nished
on account of the increased distance from the sun, the repulsive motion
due to heat is also diminished in exactly the same proportion and along
exactly the same path. If at any point in the solar system the
attractive force is doubled, then according to our repulsive theory of
heat, and the law of inverse squares, the repulsive motion is also
doubled. If the attractive force is halved, then the repulsive motion is
halved also, the repulsive motion being always and at all places exactly
proportional to the increase or decrease of the attraction of
Gravitation.

[Footnote 13: _Heat, a Mode of Motion._]

ART. 67. _First Law of Thermodynamics._--The Law of Thermodynamics is
based on two fundamental truths which have reference to the conversion
of Heat into Work, and Work into Heat. In Art. 54 we have already seen
that energy in the form of heat, light, electricity and magnetism is
capable of being converted into other forms of energy, while in Art. 59
we have seen that Joule gave us the exact relation in foot-pounds
between heat and work. He showed that when 1 lb. of water fell through
772 feet its temperature was raised one degree Fahr. Thus the principle
underlying the first law of thermodynamics states, that whenever work is
spent in producing heat, the amount of work done is proportionate to the
quantity of heat generated; and conversely, whenever heat is employed to
do work, a certain amount of heat is used up, which is the equivalent of
the work done. This principle is also in accord with the conservation of
Energy and Motion (Arts. 52 and 57), which assert that whenever energy
or motion disappears in one form, it is manifested in some other form.
Thus, from the first law of thermodynamics, we learn that wherever we
have heat we have the power to do work, and the amount of work so done
is proportionate to the heat used up. Heat, then, has a capacity to
perform work, and that power is known as the mechanical equivalent of
heat. Both Mayer of Germany, and Dr. Joule of Manchester, have worked
out this problem, and have given us the mechanical value of heat. By
experiments Mayer found out that a quantity of heat sufficient to raise
1 lb. of water one degree Fahr. in temperature was able to raise a
weight 771.4 lb. one foot high. Dr. Joule of Manchester, after making a
number of experiments which lasted over many years, came to the
conclusion that the mechanical equivalent of a unit heat was 772
foot-pounds, a u