If a stone is thrown upon a field of ice, it soon causes the ice around it to melt, forming a hole into which it sinks. A dark cloth spread upon snow acts in the same manner, and soon sinks under the influence of the sun's rays.

Water is soonest heated in a vessel whose surface is black and unpolished, because the vessel in this state is best adapted, to absorb the heat which is applied to it, but on removing it from the fire, the water cools rapidly. To retain heat in liquids, they should be confined in dense and polished vessels, as these are poor radiators. Hence, for boiling and cooking, rough and black vessels should be employed, but to keep the articles warm, dense and polished vessels should be used. It is for this reason that a silver teapot is better than an earthen one. But as silver is a good conductor of heat, the handle should be insulated by interposing between it and the vessel some non-conducting substance, as ivory or bone.

́Stoves, being intended to radiate heat, should be rough and black, but fire-places, being intended to reflect heat into the room, should be lined with white, dense, and polished substances, like glazed earthenware, or glazed fire-bricks.

204.

Conductibility of Solid Bodies.

CONDUCTIBILITY is that property of bodies by virtue of which they transmit heat. Those bodies that transmit heat readily, are called good conductors; those that do not transmit it readily, are called bad conductors.

INGENHOUSZ showed that solid bodies possess different degrees of conductibility, by means of an apparatus shown in Fig. 142. It consists of an oblong vessel to contain water, from one side of which projects a system of short tubes for receiving rods of different kinds of solids, such as metals, marble, wood, glass, and the like.

INGENHOUSZ Coated the different rods with a soft wax that

Explain the effect of a stone thrown upon ice? Of a dark cloth upon snow? Why is water soonest heated in black and unpolished vessels? In what vessels is it best kept hot? Of what material should stoves be constructed? Fire-places? Why? (204.) What is Conductibility? Good conductors? Bad conductors? Explain INGENHOusz' apparatus?

would melt at about 140° F., and then filled the vessel with boiling water. Upon some of the rods the wax melted rapidly, upon some more slowly, and upon others not at all. This showed that the rods varied in their conductibility.

It has been shown that metals are the best conductors, after which comes marble, then porcelain, bricks, wood, glass, resin, &c.

Conductibility of Liquids. - Convection.

205. Liquids are bad conductors of heat, except mercury, which is a metal. They are such bad conductors that RUMFORD asserted that water is not a conductor at all. More careful experiments have shown that all liquids are conductors, but all are extremely bad ones.

Liquids are heated by a process of circulation amongst their particles, called convection, the heat being applied from below, as shown in Fig. 143. When the particles at the bottom become heated, they expand, and as they are then lighter than the cooler particles above them, they rise to the

Explain his method of using it? What are the best conductors! What bodies come next in order? (205.) Are liquids good or bad conductors? How are liquids heated? Explain the illustration.

top of the vessel to give place to the heavier and cooler ones that supply their places. In this way a double current

of particles is set up, as

shown in the figure by the arrows, the hot ones rising and the cool ones descending. This process of circulation goes on till a uniform temperature is imparted to all of the liquid.

The circulation of particles may be shown by putting into the vessel particles of a

substance of nearly the same density as the liquid; as,

for example, oak sawdust.

These particles will partake

of the motion of the fluid, rising up in the centre, and descending along the walls of the vessel as shown in the figure

Conductibility of Gases.

206. Gases are bad conductors of heat, but, on account of the extreme mobility of their particles, it is difficult to establish the fact by direct observation.)

Gases are heated by convection, in the same manner as liquids.

Applications of the preceding principles.

207. If the hand be placed upon different articles in a cold room, they convey different sensations. Metals, stones, bricks, and the like, feel cold, whilst carpets, curtains, and the like, feel warm.

How may the circulation of particles be demonstrated? (206) Are gases good or bad conductors? How are they heated? (207.) Explain the different sensa tions experienced on touching bodies in a room.

The reason of this is, that the former are good conductors, and readily abstract the animal heat from the hand, whilst the latter are bad conductors, and do not convey away the heat of the hand.

Wooden handles are sometimes fitted to metallic vessels which are to contain heated liquids. This is because wood is a bad conductor, and therefore does not convey the heat to the hand. For a similar reason, when we would handle any heated body, we often interpose a thick holder of woollen cloth, the latter being a bad conductor.

To preserve ice in summer, we surround it with some bad conductor, as straw, sawdust, or a layer of confined air. The same means are adopted to preserve plants from the action of frost. In this case, the non-conducting substance prevents the radiation of heat.

Cellars are protected from frost in winter by a double wall inclosing a layer of air, which is a non-conductor. It is the layer of confined air that renders double windows so efficient in excluding frost from our houses.

The feathers of birds and the fur of animals are not only in themselves bad conductors, but they inclose a greater or less quantity of air, which renders them eminently adapted to the exclusion of cold. The bark of trees is a bad conductor, and so serves to protect them from the injurious effects of heat in summer, and cold in winter.

Our warmest articles of clothing are composed of non-conducting substances, inclosing a greater or less quantity of air. Such are furs, woollen cloths, and the like. It is not that these are warm of themselves, but they serve as non-conductors, preventing the escape of animal heat from our bodies.

V.-LAWS OF EXPANSION OF SOLIDS, LIQUIDS, AND GASES.

Laws of Expansion of Solids.

208. Numerous experiments have been made to determine the exact amount of expansion which bodies experience

Why are wooden handles attached to metallic vessels? How is ice preserved in summer? Why? How are plants protected? Why? How are cellars protected from frost? Why? Why are feathers adapted to exclude cold? Bark of trees? What substances form the warmest clothing? Why?

by the addition of a given amount of heat.

As in a former

article, it will be found convenient to consider first, linear expansion, and afterwards, expansion in volume.

1. Linear expansion. In order to compare the rate of linear expansion of different bodies, we take for a term of comparison, the expansion experienced by a unit of length of cach body when heated from 32° F. to 33° F. This is called the coefficient of linear expansion.. )

The coefficients of linear expansion for a great number of bodies were determined in the latter part of the last century by LAVOISIER and LAPLACE. (They reduced the substance to be experimented upon to the form of a rod or bar, then exposed it for a sufficient time to the temperature of melting ice, and measured its exact length. They next exposed the bar to a temperature of boiling water, and again measured its length. The increased length, divided by 180, gave the increase in length of the whole bar for 1° F. This result, divided by the length of the bar at 32° F., gave the linear expansion of a unit of length, and for an increase of temperature of 1° F., that is, the coefficient of linear expansion.

The following are some of the results thus obtained :

From the above table, it is seen that the amount of expansion is always very small.

(208.) What is the coefficient of linear expansion of solids? How determined by LAVOISIER and LAPLACE? Give some of the results?