Since Mercury wears no concealing veil of atmosphere, and displays markings that can be identified and followed, a surprising circumstance has come to light. In 1889, Schiaparelli discovered that Mercury, instead of rotating on its axis in about 24 hours like the Earth and Mars, rotates in 88 days; that is to say, it always turns the same face towards the Sun, just as the Moon turns the same face towards the Earth. This fact, confirmed theoretically by Prof. G. H. Darwin in his development of the theory of tidal friction, puts the condition of Mercury in quite a new light. No alternation of day or night refreshes and restores the little world; one hemisphere is for ever exposed to the blasting heat of the Sun, seven times hotter for it than for the Earth; the other hemisphere is for ever exposed to the darkness and cold of outer space, a range from something like 390° C. above freezing-point, to 270° C. below. It is true that between the two hemispheres there is a "debatable land," for, owing to the ellipticity of the orbit, the face turned to the Sun is not exactly the same at all times, and a region about 47° in width on each side of the planet, that is to say, rather more than a quarter of its entire surface, has one day and one night in each period of 88 days, but these more favoured sections can scarcely be considered more habitable than the rest. The conditions of Mercury are so unfavourable for life that, even if this remarkable relation of rotation period to revolution did not hold good, it would still be impossible to regard it as a world for habitation. But its case shows that a further condition of habitability has to be satisfied by a planet. Size and distance from the Sun afford the first two conditions; a suitable rotation period is now seen to be a third. And it is possible that in this very particular Venus fails to qualify. Schiaparelli, the first observer of his time, assisted by the clear Italian sky, believed that he had demonstrated that Venus, like Mercury, rotates once in her year; her day being thus equal in length to 225 of ours, and the face that she turns to the Sun being always the same. And in her case this statement requires practically no qualification, for, her orbit being nearly circular, there is hardly any libration; a place that has the Sun in its zenith has it so for ever; one on the night side of Venus can never have a sunrise, or gladden in the daylight. The side exposed to the Sun will wither in a temperature of about 227° C., in which all moisture will be evaporated; the side remote from it will be bound in eternal ice. In neither hemisphere will water exist in the liquid state; in neither hemisphere will life be possible.