If a piece of charcoal or stick of sulphur is burning only at one end, we shall find that we can painlessly lift it up by the other end with our fingers; because charcoal and sulphur are bad conductors of heat; that is to say, their respective powers of transmitting heat through their mass are feeble. On the other hand, the conductivity of metals is high, as we may learn by placing our hand on the middle part of a poker which has one end red hot.

Knowledge of the conductive powers of substances can be utilised in various practical ways. Thus, by enveloping a piece of ice in several folds of a blanket or woollen rug, we prevent the ice to a great extent from melting, even during a summer's day; because the protecting material is unable to conduct from the outside a sufficiency of heat to melt the ice. An instance of obstruction to the outward transmission of heat is afforded by the "Norwegian stove," which consists of a box lined with thick felt, and which is used to keep food hot for several hours. It loses its heat so slowly that if a pot containing the raw materials of a stew or soup is brought to boiling point and then put into this box, the process of cooking will proceed without interruption, and will become completed in a few hours with only a small loss of temperature.

The transmission of heat is similar to that of light and sound, in that its intensity varies inversely as the square of the distance travelled. Thus, the light of a candle or the sound of a note of music is four times as strong at a distance of 1 yard as it is at a distance of 2 yards. Consequently, we may infer that a rug 1 in. thick would be four times as warm as a rug half an inch thick, both being of the same material.

The nature of the sensation of heat or cold produced by contact is greatly dependent on the conductivity of the body that is touched. For instance, among substances of the same temperature, a bad conductor of heat, such as wool or wood, will feel warmer to the touch than a better conductor, such as iron or stone; a fact which accounts for the not uncommon idea of persons unacquainted with physics, that the former class of objects are habitually of a higher temperature than the latter, under similar conditions of heat.

The following table of the comparative heat-conducting powers of certain substances may be studied with advantage : -

"The conductivity of snow is about 1/7-th that of moist clay" (Ganot). The order of superiority in conductive power possessed respectively by linen, cotton, silk, and wool, is the same as that in which they are here written. The conductivity of hair may be regarded as the same as that of wool; for the latter substance is merely a variety of the former which has undergone a modification of shape, but not of composition or structure.

The great difference between the respective conductive powers of snow and water seems to be due chiefly to the large quantity of air contained in snow. Bark has less conductive power than wood, apparently from the fact of its containing more air.

Ordinary manufactured silk conducts heat better than pure silk, owing to its adulteration with certain agents such as salts of iron and tin; and common horse rugs are better conductors of heat than those which are "all wool," on account of their being made partly of cotton. The presence of water and air modifies the apparent powers of conduction of many substances. For instance, the fact that a woollen cloth holds imprisoned between its fibres a larger quantity of air than a cotton cloth of similar size and weight would do, makes the former material a worse conductor of heat, as compared to the latter, than the knowledge of the respective conducting powers of wool and cotton would lead us to suppose. As air conducts heat more feebly than wool, it follows that the more air a piece of cloth retains, the less will be its conducting power; therefore, very closely woven cloth is a better conductor of heat than it would be if its texture were more porous. It is evident that the limit of porosity in this case must not be so great as to allow air to circulate freely through the material. We find that in the case of "cellular cloth" the heat-retaining capacity of the cotton is much heightened by having the cloth loosely woven, and that, with a given weight of clothing, we obtain a somewhat similar effect by increasing the number of thicknesses, between each of which a more or less perfect layer of air is imprisoned. Thus, two rugs will be warmer than one which is as thick as both of them put together, and which is of similar material and structure. Probably most men know from experience that the addition of an ordinary cotton shirt worn over a flannel one will be almost, if not quite, as effective in protecting the body from cold as that of a second flannel shirt. In our own cases, as a protection from cold, it is better to put a cotton shirt over a flannel shirt than a flannel shirt over a cotton shirt, which would be apt to stick to the skin, and would thus prevent the formation of a layer of air between the skin and the shirt. By employing cotton with horses' clothing in the manner indicated, economy in weight and expense may be obtained without much loss of heat-retaining power.

When two bodies are in contact, each of them transmits heat to the other independently of any difference there may be between the respective temperatures. If one of these bodies be colder than the other, the balance of heat transmission gained by it, at any given moment, will be equal to the difference of temperature between the two. Hence, the rate of loss of heat by the warmer body will decrease according as the temperature of the colder one becomes raised.

Thus we find that when a horse is placed in a cold atmosphere he will lose heat faster when the air is in motion than when it is still; because, instead of one layer of air remaining close to the skin, and becoming gradually raised in temperature with comparatively little loss of heat by the animal, there will be presented to his skin a more or less rapid succession of layers of cold air. We have this effect heightened in the animal body by the stimulus given to the evaporation of moisture from the skin. Agreeably to these facts, we find that a temperature of - 20° F. during a Russian winter, for instance, is easier to bear when the atmosphere is still, than one of, say, 10° F. (300 higher), when a strong wind is blowing. Under the former condition, one experiences no inconvenience from walking about in ordinary clothes and without an overcoat, provided one's hands and ears are well covered; for the circulation of blood is comparatively torpid in these parts of the system. When the temperature in the shade is higher than that of the body (normal temperature of man being about 98.50 F., and that of the horse about 99.50 F.), the substitution in question is that of air which is hotter, but also is drier than the removed air. The cooling effect of the stimulus to evaporation of perspiration given by the comparatively dry air is, however, so great, that when the temperature in the shade is even at 1200 F., the effect of a current of air is very refreshing . to man and beast.