amount of heat precisely equivalent to that which was absorbed from the sunbeams. Thus it is that the heat now radiating from our fireplaces has at some time previously been transmitted to the earth from the sun. If it be timber that we are burning, then we are using the sunbeams that have shone on the earth within a few decades. If it be coal, then we are retransforming to heat the solar energy which arrived at the earth millions of years ago.
The question as to the continued existence of man on this globe resolves itself eventually into an investigation as to the permanence of the heat- supply. Doubtless human life requires many other conditions, but of this we may feel assured, that if the heat fail and if nothing else be forthcoming which can be transformed into heat, then most assuredly from this cause alone there is a term to human existence. Before discussing the prospect of the duration of sunbeams we may first consider a few other less important sources of heat. So far as the coal goes, we have already observed that as it is limited in quantity it can offer no perennial supply. Doubtless there is in the earth some quantity of other materials capable of oxidation, or of undergoing other chemical change; in the course of which, and as an incident of such change, heat is evolved. The amount of heat that can possibly arise from such sources is strictly limited. There is in the entire earth just a certain number of units of heat possible from such chemical combinations, but after the combination has been effected there cannot be any more heat from this source.
Then as to the internal heat of the earth due to the incandescent state of its interior. Here there is no doubt
