room; when placed Fig. 23.

flame, when held above, at c (Fig. 23), is blown from the below at a, it is blown into it; consequently, the light warm air above rushes out of the room, and is replaced by heavier and colder air from below. A draught of air is also noticed in passing from the sunshine into the shade; where the sun shines, the warmer air ascends, and the colder air from the shade supplies its place. For the same reason, a current of air is produced wherever a fire is burning, in every stove, and round every lamp. To ventilate a room properly, the hot and foul air should be allowed to escape from some aperture near the ceiling, while cool and fresh air is admitted near the ground.

The air-balloons, first constructed by Montgolfier, strikingly show how buoyant air may be rendered by heat; these are caused to ascend merely by filling them with air, kept continually hot by a fire beneath.

Radiation of Heat.-By conduction, bodies can communicate or abstract heat only when in contact. But heat is felt even at some distance from a fire or from a heated stove, and the earth is warmed by the sun, although a space of millions of miles is between them. This sort of heating is called radiation of heat.

Experiment 3.-Envelop three tumblers with paper, one with silver paper, another with white, and a third with dull black paper, and place them in the sun; a thermometer will indicate that the tumbler with the black paper is heated the most, and that with the silver paper the least, and yet all these vessels have been equally exposed to the rays of the sun. This difference is explained on the principle, that some substances have the power of absorbing the rays of heat, while others throw them off again, or reflect them. Good reflectors of heat, such as bright metals, are the worst absorbers, for a thing cannot absorb the heat and also reject it.

If hot water is poured into the tumblers enveloped with paper, and the cooling of it noted by the thermometer, the

contrary effect will be observed; the glass covered with black paper will first become cold, and that wrapped in silver paper the last; because bodies with dull surfaces radiate the heat more rapidly than those with polished surfaces. Good absorbers of heat are therefore seen to be good radiators, and good reflectors bad radiators. For this reason, coffee retains heat longer in a bright than in a tarnished pot; a stove of glazed Dutch tiles remains hot longer than another of unglazed tiles; a smooth sheet-iron stove, longer than a similar one of rough cast-iron, &c.

The radiation of heat enables us to explain some of those common natural phenomena which otherwise would remain obscure. Why do not the rays of the sun, even in the hottest summers, melt the snow upon the tops of high mountains, which are nearer than the level portions of the earth to the sun? Because they only heat those bodies which can absorb their warmth, as the rough surface of the earth. The snow is indeed struck by the rays of the sun, but being a white and shining body it reflects them and remains cold.

Formation of Dew.-When the surface of the earth has become warm, the air is heated by it; hence, during the day the lower strata will always be warmer than the upper. But a change takes place after the sun has gone down. The earth continues to radiate heat without receiving any in exchange, and its temperature consequently diminishes. On the other hand, the air does not so readily part with its heat, and therefore it retains during the night a higher temperature than the surface of the earth; it is only cooled where it comes in contact with the colder earth. If this cooling should reach the dew-point of the air, then the vapours are condensed, on the soil or on the plants growing upon it, in the form of small drops, just as a tumbler is covered with vapour when brought from a cold into a warm room,-dew forms. If the temperature of the earth sinks in the night to the freezing point, or below it, the aqueous vapour is deposited in a solid form, and is called hoar-frost.

The radiation of heat from the earth is greatest when the weather is clear and serene, but it is obstructed by clouds and wind. Hence the most copious deposit of dew takes place only in clear and quiet nights. The clouds serve as a screen

in reflecting back the rays of heat to the earth, so that it is but slightly cooled. The same effect is produced by the mats, straw, and boards with which the gardener covers his young plants, to protect them from the late frosts of spring, or from freezing. The annexed figure, in which arrows denote the direction of the rays of heat, will serve to render this process more intelligible.

SOLUTION AND CRYSTALLIZATION.

These subjects are so closely connected with that of heat that they may conveniently be studied in this place.

Solution.-Water can dissolve many bodies, and unite intimately with them, without losing its transparency. Such combinations are called solutions. If rain-water meets with soluble substances, either in the earth or in the rocks through which it oozes, it dissolves them; and this explains why almost all spring-water, on evaporation, yields an earthy or saline residue. Frequently this residue, particularly when it contains lime, is so altered during evaporation, that it can no longer be dissolved in water, and forms a hard incrustation round the inner sides of the vessels used in cookery. The springs of Carlsbad deposit so much residue, that articles immersed in them appear in a short time to be externally petrified or incrusted. If water is unusually rich in soluble

substances, especially such as possess medicinal properties, as, for example, iron, sulphur, &c., it is called mineral water, and the springs from which it issues are called mineral springs. A pound of sea water contains about half an ounce of substances in solution.

Experiment 1.-Pour a teaspoonful of slaked lime into a bottle, and fill it with water; cork it up, and, after shaking it for some minutes, let it stand until the water has become perfectly clear. By carefully inclining the bottle, most of the liquid may be poured off free from the sediment. This operation is called decantation, and the clear liquid is limewater. Lime is but slightly soluble in water, seven hundred and seventy-eight ounces of water being required to dissolve one ounce of lime; the excess remains undissolved, and as lime is heavier than water, it settles at the bottom. That a portion of it has been dissolved is known by the peculiar taste imparted to the liquid. This taste is called alkaline.

Keep a part of the lime-water in a well-stopped bottle for future use; it will remain transparent and clear. Pour the remainder into a tumbler, and expose it to the air; the water soon becomes turbid, and covered with a film, which gradually grows thicker, and settles at the bottom. When, after some days, the water has become clear again, it will have lost its alkaline taste; the lime dissolved in it, having been chemically changed by the air and rendered insoluble, will be found as a powder at the bottom of the tumbler.

Fig. 25.

Fig. 26.

Experiment 2.-Put into a flask half an ounce of litmus; pour over it three ounces of water, and let it remain in a warm place until the liquid has obtained a dark-blue colour. Litmus consists of a blue colouringmatter, which is soluble in water, and is hence taken up by it; it also contains some earthy matter which is insoluble, and is deposited as a slimy mass. These two substances might be separated from each other, as in the former experiment, by decantation, but it can be done more readily by fil

tration. For this purpose cut a piece of blotting-paper into a

E

circular shape, fold it together twice, and then place this filter into a glass funnel. That the paper and the glass may not come into too close contact, place between them thin pieces of wood or glass; a piece of string must also be inserted between the funnel and the neck of the flask into which the liquid is to be filtered, to allow an opening for the escape of the air from the flask, as otherwise the fluid could not flow in. The filter, which must never be higher than the top of the funnel, is first moistened with water before the fluid is poured upon it. Blotting-paper consists of fine linen or cotton-fibres matted together, between which are small interstices or pores, through which liquids, but no fine solid particles, can pass; these remain on the filter. Writingpaper cannot be used for filtration, as its pores are filled up by size or starch.

Experiment 3.--Pour a part of the solution obtained into a saucer, and pass strips of fine blotting or of letter-paper one or more times through it, until they have acquired a distinct blue colour. Preserve these strips, after being dried in a box; they are called blue litmus, or test-paper; they are reddened by vinegar, lemon-juice, and all acid fluids, and serve to test a liquid, to ascertain whether it is acid (has an acid reaction).

Experiment 4.-Mix cautiously another portion of the solution with lemon-juice, until the blue colour appears distinctly red; this also serves to colour paper. The red testpaper is used for the purpose of recognising a class of substances opposed to acids-that is, alkaline or basic bodies; these restore the original blue colour of the paper, as can be seen by bringing it into contact with lime-water or solution of caustic potash.

Experiment 5.- Add gradually, with constant agitation, to one ounce of cold water, powdered saltpetre (potassium nitrate), as long as it continues to be dissolved-perhaps about a quarter of an ounce; if more is added than is necessary, it will remain undissolved at the bottom of the vessel. This solution is called a cold saturated solution. If this mixture be boiled, and saltpetre again be added, then about three ounces more will be required to saturate the water. A thermometer held in this boiling saturated solution will indi cate about 240° F. (115.5° C.), while simple boiling water