CHAPTER LXI. THEORIES.

339. The La Place Theory.--This theory supposes that at one time
the earth and the other planets, together with the sun,
constituted a single mass of vapor, extending billions of miles
in space; that it rotated around its center; that it gradually
shrank in volume by the transformation of potential into kinetic
energy; that portions of its outer rim were thrown off, and
finally condensed into planets; that our sun is only the
remainder of that central mass which still rotates and carries
the planets around with it; that the earth is a cooling globe;
that the other planets are going through the same phases as the
earth; and finally that the sun itself is destined like them to
become a cold body.

340. A Cooling Earth.--The sun's temperature is variously
estimated at many thousands, or even millions o£ degrees. Many
metals which exist on the earth as solids -e.g. iron- are gases
in the dense atmosphere of the sun. Thus the earth, in its early
existence, must have been composed of gases only, which in after
ages condensed into liquids and solids. So intense was the heat
at that time, that substances probably existed as elements
instead of compounds, i.e. the temperature was above the point of
dissociation. We have seen that Al2O3, CaO, SiO2, etc., are
dissociated at the highest temperatures only. If the temperature
were above that of combination, compounds could not exist as
such, but matter would exist in its elemental state. On slowly
cooling, these elements would combine. It is, then, a fair
inference that such compounds as need the highest temperatures to
separate them, as silica, silicates, and some oxides, were formed
from their elements at a much earlier stage of the earth's
history than were those compounds that are more easily separable,
such as water, lead sulphide, etc., and that the most infusible
substances were solidified first.

341. Evolution.--As the earth slowly cooled, elements united to
form compounds, gases condensed to liquids, and these to solids.
At one time the entire surface of our planet may have been
liquid. When the cooling surface reached a point somewhat below
that of boiling water, the lowest forms of life appeared in the
ocean. This was many millions of years ago. Most scientists
believe that all vegetable and animal life has developed from the
lowest forms of life. There is also a theory that all chemical
elements are derivatives of hydrogen, or of some other element,
and that all the so-called elements are really compounds, which a
sufficiently high temperature would dissociate. As evidence of
this, it is said that less than half as many elements have been
discovered in the sun as in the earth, and that comets and
nebula, which are less developed forms of matter than the sun,
have a few simple substances only.

It is easy to fancy that all living bodies, both animal and
vegetable, are only natural growths from the lowest forms of
life; that these lowest forms are a development, with new
manifestations of energy, from inorganic matter; that compounds
are derived from elements; and that the last are derivatives of
some one element; but it must be borne in mind that this is only
a theory.

342. New Theory of Chemistry. We have seen that heat lies at the
basis of chemical as well as of physical changes. By the loss of
heat, or perhaps by the change of potential into kinetic energy,
in a nebulous parent mass, planets were formed, capable of
supporting living organisms. Heat changes solids to liquids, and
liquids to gases; it resolves compounds, or it aids chemical
union. In every chemical combination heat is developed; in every
case of dissociation heat is absorbed. Properly written, every
equation should be: a + b = c + heat; e.g. 2 H + 0 = H2O + heat;
or, c - a = b - heat; e.g. H2O - 2 H = 0 - heat. Another
illustration is the combination of C and O, and the dissociation
of CO2, as given on page 82. C + O2 = CO2 + energy. CO2 - O2 = C
- energy. In fact, there are indications that the present theory
of atoms and molecules of matter, as the foundation of chemistry,
will at no distant day give place to a theory of chemistry based
on the forms of energy, of which heat is a manifestation.