same fact may be proved by exposing the vessel again to cold, when the very same weight of ice will again be obtained as was originally placed in the vessel. Hence, it is very far from being a matter of course that water should be found in a fluid state. The limits of temperature, between which that condition is fulfilled are very small. Had the heat of the earth beer comparatively but little less than it is, water would have existed, naturally, only as a solid substance; the ocean would have been a mass of ice. Had the heat of the earth been much greater, every drop of water would have been dissipated into vapour.

All

There is another very remarkable circumstance connected with the communication of heat to water. fluids are expanded by the addition of heat; and we have already seen that this property, in mercury, enables us to measure the quantity of sensible heat by the degree of expansion. If mercury be gradually heated, it continues to expand very nearly equally, till it reaches a temperature of 660° of Fahrenheit, and boils.

60%

Other fluids expand also, although not so equally, by the addition of heat, and contract by being cooled; but in water there is a striking deviation from this otherwise general law. Suppose a large thermometer-tube, A T, to have been filled with boiling distilled water, and then hermetically sealed, or closed by means of the blow pipe, at A, and that, at the temperature of 60°, the water stands at the point marked in the figure. If the bulb be now plunged into a freezing mixture, the fluid will be observed to contract, until it has attained a temperature o. about 40°. After the degree of coolness has been reached, the water will be observed to rise again in the tube, indicating an expansion in the fluid, until just below it is cooled down to the freezing point, 32°, it stands at the same height as it did at the temperature of 48°.

T

4的
400

82

Fig. 2.

In the act of freezing, water expands with great

rapidity, and if confined, with irresistible force. Every one must have had experience of the breaking of a bottle, or other vessel, by the freezing of water in it; and an iron bombshell has been burst by the

same means.

The Florentine academicians succeeded in bursting a brass globe, the cavity of which was an inch in diameter, by filling it with water and freezing it. The force necessary to produce this effect was calculated at 27,720 lbs. The quantity of expansion is such, that eight cubic inches of water form about nine cubic inches of ice.

The deviation from the ordinary law of expansion in the case of water is a fact of immense importance. If water continued to be compressed until it froze, as in the case of other liquids, large bodies of water, instead of being covered with a coating of ice, would be converted into solid masses; a state which would destroy the existence of almost all living creatures, which now pass the winter under water in security and comfort. The cold, which congeals water, is usually applied at the top: as soon as a small quantity of water is cooled, it becomes specifically heavier than the rest, and sinks, thereby exposing a fresh surface to the action of the atmosphere. Thus a constant current is kept up, the cooler water descending, and the warmer ascending, until the whole reaches the temperature of 40° (or S less than freezing). After this point, the cooler stratum of water at the surface expands, and becomes specifically lighter than that below; it therefore floats, and so continues, until a sheet of ice is formed at the top, while the temperature of the water below may be seven or eight degrees warmer,--a degree of heat quite sufficient for fish and other water animals.-S. M.

BRITISH FREEDOM.

It is not to be thought of that the Flood
Of British Freedom which to the open sea
Of the world's praise from dark antiquity
Hath flowed, "with pomp of waters, unwithstood,"
Roused though it be full often to a mood

Which spurns the check of salutary bands
That this most famous stream, in bogs and sands
Should perish, and to evil and to good

Be lost for ever. In our halls is hung Armoury of the invincible knights of old

We must be free or die, who speak the tongue That Shakespeare spake, the faith and morals hold Which Milton held. In every thing we are sprung Of Earth's first blood-have titles manifold.

-Wordsworth.

THE LEVEL SURFACES OF LIQUIDS. THERE is not presented to us in the whole range of our experience any instance of a level surface more perfect than the gentle surface of a liquid. A still lake or pond, or the water in our cisterns and water-jugs, are all equally level at the surface. They require none of man's agency to make them so, for they become so whether he wills it or not. We take advantage of this property to assist us in determining the level of a piece of ground in surveying. A bubble of air is enclosed in a tube, containing a liquid; and the surveyor places thy tube horizontally, and knows when it is quite level be observing that the bubble of air is in the middle of the length of the tube. The liquid has then no tendency to drive the air bubble in one direction more than another, and therefore the bubble remains in the middle, from which circumstance the surveyor knows that any piece of wood or other surface on which the tube is resting is level. This tube he calls a spirit level. The liquid employed in the tube is usually coloured spirit, because

pure spirit, or alcohol, is never frozen, however great the cold may be to which it is exposed.

The reason of this perfect level to which the surface of a liquid attains, is that liquids, like every other body, tend towards the centre of the earth; and that as the particles of water move freely among themselves, any unequal pressure is soon communicated to surrounding parts. Suppose now that we had a large cistern of water, and that the surface of the water were three or four inches higher near one end of the cistern than near the other, there would be a larger amount of water, a greater number of particles pressing on the bottom of the cistern at the former part than at the latter. Each particle presses on that which is beneath it; and as there would be a loftier column of particles at the one part of the cistern than at another, those particles which are near the bottom of the heavier column would press on those that surround them, and force them upwards, in order in fact, to allow room for themselves to escape from some of the pressure which they experience. There continues to be this pressure until the surface is level in every part, when, as all parts of the liquid near the bottom are equally pressed, 'no one can yield to another, and they all remain in equilibrium (a word which means equally balanced). It appears, therefore, that as soon as the surface of the liquid becomes, from any cause, out of the level direction, a

commotion and a kind of a struggle takes place, and does not cease until the level is again attained. There is a very good experiment which shews this tendency of liquids to maintain a perfect lovel, and to descend whenever an opportunity offers for so doing.

Fig. 1 represents the section of a vessel or basin, which exhibits the paradoxical property of never becoming full, however much water may be poured into it. vessel looks neither like a sieve nor a cullender*; no holes

Fig. 1.

*More correctly spelt colander.

The

can be seen in it, and no water is seen to flow from it. A little inspection of the construction will, however, enable us to solve this riddle. It may be seen that the vessel is sufficiently thick to have a groove or channel cut in it. At the bottom of the vessel, at a, there is a small opening, which leads into a channel ascending to the point b, and from thence descending to the point c, where it is enclosed in the foot or stand of the vessel, which is hollow. Now, when water is poured into this vessel, some of it enters the little channel at a, and ascends as fast as more water is poured into the vessel; just before the vessel is quite full, the water ascends to that part of the channel which begins to turn downwards, and immediately on attaining that level, the water flows down the outer channel as fast as it is poured into the vessel, provided that is not done too quickly. There is a conveyance for carrying off the water from the lower cistern at c, by a concealed pipe, not shewn in the figure.

Here, then, we have an instance of the tendency of water to keep a constant level, whether it branch out into two or three streams, or remain in one body. The water ascends in the narrow channel just as fast as in the broad open part of the vessel; and when it arrives at the level of the bend in the channel, B

instead of rising still higher in the vessel, it turns into the descending part of the channel, and so flows out.

Fig. 2.

This property has been made the groundwork of an amusing experiment. Fig 2 is a philosophical toy, called the Cup of Tantalus. A little figure of a man or boy is sitting in the cup, and his face is made to express great anxiety to obtain something to drink; but that he can never obtain. If we pour water into the cup it will rise just to the level of his chin, but no higher, and the little martyr to thirst is obliged to keep his lips dry, whether he will or no. The mystery is ingenious, and is very similar to the experiment which we last described.