In a former lesson you read a description of the thermometer, a useful instrument which enables us to estimate the temperature, or sensible heat, of substances.

All bodies, even the coldest, contain heat; and they have also a tendency to part with their heat to colder substances around them, until all have the same temperature. Thus, when you lay your hand on a block of iron or marble, heat leaves your hand to enter the less warm material and raise its temperature; and it is this abstraction of heat that produces in you the sensation of cold.

There is, then, a constant communication or transmission of heat from one body to another. This communication is effected chiefly in two ways—by conduction and by radiation. In conduction, the bodies are in contact; in radiation, they are at some distance apart.

If you push one end of a cold poker into the fire, that end will soon become warm, and the heat will be propagated from particle to particle through the poker, until the end most distant from the fire becomes too hot to be touched without injury. This mode of transmission is called conduction. Different substances possess this power in very different degrees. Thus, if instead of a poker you thrust into the fire a bar of wood of equal length and thickness, you will find that, even when the inserted end is in flames, the other remains comparatively cold, and may be handled with impunity. Hence we say that iron is a good conductor, and wood a bad conductor of heat.

The conducting power of bodies depends in a great measure on the closeness of their particles—dense, solid substances being much better conductors than those which are light and porous. The metals are the best conductors, but they differ very much among themselves. The best is silver; the others stand in this respect in the following order—copper, gold, brass, tin, iron, steel, lead.

You will now understand why metals feel cold to the touch: it is because, being good conductors, they carry the heat rapidly away from that part of our body with which they are in contact.

Among the bad conductors of heat are fur, wool, cotton, silk, and linen; straw, paper, feathers, wood, earth, snow, water, and air; and loose bodies, such as sawdust and shavings, which contain a large amount of air in the spaces between their particles.

Our clothing, as you know, is made of wool, cotton, or linen. Can you tell why such materials are selected for the purpose? It is not, as many ignorant people suppose, because they are best adapted to impart warmth. The true reason is that, being bad conductors, they prevent the cold air and other objects around us from robbing us of the heat which is produced within our bodies.

When once you understand what is meant by conduction of heat, and can distinguish between substances which are good conductors and those which are not, you will be able to give a reason for many facts that must appear strange to every one who does not possess such information. A little reflection, for instance, will enable you to explain why a linen garment feels colder to the skin than one made of cotton or wool; why a silver spoon becomes hot when the bowl is left for a few minutes in a cup of hot liquid; why a metal tea-pot or kettle is commonly furnished with a handle of wood or ivory; why ice may be preserved by being wrapped in flannel or covered with sawdust; why a pump, in frosty weather, should be encased in straw or matting; and why the farmer welcomes the snow, and regards it as a protection to his crops.

Every warm body has the power of sending out rays of heat, as a luminous body gives out rays of light. This mode of communicating heat is called radiation, and it serves, as we shall see, a very important purpose in the economy of nature.

When you stand before a fire, heat-rays stream forth from the burning fuel, and create in you the sensation of warmth. In this case, the heat of the fire is communicated to you, not by conduction, but by radiation.

Everything in nature is constantly radiating heat from its surface. If a body be surrounded by objects hotter than itself, it becomes heated by radiation; if it be exposed to the influence of objects colder than itself, it becomes cooled by radiation; and if the objects around it are neither hotter nor colder than itself, its temperature remains unaltered.

But though all bodies radiate heat, they have not all the same radiating power. Some substances possess this power in a far greater degree than others. The metals, though they are the best conductors, are the worst radiators. This is particularly the case when they are polished. Dull, dark substances, and especially those which have a rough or scratched surface, are good radiators; light-colored and smooth substances, on the other hand, are bad radiators; and this explains why, as every good housewife knows, a polished metal tea-pot keeps tea warmer than a black earthen one—it does not part with its heat so readily.

Bodies which radiate freely have the power in an equal degree of absorbing heat; that is, they are as ready to take it in, as they are to throw it out again. Dark substances, therefore, must be good absorbers, as they are good radiators of heat. A very simple experiment will illustrate this fact very clearly. If you spread upon snow, in a place exposed to the sunbeams, two pieces of cloth of the same texture, one black and the other white, you will find, after some time, that under the black cloth the snow has been melted, but under the white cloth it remains as it was at first. The black material has been heated quickly and intensely; the white has not been heated at all: the former has absorbed the sun’s rays; the latter has reflected them.

The laws of absorption and radiation enable us to explain many curious facts. Thus, dark-colored clothes are cold in the shade, because they are then radiating heat from our bodies; but they are warm in the sunshine, because they are then absorbing the heat that falls upon them from the sun. On the other hand, light colored clothes are warm in the shade, and cool in the sunshine. Again, a dish-cover or metal tea-pot is kept as brightly polished as possible, in order to prevent the escape of the heat by radiation; a black earthenware tea-pot, on the contrary, has a dull and dark surface, so that it may be placed on the hob and absorb the heat. So, too, if a kettle is to heat quickly, the bottom and sides should be covered with soot, to absorb the heat; while the upper part should be bright, to prevent radiation.

It is radiation, also, that accounts for the deposition of dew. It is a common error to suppose that dew falls in the same manner as rain or mist, only in much finer particles. Dew, in fact, does not fall, but is formed on the surface of bodies by the condensation of the moisture of the atmosphere.

The air around us contains at all times a quantity of moisture in the form of vapor. Now this vapor has been formed from water by the action of heat; and it may again be turned into water by being brought in contact with objects that are cold. And this is just what takes place in the formation of dew. When the sun has set, the trees and grass and other objects on the earth’s surface immediately begin to radiate the heat which they have absorbed from its beams during the day. The best radiators, of course, become cool most rapidly, and quickly condense the vapor that floats in the air around them; and in the morning we find those objects which radiate freely, such as blades of grass, leaves of plants, and floating cobwebs, covered with this condensed vapor in the form of glittering dewdrops.

Clouds, in a great measure, prevent radiation, and hence the dew will be most plentiful on a clear and cloudless night. If the radiation continues till the temperature of the ground is very low, the dew freezes as it is deposited and forms hoar-frost.

You may see a smooth road or gravel walk quite dry in the morning, while the grass or box by its side is thickly coated with moisture. Why is this? It is simply because the road or walk is a bad radiator and cools slowly, while the grass and box, being good radiators, become rapidly cold and condense the vapor of the passing air into dew.

Thus by a wise arrangement, the cultivated fields receive an abundance of precious moisture, while not a drop is wasted on the bare rock, or the sterile sands of the desert.