(i6) 
Now let us leave the solar system, and confine our thought to 
our own earth. At this point, it is well to explain that by heat 
we mean merely a peculiar kind of motion. The molecules of 
which a body is composed are in a state of constant motion or 
vibration : the greater the motion of these particles, the hotter the 
body seems to us, and the less violent the vibration, the colder the 
whole mass seems. Now, the chemic force by which atoms com¬ 
bine is exerted only between atoms which are infinitesimally 
near each other, and so the intense heat which, at one time, pre¬ 
vailed upon the earth, consisting in a violent agitation of the 
atoms, prevented their remaining sufficiently long in close contact 
for combination to occur, so that, at this time, the earth consisted 
of free or uncombined atoms of matter. 
At a later stage, the earth having grown somewhat cooler, i. e. 
the energy of vibration having lessened, it became possible for 
these primitive atoms, of identical structure and properties, to 
combine themselves into simple compounds of widely differing 
qualities. These earliest combinations were, doubtless, the sub¬ 
stances which we now term elements—as oxygen, hydrogen, gold, 
mercury, lead, etc. The so-called elements were believed until 
very recently to be ultimate constituents of the material universe, 
simple, uncompounded, indivisible—but, through the spectro¬ 
scope, it has become almost certain that they are really com¬ 
pounds, quite separable into their components by intense heat. 
Thus, in the laboratory, by the heat of the Bunsen burner, several 
of the elements can be resolved, and the constituents recognized 
by the spectroscope ; others by the more intense heat of the elec¬ 
tric arc ; while still others of our elements may be discerned in 
the sun as undergoing through its fervent heat a dissociation which 
could never be accomplished amid the conditions known to us 
upon earth. 
As the primitive temperature lessened .still further, the kt -ele- 
ments” began to combine, and the resulting compounds gradually 
became more and more complex : first such simple and more 
stable compounds, as water, H2O, and the inorganic acids, 
carbonic acid, CO3, nitric acid HNO3, sulphuric acid, H2 
SO4; then the more complex salts, as sulphate of calcium, 
Ca SO4, chalk, Ca CO3 ; then the more complicated compounds 
