Even if it is supposed that, in the case of Venus, its perpetual cloud-canopy may keep down the surface temperature within the limits necessary for animal life, the extraordinary turmoil in its atmosphere caused by the excessively contrasted temperatures of its dark and light hemispheres must be extremely inimical to life, if not absolutely prohibitive of it. For on the greater part of the hemisphere that never receives a ray of light or heat from the sun all the water and aqueous vapour must be turned into ice or snow, and it seems almost impossible that the air itself can escape congelation. It could only do so by a very rapid circulation of the whole atmosphere, and this would certainly be produced by the enormous and permanent difference of temperature between the two hemispheres. Indications of refraction by a dense atmosphere are visible during the planet's transit over the sun's disc, and also when it is in conjunction with the sun, and the refraction is so great that Venus is believed to have an atmosphere much higher than ours. But during the rapid circulation of such an atmosphere, heated on one half the planet and cooled on the other, most of the aqueous vapour must be taken out of it on the dark side as fast as it is produced on the heated side, though sufficient may remain to produce a canopy of very lofty clouds analogous to our cirri. The occasional visibility of the dark side of Venus may be caused by an electrical glow due to the friction of the perpetually overflowing and inflowing atmosphere, this being increased by reflection from a vast surface of perpetual snow. If we consider all the exceptional features of this planet, it appears certain that the conditions as regards climate cannot now be such as to maintain a temperature within the narrow limits essential for life, while there is little probability that at any earlier period it can have possessed and maintained the necessary stability during the long epochs which are requisite for its development. Before considering the condition of the larger planets, it will be well to refer to an argument which has been supposed to minimise the difficulties already stated as to those planets which approach nearest to the earth in size and distance from the sun. THE ARGUMENT FROM EXTREME CONDITIONS ON THE EARTH