beyond which human existence, or indeed, organic existence of any type whatever, cannot any longer endure on the earth.
We may say once for all that the sun contains just a certain number of units of heat actual or potential, and that he is at the present moment shedding that heat around with the most appalling extravagance. No doubt the heat-hoard of the sun is so tremendous that the consequences of his mighty profusion do not become speedily apparent. They are indeed, it must be admitted, hardly to be discerned within the few brief centuries that the sun has been submitted to human observation. But we have grounds for knowing as a certainty that the sun cannot escape from the destiny that sooner or later overtakes the spendthrift. In his interesting studies of this subject, Professor Langley gives a striking illustration of the rate at which the solar heat is being squandered at this moment.
He remarks that the great coalfields of Pennsylvania contain enough of the precious mineral to supply the wants of the United States for a thousand years. If all that tremendous accumulation of fuel were to be extracted and burned in one vast conflagration, the total quantity of heat that would be produced would no doubt be stupendous, and yet, says this authority, who has taught us so much about the sun, all the heat developed by that terrific coal fire would not be equal to that which the sun pours forth in the thousandth part of each single second. When we reflect that this expenditure of heat has been going on not alone for the centuries during which the earth has been the abode of man, but also for those periods which we cannot estimate, except by saying
