But we have no right to forbid Nature to act differently in worlds from which carbon is absent. A world, for example, in which silica replaces carbon, silicic acid carbonic acid, might be inhabited by organisms absolutely different from those which exist on the Earth, different not only in form, but also in substance. We already know stars and suns for which spectral analysis reveals a predominance of silica, _e.g._, Rigel and Deneb. In a world where chlorine predominated, we might expect to find hydrochloric acid, and all the fecund family of chlorides, playing an important part in the phenomena of life. Might not bromine be associated in other formations? Why, indeed, should we draw the line at terrestrial chemistry? What is to prove that these elements are really simple? May not hydrogen, carbon, oxygen, nitrogen, and sulphur all be compounds? Their equivalents are multiples of the first: 1, 6, 8, 14, 16. And is even hydrogen the most simple of the elements? Is not its molecule composed of atoms, and may there not exist a single species of primitive atom, whose geometric arrangement and various associations might constitute the molecules of the so-called simple elements? In our own solar system we discover the essential differences between certain planets. In the spectrum of Jupiter, for instance, we are aware of the action of an unknown substance that manifests itself by a marked absorption of certain red rays. This gas, which does not exist upon the Earth, is seen still more obviously in the atmospheres of Saturn and Uranus. Indeed, upon this last planet the atmosphere appears, apart from its water vapor, to have no sort of analogy with our own. And in the solar spectrum itself, many of the lines have not yet been identified with terrestrial substances. The interrelation of the planets is of course incontrovertible, since they are all children of the same parent. But they differ among themselves, not merely in respect of situation, position, volume, mass, density, temperature, atmosphere, but again in physical and chemical constitution. And the point we would now accent is that this diversity should not be regarded as an obstacle to the manifestations of life, but, on the contrary, as a new field open to the infinite fecundity of the universal mother. When our thoughts take wing, not only to our neighbors, Moon, Venus, Mars, Jupiter, or Saturn, but still more toward the myriads of unknown worlds that gravitate round the suns disseminated in space, we have no plausible reason for imagining that the inhabitants of these other worlds of Heaven resemble us in any way, whether in form, or even in organic substance.