The simpler the organism, the simpler are its conditions of existence and the greater are in general the possibilities of supporting unfavourable conditions. Animals and plants living in caves or at great depths under water are deprived of light; they have accommodated themselves to that privation and do not suffer by it. Animals require oxygen in air and in water; plants also require a small quantity of carbonic acid for building up their tissues. There are even animalcule in existence for whom oxygen is a poison. As a general rule, temperatures above 50 degrees centigrade are insupportable. This is due to the fact that at that temperature albumen, one of the most important substances in the animal organism, coagulates. Inferior beings can resist higher temperatures and even for a short time 100 degrees centigrade, which is the boiling-point of water, but they could not live long. In its liquid form water is indispensable to organic life; life of any duration below zero is impossible because the water contained in every organism would solidify and the parts composing the organism would lose their mobility. Lack of water does, however, not inevitably imply death, and plants particularly can preserve for a long time a latent vitality while deprived of water. Cereals furnish a striking example, for when dried they can preserve their germinating power for years. Although living matter can preserve its vitality so long, it is none the less true that during that period all manifestations of life cease. If therefore the lack of water is perpetual, life must be considered as really suppressed. Even for the lowest forms of life three conditions must be regarded as essential: water, an atmosphere containing oxygen and carbonic acid, and a temperature between the limits indicated above. It is therefore really from these three points of view that we must study the heavenly bodies in order to be in a position to judge whether organic life as we know it is possible on them or not. As regards knowing whether that life presents itself under forms comparable to those which we see here, whether there are, for instance, beings analogous to humanity, that is quite another question. The means at the disposal of astronomy for determining the constitution of the heavenly bodies are of various kinds. We can take into account phenomena which at first sight do not seem adapted to that end.