Most combinations of silicic acid with metals are insoluble in water. Some, however, and especially a combination with potassium known by the name of water-glass, occurs in a soluble form. If into such a solution we introduce a small quantity of any soluble metallic salt such as chloride of copper or silver nitrate, decomposition takes place and the chloride of copper is converted into silicate of copper and chloride of potash. A very curious phenomenon then takes place. At the moment when the first traces of the metallic salt are dissolved, the decomposition mentioned above takes place and the molecule of salt is covered with an extremely fine skin of insoluble silicate. It is across this skin that the liquids are then exchanged by a process of osmosis which, as we know, plays a great part in the life of plants and animals and which here takes place as follows:
More liquid enters the membrane than issues from it; consequently the internal pressure increases until it is great enough to break through the membrane, somewhere allowing a drop of the liquid which is a solution of metallic salt to escape. On account of the chemical decomposition, this drop immediately surrounds itself with a new skin, and the process of rupture and reformation is repeated until the metallic salt inside the membrane is completely decomposed. We thus see arising under our eyes in a few minutes a marvellous arborescent structure which could at first sight be taken for some plant of inferior order. The rupture of the membrane always takes place in the most recently formed portion, because there the membrane, which thickens gradually, has as yet the least thickness; that is generally the upper portion, so that this offers another analogy to plant life. The coloration of the cells differs with the nature of the metallic salt, but we cannot trace any influence upon the forms obtained. By throwing into the solution several different metallic salts, one can form a many-coloured garden in a small bottle.
We are far from seeing in this experience anything but a quite external resemblance to the phenomena of living matter. Yet it leads us to other reflections, especially as we do not yet know anything concerning the real nature of living matter.
Among plants and inferior species of the animal world we cannot imagine a life conscious of itself. But in common with our silica creature they show growth and the consumption of chemical substances. In both cases that growth is arrested wherever and whenever the food gives out. This happens in the silica solution when the provision of metallic salt is exhausted; and these are not the only analogies which one can find.
But it will be said that there is in reality a capital difference: in one case we have to do with real life for which we have no explanation; in the other we have to do with a simple chemico-physical phenomenon.
We reply that a few centuries ago this chemical vegetation would have been an enigma as life itself is still to us, and nobody would then have doubted that he was observing the development of some strange plant. Would they not then have taken the silicon cell for an organic cell? What follows? That the idea of living nature is quite relative, that it changes with our knowledge, and that an imaginative spirit is quite at liberty to endow the stare which we just excluded from life with a life quite different from ours.
Apart from these possibilities, the somewhat discouraging result of our previous study was that of all celestial bodies visible to us only two or three could be described with any probability as fit to support something resembling our terrestrial organic life. We cannot object to this conclusion so long as we take it literally, but we wish to point out that all depends upon the little restriction contained in the words “visible to us.”
We must therefore include the heavenly bodies which we cannot see and concerning which we know very little. But here we encounter a very peculiar paradox. We know nothing about these invisible stars, and yet as far as our interests are concerned we know more about them than the others. For mathematics comes to our aid, and if we rely on the calculus of probabilities we arrive, as we shall see, at very clear results.
Our Sun has created for itself, without counting the asteroids, a retinue of eight planets, which on account of their respective distances from the central body are placed in the most diverse conditions as regards temperature. Out of these eight planets one, the Earth, is undoubtedly inhabited, and two others, Mars and Venus, very probably. From the fact that the Sun has produced not one planet but eight, we can conclude that very probably the other suns or fixed stars have also produced one or several planets, and that those which escape from that law are the exception. We must also admit that among these stellar planets there may be some in such a condition and at such a distance from their central sun that organic life is possible on their surfaces.
We shall make this calculation with figures so modest that we shall obtain results obviously below the truth.