2 PHYSICS. 
those peculiarities which form the essence of what is known as body, matter, 
material, which thus apply to all bodies without any exception. Among 
these peculiarities may be first mentioned extension and impenetrability. A 
body must have a certain extension, that is, must occupy a certain space ; 
it must nevertheless be impenetrable, or must fill this space in such a man- 
ner, that no second body can also occupy it at the same instant of time. 
We must not fall into the error of supposing that one body can penetrate 
another, as a nail can a board, in the physical sense of the word. As the 
nail is driven through the board by mechanical force, it pushes aside the 
fibres of the wood, and occupies their place; the particles of the wood 
and iron are therefore contiguous, but not in the same place. Penetration, 
in the physical sense of the word, is the destruction of one substance by 
another, not a mere displacement. In the latter case, there is not neces- 
sarily an increase in bulk, as the board with the nail occupies no more space 
than without it; and a measure of water mixed with a measure of sulphuric 
acid will not fill two measures: penetration, nevertheless, has not taken 
place, no atom having been annihilated, as may be proved by weighing. 
Divisibility is another general property of bodies, by means of which they 
are supposed to be capable of division into smaller and smaller portions— 
atoms. The pulverization of solid bodies, the small globules of fluids, as 
the blood globules, whose diameter is only ;1, of a line, and the great 
space which gaseous bodies can occupy, show this property on a large 
scale, while the atomic theory follows it to the smallest molecules. Nearly 
allied to divisibility, are two other properties of bodies, extensibility and 
compressibility, which are opposed to each other. “By these terms is meant 
an increase or diminution of the space which a body, under certain cir- 
cumstances, occupies, without the connexion of its molecules or atoms 
being thereby affected. As these atoms are supposed to be unchangeable, 
this change of space must necessarily be referred to an expansion or con- 
traction of the interspaces which exist between these atoms, in the natural 
state of the body. This extension is the result of a stretching or heating ; 
the contraction takes place under the influence of cold or pressure. 
The mention of interspaces between the individual atoms of a body, leads 
us to the consideration of another property of bodies, called porosity, pos- 
sessed, as far as we know, by all. In ordinary language, however, the term 
pore, which may be considered, scientifically, as referring to an interspace 
infinitely small, is applied to those only which are large enough to allow the 
passage of fluids or gases. It is by means of these pores that the parts of one 
body penetrate between those of another, as water a sponge. In other 
bodies the pores are so small as not even to admit the entrance of gases, as. 
for instance, glass. 
The atoms of which a body is composed are not always homogeneous, 
and hence the different kind of bodies; thus cinnabar is composed of atoms 
of sulphur and mercury ; water, of oxygen and hydrogen atoms, &c.; such 
bodies being called compound, as distinguished from simple (elementary or 
elements), in which the atoms are homogeneous. These investigations, 
however, belong to the department of chemistry, and as such, do not belong 
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