of activity, we have an increased amount of
carbonic acid expired from the lungs. Dr.
Edward Smith, has shown that the quantity of
this gas expired during periods of great exertion,
and consequently of great bodily heat, may be
five times that expired in a state of repose

In his lectures, Professor Tyndall illustrated
his subject by a series of experiments which must
have been most striking to witness, but which still
retain their force and interest when read. He was
anxious that his audience should see, with their
own eyes, the facts on which his philosophy is
based; and for this purpose an ordinary
thermometer was useless. He therefore resorted to
a delicate little instrument which is called a
thermo-electric pile. The heat which this instrument
receives generates an electric current. Such
a current has the power of deflecting a freely-
suspended magnetic needle, to which it flows
parallel. Within view of the audience was such a
needle, suspended by a fibre of unspun silk, and
protected by a glass shade from any disturbance
by currents of air. To one end of the needle was
fixed a piece of red, and to the other a piece of
blue paper. All could see those pieces of paper;
and when the needle moved, its motion was clearly
visible to the most distant person in the room.

The needle being quite at rest, the instrument
was tested by the lecturer's breathing for an
instant against the naked face of the pile—a
single puff of breath was sufficient for the
purpose. Instantly, the needle started off, passing
through an arc of ninety degrees, or a quarter of
a circle. It would have gone further, had not
its swing been limited by a thin plate of mica
fixed, edgewise, at ninety degrees. Attention
was called to the direction of the deflection: the
red end of the needle moved from the lecturer
towards the audience, as if it disliked him, and
had been inspired by a sudden affection for them.
This action of the needle was produced by the
small amount of warmth communicated by his
breath to the face of the pile; no ordinary
thermometer could give so large and prompt an
indication.

Allowing the heat to waste itself (which it
did in a very short time), they noticed that, as
the pile cooled, the needle returned to its first
position. To show the effect of cold on the face
of the pile, he cooled a plate of metal by placing
it on ice. He then wiped the chilled metal, and
with it touched the face of the pile. The result
was, that a moment's contact sufficed to produce
a prompt and energetic deflection of the needle,
but in a direction opposite to the former
movement. When the pile was warmed, the red end
of the needle moved from him towards them; its
likings were now reversed, and the red end
moved from them towards him. The important
point to establish was, that from the direction
in which the needle moves we can, with certainty,
infer whether cold or heat has been communicated
to the pile; and the energy and promptitude
with which the needle moves, give us some
idea of the comparative quantity of heat or cold
imparted to it in different cases.

And now for an experiment or two, to
connect heat with the more familiar forms of force.
The lecturer had placed in the next room some
pieces of wood, which his assistant now hands
to him. The temperature of that room being
lower than the temperature of the lecture-room,
the wood is consequently slightly colder than
the face of the pile. To prove it, the piece of
wood is placed against the face of the pile. The
red end of the needle moves from them to him,
thus showing that the contact has chilled the
pile. He now carefully rubs the face of the pile
along the surface of the wood—carefully, because
the pile is a brittle instrument, and rough usage
would destroy it. Mark what occurs. The
prompt and energetic motion of the needle
towards the audience declares that the face of
the pile has been heated by this extremely small
amount of friction.

Experiments illustrating the development of
heat by mechanical means should be to students
in natural philosophy what a boy's school
exercises are to him. In order to fix them in our
minds, and obtain due mastery over them, we
must repeat and vary them in many ways. The
lecturer, therefore, takes in his fingers a flat
piece of brass attached to a cork, preserving
the brass from all contact with his warm hand.
He places the brass in contact with the face of
his pile; the needle moves, showing that the
metal is cold. He now rubs the brass on the
surface of a cold piece of wood, and lays it once
more against the pile. It is so hot, that if
allowed to remain in contact with the instrument,
the current generated would dash the
needle violently against its stops, and probably
derange its magnetism. Indeed, when a boy at
school, he had often blistered his hand by a
brass button which he had rubbed energetically
against a form.

Here is a razor, cooled by contact with ice;
and here is a hone, without oil, along which he
rubs the cool razor, as if to sharpen it. He
now places the razor against the face of the
pile; and the steel, which a minute ago was
cold, is now proved to be hot. Similarly, he
takes a knife and knife-board, which are both
cold, and rubs the knife along the board. The
knife, placed against the pile, declares itself
hot. He passes a cold saw through a cold
piece of wood, and places, in the first instance,
the surface of the wood against which the saw
has rubbed, in contact with the pile. The
needle instantly moves in a direction which
shows the wood to be heated. He allows the
needle to return to zero, and then applies the
saw to the pile. It is also hot. These are the
simplest and most common-place examples of
the generation of heat by friction; and for this
reason they were chosen. Mean as they appear,
they are illustrations of a principle which
determines the polity of the whole material universe.

To illustrate the development of heat by
compression, a piece of deal wood is taken,
cooled below the temperature of the room,
and giving, when placed in contact with the pile,
the deflection which indicates cold. The wood
is placed between the plates of a small hydraulic