before they can be used by animals is found in the Ca3(PO4)2 of
bones. This is rendered soluble; plants absorb and transform it;
animals eat the plants and obtain the phosphates. Thus man is
said to "eat his own bones." The food of mankind may be divided
into four classes (1) proteids, which contain C, H, O, N, and
often S and P; (2) fats, and (3) amyloids, both of which contain
C, H, O; (4) minerals. Examples of the first class are the gluten
of flour, the albumen of the white of egg, and the casein of
cheese. To the second class belong fats and oils; to the third,
starch, sugar, and gums; to the fourth, H2O, NaCl and other
salts. Since only proteids contain all the requisite elements,
they are essential to human food, and are the only absolutely
essential ones, except minerals; but since they do not contain
all the elements in the proportion needed by the system, a mixed
diet is indispensable. Milk, better than any other single food,
supplies the needs of the system. The digestion and assimilation
of these food-stuffs and the composition of the various tissues
is too complicated to be taken up here; for their discussion the
reader is referred to works on physiological chemistry.

338. Conservation.--Plants, in growing, decompose CO2, and
thereby store up energy, the energy derived from the light and
heat of the sun. When they decay, or are burned, or are eaten by
animals, exactly the same amount of energy is liberated, or
changed from potential to kinetic, and the same amount of CO2 is
restored to the air. The tree that took a hundred years to
complete its growth may be burned in an hour, or be many years in
decaying; but in either case it gives back to its mother Nature,
all the matter and energy that it originally borrowed. The ash
from burning plants represents the earthy matter, or salts, which
the plant assimilated during its growth; the rest is volatile. In
the growth and destruction of plants or of animals, both energy
and matter have undergone transformation. Animals, in feeding on
plants, transform the energy of sunlight into the energy of
vitality. Thus "we are children of the sun."

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 oŁ 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.

Chapter, LXII.

GAS VOLUMES AND WEIGHTS.

343. Oxygen.

Experiment 134.--Weigh accurately, using delicate balances, 5 g.
KClO3, and mix with the crystals 1 or 2 g. of pure powdered MnO2.
Put the mixture into a t.t. with a tight-fitting cork and
delivery-tube, and invert over the water-pan, to collect the gas,
a flask of at least one and a half liters' capacity, filled with
water. Apply heat, and, without rejecting any of the gas, collect
it as long as any will separate.

Then press the flask down into the water till the level in the
flask is the same as that outside, and remove the flask, leaving
in the bottom all the water that is not displaced. Weigh the
flask with the water it contains; then completely fill it with
water and weigh again.

Subtract the first weight from the second, and the result will
evidently be the weight of water that occupies the same volume as
the O collected. This weight, if expressed in grams, represents
approximately the number of cubic centimeters of water,--since 1
cc. of water weighs lg,--or the number of cubic centimeters of O.

At the time the experiment is performed the temperature should be
noted with a centigrade thermometer, and the atmospheric pressure
with a barometer graduated to millimeters.

Suppose that we have obtained 1450 cc. of O, that the temperature
is 27 degrees, and the pressure 758 mm.; we wish to find the
volume and the weight of the gas at 0 degrees and 760 mm.

According to the law of Charles--the volume of a given quantity
of gas at constant pressure varies directly as the absolute
temperature. To reduce from the centigrade to the absolute scale,
we have only to add 273 degrees. Adding the observed temperature,
we have 273 degrees + 27 degrees = 300 degrees. Applying the
above law to O obtained at 300 degrees A, we have the proportion
below. Since the volume of O at 273 degrees will be less than it
will at 300 degrees, the fourth term, or answer will be less than
the third, and the second term must be less than the first. 300 :
273 :: 1450 : x. This would give the result dependent upon
temperature alone.

By the law of Mariotte - Physics, - the volume of a given
quantity of gas at a constant temperature varies inversely as the
pressure. Applying this law to the O obtained at 758mm, we have
the following proportion. The volume at 760mm will be less than
at 758mm; or the fourth term will be less than the third; hence
the second must be less than the first. 760: 758:: 1450: x. This
would give the result dependent on pressure alone.

Combining the two proportions in one:--

	300: 273 ):: 1450: x = 1316cc.
	760: 758 )

1316cc=1.316 liters. It remains to find the weight of this gas. A liter of
H weighs 0.0896g. The vapor density of O is 16. Hence 1.316 liters of O
will weigh 1.316 X 16 X 0.0896 =1.89g.

                    (KClO3 = KCl + O3)
From the equation   (122.5         48) we make a proportion,
                    (   5           x)

122.5: 5:: 48: x = 1.95, and obtain, as the weight of O contained in
5g of KClO3, 1.95g. The weight we actually,obtained was 1.89g. This
leaves an error of 0.06g, or a little over 4 per cent of error (0.06 / 1.95
= 0.03 +). The percentage of error, in performing this experiment,
should fall within 10.

Some of the liabilities to error are as follows:--


1. Impure MnO2, which sometimes contains C. CO2 is soluble m H2O.

2. Solubility of O in water.

3. Escape of gas by leakage.

4. Moisture taken up by the gas.

5. Difference between the temperature of the gas and that of the
air in the room.

6. Errors in weighing.

7. Want of accuracy in the weights and scales.

344. Hydrogen.

Experiment 135.--Weigh 5g, or less of sheet or granulated Zn, and

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