of Newton, has
given to the universe an unity from the philosophical standpoint that it
did not possess before.

Some matter may, or may not be elastic; it may, or may not be solid, or
liquid, or gaseous; but there is this fact regarding matter which is
absolutely undeniable, and that is, "All matter is gravitative."

That this is true of each and all kinds of matter has been proved by
direct experiment times without number, and the constant application of
the law to all forms of matter is a fact observable from the phenomena
incidental to every-day life. Astronomical observation teaches us also,
that all stars, suns, planets, satellites, and comets are subject to
this great Law of Gravitation, as indeed they must be if they are
composed of matter. That they are all composed of exactly similar
elements of which the earth is composed, has been proved again and again
by spectroscopic analysis, which teaches that hydrogen, iron, and
calcium, etc., are to be found in distant stars and nebulae, as they are
equally to be found in the composition of the earth. Thus throughout the
wide universe so far as observation and experiment can teach us, we
learn that without any exception, everything that is termed matter is
subject to this universal Law of Gravitation.

ART. 38. _Matter possesses Density._--Density is that property of matter
which decides the weight of a body per unit of volume.

The density of any substance may be shown in several ways. It may
denote, first of all, the number of molecules in a given body. Let us
take as an illustration, the case of air being forced into a vessel of a
given size, say one cubic foot capacity. We will suppose that in such a
vessel there are 1,000,000 molecules. If we pump in a quantity of air
equal to the amount it contained at first, then it is obvious that we
have doubled the number of molecules in the same vessel, and therefore
we say we have doubled the density. Not only so, but the weight of the
air in the vessel will have been doubled. Looked at from this
standpoint, density means the number of molecules in unit volume such as
a cubic inch, or cubic centimetre.

Again, as has already been shown in Art. 35, the different elements have
different atomic weights. Thus an atom of carbon weighs twelve times as
much as an atom of hydrogen, that is to say, there are twelve times as
much matter by weight in an atom of carbon as there is in an atom of
hydrogen, so that it would take