—this is air contained in the water. As it takes the temperature of the room these airbubbles form themselves, expand as they rise, come suddenly to the top, the water being of equal temperature throughout. Why does the bubble expand as it rises ? The pressure upon its surface varies as the depth ; and therefore the nearer the surface the less the pressure.

How is it then, if you place water in an open saucepan on the fire to heat, we see at first bubbles form themselves at the bottom, like pieces of glass, rise up a little way, and are then lost before coming to the surface. .

The air in that part of the water in contact with the bottom of the saucepan, immediately it begins to feel additional warmth, forms a bubble, rises up a little way, and although the pressure is diminished, it becomes again compressed, in consequence of coming in contact with cooler water as it rises. This it is, I believe, which causes what is called the hissing of the kettle.

If you were to boil a quart of water until it has all, as you call it, boiled away, what has become of it ?- All turned into steam.-If water with chalk or salt in it?

The water would go into vapour, and the chalk or salt be left behind at the bottom of the kettle.

Did yon ever see a white crust at the bottom of your tea-kettle? Yes, Sir; but we don't know what it is ? Don't you know we live upon what is called a chalk soil here, and the rain that falls makes its way through the chalk and comes out underneath it, having taken up some of the chalk in its way through. If our hills had been of iron ore, lead, or salt, the water would have taken up some of these substances in passing through them, as it always takes up some of the earth through which it filters--as it is a fluid in which many things are soluble ; thus, we get water with chalk in it—when you boil it, the pure water goes off in vapour, and leaves the chalk behind, which falls to the bottom of the kettle: besides this, although hot water will hold up or melt more sugar or salt than cold, yet it will not hold more chalk, on the contrary, less, as the heating drives off a particular gas or air (called carbonic acid gas), which has a great liking for the chalk, and holds it up in the water, so that what falls to the bottom partly belongs to the water which is driven off, and partly to that which is left in the kettle. These are two reasons, therefore, why your kettle has a white mass of chalk at the bottom.

Taking off the lid of a kettle when the water is boiling, turning it up, what do you observe ? Drops of water. These are formed by the steam coming against the lid, cooling it down so that it becomes water- the lid being in contact with the atmosphere conducts off the heat from the steam—this is distilled water or pure water, containing no lime, salt, etc.

Two fluids mixed together, which become vapours at different temperatures, may be easily separated-thus a mixture of spirit and water ; heat the mixture up to the temperature at which spirit becomes vapour, it goes off and may be collected, the water remaining behind,

That the boiling point of water or any other fluid varies with the atmospheric pressure --- how this may be applied to find the altitude of mountains—that water at the top of Mont Blanc, for instance, boils at a temperature of about

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