han those sought for in vain by Mr. Huxley. These
particles, as already stated, must have been less than 1/100000th of
an inch in diameter.

And now I want you to consider the following question: Here are
particles which have been growing continually for fifteen minutes, and
at the end of that time are demonstrably smaller than those which
defied the microscope of Mr. Huxley--_What must have been the size of
these particles at the beginning of their growth?_ What notion can you
form of the magnitude of such particles? The distances of stellar
space give us simply a bewildering sense of vastness, without leaving
any distinct impression on the mind; and the magnitudes with which we
have here to do, bewilder us equally in the opposite direction. We
are dealing with infinitesimals, compared with which the test objects
of the microscope are literally immense.

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 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 300,000 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 distance 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 should have the deep
blue firmament. Let the atmospheric space beyond the shell be swept
clean, and the sky-matter properly gathered up. What would be its
probable