"Another point deserves attention. Oxygen is an element having a wide range of very strong and active affinities. It is ready to unite with every known substance, fluorine excepted. What if some other equally active element were mingled with oxygen to form the atmosphere? What if, in place of nitrogen, vapor of sulphur were substituted? What if hydrogen were put in the place of nitrogen? The two elements would combine in sudden combustion or explosion, and the atmosphere itself would perish. But nitrogen is a substance so sluggish and inert that it can be brought into union with oxygen only by indirect processes. Because the air is composed of one so inert element as nitrogen, the atmosphere is preserved, and, what is almost as important, it is kept, as it now is, composed of simple elements, and hence _diathermic_. If our atmosphere were a compound gas, the world would perish with cold. "The temperature of the earth depends also upon certain qualities of the earth's surface. I should be glad to have Mr. Hume explain this." "I suppose," answered Mr. Hume, "that you refer to the qualities of the earth as an absorbent and conductor of heat. The earth must needs have the capacity of receiving and retaining the heat which falls upon it from the sun. If the earth's surface were polished and brilliant, the heat of the sun would be reflected into space as from the surface of a mirror, and very small advantage would the earth receive from the solar heat. A dark soil absorbs heat more readily than a soil of lighter color, and a wet soil, on account of the high specific heat of water, requires more heat to raise its temperature than a dry soil. The mineral elements of the soil and its compactness or porosity also help to make up its capacity for receiving and retaining heat. The color and constitution of the soil sometimes go far toward making the climate of a region. The conducting qualities of the earth's crust in its profoundest depths also must be taken into account. If the crust of the earth were composed of silver, or any other substance of like conducting quality, and the interior of the earth were molten rock, as it now is, the interior heat would be so rapidly conducted to the surface that everything upon the earth would be consumed." "Upon so many circumstances wisely adjusted and nicely blended," said Mr. Wilton, "does the temperature of the earth depend. The intensity of the sun's heat, the dimensions of the earth's orbit, the constitution of our atmosphere in the subtlest qualities and relations of its elements, and the material, structure, and color of the earth's crust,--on all these and many other things which I cannot stop to mention depends the temperature needful for the well-being of the inhabitants of this globe. I beg your pardon, Mr. Hume, but allow me to ask whether such a combination of agencies and conditions, uniting to work out good for man, does not seem to you quite superhuman and worthy of a wise and good Creator?"