Not only do dissimilar bodies radiate heat in different degrees, but they conduct it also with constantly varying rates. Heat passes along silver or copper with readiness, compared to its progress through platinum. It is conducted by glass but slowly, and still more slowly by wood and charcoal. We receive some important intimations of the molecular structure of matter, from those experiments which prove that heat is conducted more readily along some lines than others. In some planes, wood and other substances are better conductors than in others. The metallic oxides or earths are bad conductors of heat, by which provision the caloric absorbed by the sun’s rays is not carried away from the surface of this planet so rapidly as it would have been had it been of metal, but is retained in the superficial crust to produce the due temperature for healthful germination and vegetable growth. The wool and hair of animals are still inferior conductors, and thus, under changes of climate and of seasons, the beasts of the field are secured against those violent transitions from heat to cold which would be fatal to them. Hair is a better conductor than wool: hence, by nature’s alchemy, hair changed into wool in the animals of some countries on the approach of winter, and feathers into down. It is therefore evident that the rate at which solar heat is conducted into the crust of the earth must alter with the condition of the surface upon which it falls. The conducting power of all the rocks which have been examined is found to vary in some degree.[60] It follows, as a natural consequence of the position of the sun to the earth, that the parts near the equator become more heated than those remote from it. As this heat is conducted into the interior of the mass, it has a tendency to move to the colder portions of it, and thus the heat absorbed at the equator flows towards the poles, and from these parts is carried off by the atmosphere, or radiated into space. Owing to this, there is a certain depth beneath the surface of our globe at which an equal temperature prevails, the depth increasing as we travel north or south from the equator, and conforming to the contour of the earth’s surface, the line sinking under the valleys and rising under the hills.[61]