The expansion of liquids by heat has been used in the formation of the Thermometer, or heat (thermo) measure (meter).

Quicksilver, though a solid metal at the North Pole, is a liquid, and can be poured out like water in our climate. It is nearly fourteen times as heavy as water.

* Potential energy used to be called "latent heat".

It is used for thermometers instead of spirits of wine, because it bears a greater degree of heat before it boils. The instrument is made in the following manner: -

A glass tube with a small bore, sufficiently so to be capillary, is selected with care, in order to secure the same diameter throughout. The bores of some tubes are like an elongated cone, and, if they were used, the mercury would expand much more in some parts of the tube than in others, and hence the indications of such a thermometer would be incorrect. A little mercury, amounting to an inch in length, is allowed to enter the tube, and being moved from one end of the tube to the other, it is soon discovered whether the mercury increases or decreases in length, or remains, as is usually the case, of the same linear dimensions in all parts. The proper length having been cut off, one end is melted and blown out into a bulb, the other being formed into a cup or funnel-shaped form, to hold the mercury, which is forced in; the tube is now inclined slightly, and the air in the bulb expanded by heat; it is afterwards allowed to cool, and, as the air cools and contracts, the mercury from the upper funnel is forced in by the pressure of the air, and enters to supply the place of the air driven out by expansion.

To get rid of the rest of the air, the mercury is alternately boiled and cooled until the bulb and part of the tube are full of mercury.

Having thus filled the bulb and one-third of the tube, the next step is to seal it hermetically, which is done by heating the mercury to the boiling-point, and at the moment the mercury is overflowing at the summit the glass is fused with a flame, urged by a blow-pipe before the mercury has had time to contract; and if this operation has been skilfully performed, a perfectly void space or vacuum, free from air., is obtained as the mercury sinks or contracts in the bulb and tube.

The marks on the thermometer are made by means of melting ice and boiling water. Ice always melts at the same temperature, and pure water invariably boils at the same temperature, when the barometer stands at 29.8 in.

It is only necessary to immerse the thermometer alternately in melting ice and in boiling water, with certain precautions, and to mark the point at which the mercurial column stands - one being called the freezing-point, and the other the boiling-point.

The instrument must be immersed in the melting ice until the mercury becomes perfectly stationary. The immersion in boiling water requires the greatest care, and a time should be selected when the barometer stands at 29.8 or 30 inches. The depth of the water in the vessel should not exceed two inches. The vessel must not be a shallow one, but sufficiently deep to contain the bulb and nearly all that part of the tube up which the mercury will rise. When placed in boiling water, distilled water should be used, and brisk ebullition maintained, and the steam allowed to escape freely, as the confinement of it would raise the temperature above that of boiling water.

The Thermomter.

The place at which the scale is marked 32°, is called freezing-point-The boiling-point is marked 212o.

F. signifies Fahrenheit's, or the English scale. C. signifies the Centigrade, or French scale.

212°. Water boils.

190°. Brandy boils.

1740. Spirits of Wine boil.

1270. Tallow melts.

110°. Summer in East Indies.

80°. Very hot in England. Common heat in East Indies.

75°. Summer heat in England.

55°. Temperate in England.

50°. Common heat of spring-water.

430. Olive oil begins to stiffen.

320. Water freezes.

200. Very cold in England.

5°. Winter in Canada.

In Iceland, the cold is sometimes 240 below 0.

At 400 below O, quicksilver freezes.

In England the interval, according to Fahrenheit, is divided into 180 farts, the zero being 320 below the freezing-point. On the Continent, the interval is divided by Celsius into 100 pans, and is called the Centigrade scale, - the zero commences with the freezing-point; sometimes into 80 parts, called Reaumur's scale, the zero, as before, being the freezing-point of water. Of the three, that of Celsius is the most simple, and will be gradually adopted throughout the civilized world. Fahrenheit's scale expresses slight variations of temperature better, therefore it is preferable; the degrees on the "Centigrade " being too large to express minute differences.