that we know, the following conditions, among others, are necessary, namely, (1) A supply of radiant energy which is intermittent or variable, (2) one or more elements which (like nitrogen) are very sensitive to changes of energy, and (3) one or more elements which (like carbon and hydrogen) are capable of alternately accumulating and dispersing energy by means of opposite chemical changes.
Since popular ideas as to the physical conditions of other worlds are generally hazy and often far from correct, it may not be amiss to recount such particulars as astronomical research has revealed in the members of the solar system.
In looking for a world where the conditions are most like our own, we naturally turn first to Venus. Her size and gravitation are nearly the same as those of the earth, and her atmosphere may therefore be expected to have a similar density and to hold approximately the same gases. As she is nearer the sun, the general temperature would be considerably higher, and the equatorial region might be too hot for our life; but there might nevertheless be a suitable temperature nearer the poles. But our speculations are damped by a suspicion, strong but not fully confirmed, that Venus has no day and night, but always keeps the same side toward the sun. If this is really the case, then the sunny side must be always burning hot and quite dry, while the opposite side must be always encased in ice—nay more, in a mixture of ice and solidified atmospheric gases. The life of such a world must be very different from any that we know.
After Venus, Mars is the planet whose conditions seem most to resemble those of our world. But there are far greater differences than are generally supposed. Mars is so small that he can not provide much heat from within, and so far from the sun that he receives comparatively little heat from without. His gravitation is so slight that the atmosphere is rare and nearly cloudless, and therefore heat must be readily lost by radiation. Thus on theoretical grounds Mars should be intensely cold: in fact his surface should be constantly in the condition of the highest mountain-tops of our world, only receiving less heat than they do from the sun. At such low temperature and pressure, water could never exist in the liquid form, though it might be solid or gaseous. But water is very possibly absent from Mars. Dr. Johnstone Stoney has calculated, by application of the dynamic theory of gases, that any water vapor introduced into the atmosphere of that planet would escape into space, the gravitation being there insufficient to retain it. Professor G. H. Bryan, calculating from slightly different data, questions Dr. Stoney's conclusions; but at all events Mars's gravitation is very near the dividing line between the ability and inability to retain water. If water is absent from Mars, then the polar caps and other seeming evidence of its presence must be due to some other fluid or gas, having heavier molecules and lower freezing and
