nor was it fit to be, the theatre of life."[143] The earth was once a molten mass heated to incandescence--a self-luminous globe. On this point there is scarcely any difference of opinion among scientific men. Furthermore, a large majority of modern scientists regard themselves as justified in the affirmation of a still anterior nebulous condition. If the nebular hypothesis is accepted, then we are required to contemplate a period when the earth did not exist, and when even the matter which now enters into its constitution was an undistinguished part of the nebula from which the whole solar system was evolved. Many exact observations and mathematical computations as to the secular cooling of the earth give results which are in strict accordance with this theory of its primitive igneous condition. The observed facts clearly indicate that the earth is becoming, on the whole, cooler from age to age, and that the natural current of events is carrying it inevitably to a state of total refrigeration.[144] The fossil remains now found within the arctic circle indicate that at a period, not extremely remote, tropical vegetation flourished, and forms of animal life subsisted there which are now confined to the torrid zone. Mammoths lived in the now uninhabited polar regions, and tree-ferns and the tropical shell-fish found there a home.[145] The surface of the earth was then warmed by internal heat which since that period has waned; that heat has been gradually dissipated in the surrounding space, as a red-hot ball suspended even in the warm air of a room must, according to the well-known laws of radiation and absorption, necessarily part with its heat. Many experiments carefully conducted in our time show that the temperature of the earth increases with the depth to which we penetrate: "In boring for the artesian well at Grenelle, which is 546 metres deep, it was observed that the temperature augmented at the rate of 1° Centigrade for every 30 metres. The same result was obtained by observations in the artesian well at Mondorf, in Luxemburg; this well is 671 metres in depth, and its waters 34° warm." As the result of many investigations in mines and borings, Sir William Thomson concludes that the average inference may be thus stated--there is on the whole about 1° Fahr. of elevation of temperature per 50 British feet of descent.[146] If this increase is uniform--and we have no reason to suppose the contrary--then at the depth of 50 miles there exists, says Helmholtz, a heat sufficient to fuse all our minerals.