The post. [volume] (Middleburg, Snyder County, Pa.) 1864-1883, 1879-07-10

,es radiato aua absorb heat even zoe,
under a temperature below it's own.
radiates heat. But oil bodies do not
absorb and radiate heat alike. Those
that are the best absorbers are also
the best radiators, but the best re-
Electors of beat arc the worst radia.
tors. by this law of radiation aud
absorption Of heat, all bodies change
constantly their condition in respect
to temperature, and also size. So
lar heat is abundantly absorbed by
all surrounding objects, but they ra-
dlnto as soon as the atmosphere is
disturbed In its equilibrium and
hence the atmosphere is kept up ut
nearly the same condition ut the sur.
face Of earth. II is only by terres-
trial radiation that heat is impaired
to atmospheric air. Tho direct in.
tluence of tho sun has no sensible
effect upon atmospheric temperature.
1t is owing to this fact that the up
per regions of the atmosphere are
intensely cold. Radiation of heat
takes place in II vacuum moro easily
than in air, ana is therefore quite in
dependent of any conducting medi-
um. Bit all bodies that rulia'o
hoat, also conduct heat from particle
TO particle. If this were not 81, the
snrftce Of bodies only would be
warmed, aul the interior never
change. iIcnco alt bodies conduct
hear also, but this is Independent of
radiation or absorption. II ono end
Of bar OF iron is heated, tho heat is
independent Of radiation or abs9rp.
tiDn, II one onl of bar Of iron is
heatol, tho hoGt is transmitted to
the other by condaotion through it's
particles. All bodies, however, do
not have the same conducting powor.
rhe metals, rs class, are good con-
ductors; while wool, wood, charcoal,
ire clny, aud similar bodies, are bad
ones. Gold is tho best conductor
known. In metals OF Il dluse texture
the condlction of heat is accompini'
cd with vibration oi their partic-
lea
The sense of touch gives n gool
idea of the conducting p1wor OF Va
rious bodies. la Il common atmos-
;here all bodies have almost precise
ly the same temperature Apply
yonr hand to one, it feels cold, to
another, aud it foels warm yetboth
have tho same Temperature. This is
owing to tho different conducting
power of the bodies good con-
iuctor abstracts tho heat quickly
from the haud, aud the body feels
cool, when, in point of fact, it is not
so. Heat obeys the same law in ra-
diation as light. 1t is transmitted
in rectilinear lines, and in its reilec-
ton tho angle of reflection Is equal
to the angle of incidence.
Rays of heat are also collected by
concave mirrors in faci They nm
dergo refraction and polarization
precisely as the rays of light. The
laws regulating heat and light are
80 much alike, that heat has been
thought to be only II modification of
light. However, there are certain
phenomenas of heat that do not ap-
pear under tho laws of light. The
modification OF bodies by their ther-
mological change, is one of the most
important laws of physical science.
All physical bodies expand by heat,
and contract by cold. The more
solid tho body, tho more powerful
the face Of dilatation, There is m
exception to this law, however, in
the case of ice. or water in certain
state. The law holds in water, in its
flid aud gaseous state, to a certain
degree OF heat, but immediately
changes below thirty-nine degrees.
Ice 10 we'll known to expand by
freezing. $40 this exception the
comfort, and even the habitability of
our globe, are in great measure,
dependent IN point of utility, the
exception to the law referred to is
truly Important.
The alterations of bodies by heat
are of two kinds. We have only
spoken of the first, that is, change OF
volume but production of u new
state Of aggregation is also discover
ed. For instance, a solid body is
reduced to fluid by change of its
temperature, aud a liquid 10 gass.
When We heat any substance, the
elevation Of its temperature is de-
ternrined solely by the portion OF
heat consumed, the rest OF which,
insensible to the thermo,n eter, is
absorbed to modify tho physical con-
stitution of the body. This is what
is meant by latent heat; that is q
portion of heat has become latent II
the body. This is called the funda.
mental law discovered by Black, and
is regarded as one Of the most bril'
liant discoveries in physical science.
Solldity aud Auidity were long re-
| garded absolute qualities OF bodies
but, properly speaking, they are onl,
| relative qualities ar. We aro even