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.