SPECIFIC HEAT—FUSION—LATENT HEAT—CONDUCTION AND CONVECTION OF HEAT—CALORESCENCE.
We have considered the effects of heat upon water, and touched upon one or two kindred experiments. But we have some other subjects to discuss, two in particular; viz., Specific Heat, and Latent Heat.
The specific heat of any substance is “the number of units of heat required to raise one pound of such substance one degree.” We can explain this farther. When heat is communicated to a body it has two or three functions to perform. Some of it has to overcome the resistance of the air in expanding the body, more of it expands, and the remainder increases the temperature of the body. So some heat disappears as heat, and is turned into energy,—“molecular potential energy,”—as it is called, and the rest remains. Of course in objects the molecules vary very much in weight and in their mutual attraction, and the heat requisite to raise equal weights of different substances through the same number of degrees of temperature will vary. This is called capacity for heat, or specific heat. The capacity of different metals for heat can easily be shown. The specific heat of water is very high, because its capacity for heat is great. We can cool a hot iron in very little water, and it takes thirty times as much heat to raise a given weight of water a certain number of degrees, as it would to raise the same weight of mercury to the same temperature. Water has greater specific heat, generally speaking, than other bodies, and it is owing to this circumstance that the climate is so affected by ocean currents.
Nearly all substances can be melted by heat, if we go far enough, or frozen, if we could take the heat away. Solid can be made liquid, and these liquids can be made gases and fly off in vapour. Similarly, if we could only get heat away sufficiently from the atoms of a substance we could freeze it. We cannot freeze alcohol, nor make ice from air, nor can we liquify it, for we are unable to take away its heat sufficiently. But we can turn water into steam, and into ice; or ice into water, and then into steam. But there is one body we cannot melt by heat, that is carbon. In the hottest fire coal will not melt, it becomes soft. We call this melting fusion, and every body has its melting point, or fusing point, which is the same at all times if the air pressure be the same.
It is a curious fact that when a body is melting it rises to a certain temperature (its fusing point), and then gets no hotter, no matter whether or not the fire be increased;—all the extra heat goes to melt the remainder of the substance. The heat only produces changes of state. So this heat above fusing point disappears apparently, and is called Latent Heat. This can easily be proved by melting ice. Ice melts at 32° Fahr., or 0° Cent., and at that temperature it will remain so long as any ice is left; but the water at 32°, into which the ice has melted, contains a great deal of latent heat, for it has melted the ice quickly, and yet the thermometer does not show it. It is just the same with boiling water.
When substances are fused they expand as a rule, but ice contracts; so does antimony. On the other hand, when water solidifies it does not contract as most things do. It expands, as many of us are aware, by finding our water pipes burst in the winter; and the geologist will tell us how the tiny trickling rills of water fall in between the cracks of rocks and there freeze. In freezing the drops expand and split the granite blocks. Type-metal expands also when it becomes solid, and leaves us a clear type; but copper contracts, and won’t do for moulding, so we have to stamp it when we want an impression on it.
There is no doubt that chemical combinations produce heat, as we can see every day in house-building operations, when water is poured upon lime; but there are also chemical combinations which produce cold. Fahrenheit produced his greatest cold by combining snow and salt, for in the act of combining, a great quantity of heat is swallowed up by reason of the heat becoming latent, as it will do when solid bodies become liquid. Such mixtures or combinations are used as Freezing Mixtures when it is necessary to produce intense cold artificially. Sulphate of Sodium and Hydrochloric acid will also produce great cold, and there are many other combinations equally or even more efficacious.
Heat is communicated to surrounding objects in three well-known ways—by conduction, by radiation, and by convection. Conduction of heat is easily understood, and is the propagation of heat through any body, and it varies very much according to the substance through which it passes. Some substances are better conductors of heat than others. Silver has a far greater conductivity than gold, and copper is a better heat-conductor than tin. Flannel is a non-conductor, or rather a bad conductor, for no substance can be termed actually a non-conductor. Flannel, we know, will keep ice from melting, and a sheep’s wool or a bird’s feathers are also bad conductors of heat; so Nature has provided these coverings to keep in the animal heat of the body. A good conductor of heat feels cold to the touch of our fingers, because it takes the heat from our hands. This can be tried by touching silver, lead, marble, wood, and wool. Each in turn will feel cold and less cold, because they respectively draw away, or conduct less and less heat from our bodies. So our clothes are made of bad-conducting substances. The bark of a tree is a bad conductor, and if you strip off this clothing the tree will die.
Solids conduct heat the better the more compact they are. Air being a bad conductor it follows that the less tightly the molecules are packed the less conductibility there will be; and even a substance powdered will be a worse conductor than the same substance in solid form; and also more readily in the direction of the fibres than crossways.
Liquids do not possess great conductivity, but they, as well as gases, are influenced by convection, or the transport of heat from the bottom layers to the top (conveho, to carry up). We have already mentioned that the heated particles of water rise to the top because they expand, and so become lighter. This is convection of heat; and by it liquids and gases, though actually bad conductors, may become heated throughout to a uniform temperature. Of course the more easily expansible the body is the more rapidly will convection take place—so gases are more readily affected than liquids. Solids are not affected, because convection of heat depends upon molecular movement or mobility, and it is obvious that the particles of solid bodies are not mobile. Professor Balfour Stewart says with reference to this that “were there no gravity there would be no convection,” for the displacement of the light warm particles by the heavier cold ones is due to gravity. The instances of convection of heat in nature are numerous, and on a gigantic scale. The ocean currents, trade winds, lake freezing, etc., while the chimney draught already referred to, is another example; and in all these cases the particles of air or water are replaced by convection. In the case of the lake freezing the cold particles at the top sink, and the warmer ones ascend, until all the lake is at a temperature of 36·2°, or say 4° above freezing. At this temperature water assumes its maximum density, and then expands, as we have seen, instead of contracting. Ice is formed, and being thus lighter than water, floats; and so unites to cover in the water underneath, which is never frozen solid, because the cold of the atmosphere cannot reach it through the ice in time to solidify the whole mass.