mechanics, now measure without seeing them, or make their existence plain without grasping them! We may mention some instances which can give us an idea of these.
According to Tyndall, when very minute solid particles, smaller than the luminous waves, are diffused in a medium traversed by light, the light is decomposed in such a way that the least waves, the blue ones, predominate in the reflected rays, and the largest ones, the red waves, in the transmitted rays. This ingenious physicist thus explains how the blue color of the sky depends and must depend on the existence of solid particles, excessively minute, diffused in infinite quantity through the atmosphere. Tyndall is not disinclined to the idea that these imperceptible atoms might very well be no other than those germs of microscopic organisms the presence of which in the atmosphere has been proved by the labors of Pasteur, as well as the part they take in the phenomena of putrefaction and fermentation. The ova of these beings, which are barely visible under the microscope after attaining full development, and of which the number, ascertained by the most decisive evidence, confounds the boldest imagination, these would be the elements of that vital ether, as we have termed it, that dust which gives its lovely blue tint to the vault of the sky. "There exist in the atmosphere," Tyndall says in closing, "particles of matter that elude the microscope and the scales, which do not disturb its clearness, and yet are present in it in so immense a multitude that the Hebrew hyperbole of the number of grains of sand on the sea-shore becomes comparatively unmeaning." And to give an idea of the minuteness of these particles, Tyndall adds that they might be condensed till they would all go into a lady's travelling-bag. Manifestly these particles escape any kind of direct measurement and observation. Their objective reality can no more be demonstrated than that of the particles of ether can be made evident. Yet there are certain facts which aid us to form a clear conception of them. Let us dissolve a gramme of resin in a hundred times its weight of alcohol, then pour the clear solution into a large flask full of pure water, and shake it briskly. The resin is precipitated in the form of an impalpable and invisible powder, which does not perceptibly cloud the fluid. If, now, we place a black surface behind the flask, and let the light strike it either from above or in front, the liquid appears sky-blue. Yet, if this mixture of water and alcohol filled with resinous dust is examined with the strongest microscope, nothing is seen. The size of the grains of this dust is much less than the ten-thousandth part of 1⁄25 of an inch. Moren makes another experiment, proving in a still more surprising way the extreme divisibility of matter: Sulphur and oxygen form a close combination, called by chemists sulphuric-acid gas. It is that colorless and suffocating vapor thrown off when a sulphur-match is burned. Moren confines a certain quantity of this gas in a receiver, places the whole in a dark medium, and sends a bright ray of light
