perhaps an inch or two above the cork. Apply heat, and the water will rise in the tube rapidly, and will presently begin to trickle out at the top. If the tube be a very thin one, merely holding the flask in your warm hands would be sufficient to make the water rise rapidly. The water might, if preferred, be coloured, in which case its expansion would be more easily watched.

EXPANSION OF GASES.-Partly fill a bladder with air, tie it up firmly, then heat it before the fire, and the air will expand and distend the bladder. This explains why you cannot blow up a football properly with your breath; for when you have, as you think, blown it quite full and tight, you forget that your breath is warm, that as it cools it will lessen in bulk, and that then your football will no longer be tight.

The force of expansion and contraction by heat and cold respectively, is enormous and almost irresistible, and is often applied to useful purposes. The tire of a wheel is fixed on red-hot, and then being plunged into water it cools, and contracts with a force far beyond human strength, so that the wheel is rendered firm and tight in all its parts. Large iron plates are riveted together with red-hot iron rivets, which on cooling bind the plates together with wonderful closeness. The walls of large buildings which had begun to bulge outwards have been straightened by passing iron bars from wall to wall; these were heated, then screwed up tight and allowed to cool. Another set of bars was then treated in the same way, and thus inch by inch the walls were restored to their proper position. For the same reason allowance must often be made for expansion by heat, or serious consequences may follow. An iron bridge seventy yards long would expand and contract more than an inch under the influence of summer heat and winter frost. The iron rails of a railway must be left a little apart, to allow of similar expansion.

The amount of expansion in any case depends upon the

nature of the body. It so happens that all gases expand equally, or nearly so, at the same temperature; but this is not so with liquids, nor with solids. Each liquid and each solid has its own power of expansion for any given increase of temperature-which is invariable for that substance. Thus if a rod of iron a yard long, when brought to a red heat expands in length a quarter of an inch, any iron rod will expand in the same proportion when raised to the same heat; but a brass rod will expand half as much again.

Use is often made of the different expansibilities of different substances. We will give one example: a clock pendulum beats rapidly or slowly according to its length; so that in summer time or in a warm room a pendulum will expand and beat more slowly than it ought to do, causing the clock to lose time. The simplest way of correcting this fault which has been invented is this: the pendulum is made of steel, with a glass vessel at the bottom containing mercury. Now, when the steel rod expands by heat, the mercury also expands twelve times as much in proportion to its length. The steel expands downwards, the mercury upwards; and thus a compensation is easily brought about by a small quantity of mercury, causing the pendulum to beat regularly.

THE THERMOMETER.

THE property, which almost all bodies possess, of expanding by heat, affords us a ready means of measuring heat. For the gradual expansion or contraction of some body, as the heat increases or decreases, will give us certain indication of the degree of that heat at any particular time. The substance most frequently used for this purpose is mercury (quicksilver)--a metal which is liquid at all ordinary temperatures.

This metallic mercury is inserted in a glass tube, having

a very small bore, with a bulb at one end. The tube is only partly filled, so as to leave room for the mercury to rise. The tube is then heated, and as the temperature increases, the mercury will gradually expand, rising higher and higher in the tube till it reaches the top, and begins to flow out. At this moment the glass at the open end is quickly heated by a blowpipe flame, which softens it like wax, so that the tube can be closed.

о

FAHR.

We have now obtained a thermometer, or instrument for measuring heat. The tube, as we said before, should be a fine one, so that when the mercury expands or contracts ever so little under the influence of a change of temperature, it will rise or fall quickly and perceptibly in the tube. The next thing is to graduate the instrument—that is, to divide the tube into small divisions, called degrees, and to number them. We will now explain how this is done.

It has been found that, if a thermometer be placed in snow or broken ice which is in the process of melting, the height of the mercury is always the same in that thermometer. It is not like your hand sometimes warmer, sometimes cooler try the experiment twenty times over, and the height of the mercury will not vary in the slightest degree from one certain point.

Here, then, we have a good starting-point.

Scratch a

mark on the glass tube where the mercury stands when the thermometer is placed in melting snow or ice, and call that the Freezing Point, because it is just the point where water begins to freeze or ice to melt.

It has also been found that, under the same pressure of the atmosphere, pure water always boils at the same temperature. Therefore plunge your thermometer into boiling water, and mark your glass with another scratch where the mercury stands. Call this the Boiling Point.

The distance between these two points must be divided into small portions (called degrees). There are three common methods of thus dividing it; but the one usually adopted in this country is that first employed by Fahrenheit. He divided the distance between the freezing point and the boiling point into 180 degrees, numbering the freezing point 32 and the boiling point 212. Thus the freezing point of water is known as 32 degrees Fahrenheit,-written briefly thus, 32°F.; and the boiling point of water by the same scale is 212°F.

The remaining parts of the tube, above the boiling point and below the freezing point, are then marked off in degrees of the same length, and are numbered accordingly. The point which is 32° below freezing point is marked o°, and is called zero.

We wonder, now, if any boy or girl who has been reading these lessons, has had the acuteness to think after this fashion:- "The book says that glass expands by heat as well as mercury; if so, when heat is applied to the thermometer, the glass bulb and tube ought to expand as well as the mercury inside. Why, then, does the mercury rise in the tube?" The answer to this is that mercury expands eighteen times as much as glass; that is to say, when the glass has expanded by heat, the mercury only spends oneeighteenth part of its expansion in filling up the expanded glass, leaving seventeen other parts of expansion to rise in the tube.

You will now, I dare say, begin to see several reasons why mercury is so suitable for thermometers. It is liquid at all ordinary temperatures, and therefore a mercurial thermometer can take the convenient form of a bulb and

tube. It does not freeze except at a very low temperature71 degrees below the freezing point of water; it does not boil except at a considerably high temperature-viz., about 662°F., or 450 degrees above the boiling point of water; consequently it has a large range of usefulness. It is bright in colour, and is therefore easily watched as it rises or falls in the thermometer. And, as you have seen, it expands or contracts very readily under the influence of a change of temperature.

Coloured alcohol is also much used for thermometers. It does not freeze at any known cold, and will therefore be of service to measure low temperatures, especially when mercury is frozen. But, on the other hand, it boils at 172° Fahr., or 40 degrees below the boiling point of water, and then, of course, ceases to be of service.

The following table may prove interesting (the minus sign is used to indicate degrees below zero):

NOTE. Water is in this table said to boil at 212°. But this is not always the case. The boiling point depends on the pressure of the atmosphere, and this again upon the elevation of the place where the water is boiled. At the top of Mont Blanc it will boil at 182°, Travellers who have climbed high mountains have been astonished to find that they cannot boil eggs there. The egg may be placed in the water, and the water may boil, but the heat of this boiling water is not sufficient to cook the egg.