272. =Sources of Heat.=--The principal of the sources of heat on our earth is the _sun_, though that body is ninety-five millions of miles distant from us. As the heat, in traveling all this long journey, is becoming more and more diffused; or, in other words, as its rays are all the way separating from each other more and more, we can have no conception of the concentrated heat that exists in the sun itself. We can, however, approximate to the idea by observing the effects of heat when some of its separated rays are gathered to a point by a powerful lens, as represented in Fig. 191. A lens which concentrated the heat ten thousand times melted platinum, gold, quartz, etc., in a few seconds. And as the heat at the sun is supposed to be thirty times more concentrated than this, none of the most solid substances of our earth would remain solid if they were there, but would be some of them liquid, and others even in a state of vapor. The heat which the sun constantly radiates to the earth pervades all substances, producing motion, and awakening life every where, so that, in the expressive language of the Bible, "There is nothing hid from the heat thereof." Another source of heat is _within the earth itself_. It has been found as we go down into the earth there is a constant increase of temperature the farther we go. This internal heat is attributed in part to subterranean fires and various chemical actions. We see here and there external evidences of the operation of these causes in the eruptions of volcanoes, the boiling springs, the jets of steam and sulphureous vapors, etc. But that the heat in our earth which comes from these subterranean sources is small compared with that which comes from the sun, is seen in the fact that the rate of increase of heat at great depths is much less than it is nearer the surface. This would seem to show that although fires within the earth may have considerable influence in heating its crust, on which we live, it derives the most of its heat from the sun, at least to a very great depth. How great a source of heat _electricity_ is we know not, but that considerable heat comes from this source is evident from the melting and burning effect which we often see resulting from the passage of the electric fluid. Another very common source of heat is _chemical action_. We see it continually produced in chemical experiments. Combustion, which, as will be shown to you in the Second Part of this Series, is nothing but an example of chemical action, is the most common of all the chemical sources of heat. Animal heat is also, for the most part, a result of chemical action.