of a few pounds in weight, which strike the earth, we have become quite familiar, while spectrum analysis has carried us a long way toward the conclusion that there is a unity in the material composition of the universe. But, as every thing is in motion, and all the celestial masses are revolving and whirling at great rates of speed, and, as there is much evidence of fractures, collisions, and transformations, it seems impossible that there should not have been going on constant abrasions and comminutions, with the production of all grades of dust down to the most impalpable, as a consequence of the general wear and tear. Such a notion would, of course, have been inadmissible in old times, when people reasoned about the universe from their a priori notions of what it ought to be; and, holding that the heavens are the type of all perfection, they would have been shocked at the notion that the gearing of the spheres could not work without the production of dirt and dust. But we have survived those prejudices, and now search for the celestial débris and waste with just as much interest as we do for more imposing and dignified objects.
It is but a few years since the world was surprised and delighted by that brilliant series of researches made by Prof. Tyndall, on the formation of artificial clouds and artificial sky by the electric light, in his vacuum-tubes. Spaces, washed clean, and apparently pure, were found to be filthy with dust, and, beginning with the thinnest transparent vapors, he was able to develop a succession of the most exquisite cloud-forms definitely related to the colors of the spectrum as the molecules grew in complexity under the chemical transformation. The firmamental blue he found to be caused by impurities in the air, so attenuated as to react only with the finest waves of the ethereal medium. How far down in the scale of minuteness beyond all previous conception the particles are, which reflect the azure light of the sky, may be gathered from the following memorable passage, taken from Dr. Tyndall's "Fragments of Science" (page 148):
"From their perviousness to stellar light and other considerations, Sir John Herschel drew some startling conclusions regarding the density and weight of comets. You know that these extraordinary and mysterious bodies sometimes throw out tails 100,000,000 of miles in length, and 50,000 miles in diameter. The diameter of our earth is 8,000 miles. Both it and the sky, and a good portion of space beyond the sky, would certainly be included in a sphere 10,000 miles across. Let us fill a hollow sphere of this diameter with cometary matter, and make it our unit of measure. To produce a comet's tail of the size just mentioned, about three hundred thousand such measures would have to be emptied into space. Now, suppose the whole of this stuff to be swept together and suitably compressed, what do you suppose its volume would be? Sir John Herschel would probably tell you that the whole mass might be carted away at a single effort by one of your dray-horses. In fact, I do not know that he would require more than a small fraction of a horse-power to remove the cometary dust. After this you will hardly regard as monstrous a notion I have sometimes entertained concerning the quantity of matter in our sky. Suppose a shell to surround the earth at a height above the surface which would place it beyond the grosser matter that hangs in the lower regions of the air—say at the height of the Matterhorn or Mont Blanc. Outside this shell we have the deep-blue firmament. Let the atmospheric space beyond the shell be swept clean, and let the sky-matter be properly gathered up. What is its probable amount? I have sometimes thought that a lady's portmanteau would contain it all. I have thought that even a gentleman's portmanteau—possibly his snuffbox—might take it in. And, whether the actual sky be capable of this amount of condensation or not, I entertain no doubt that a sky quite as vast as ours, and as good in appearance, could be formed from a quantity of matter which might be held in the hollow of the hand."
Whatever may be the validity of these quantitative speculations, all the lines of investigation seem to converge
