PROPERTIES OF MATTER. 5 
whenever a solid which has been put in a liquid comes out wet. This 
attraction affords an explanation of what often happens when water is 
poured from a vessel—the water runs down the outside of the vessel, 
instead of flowing as desired. (3) Adhesion between solids and gases.— 
Ifa piece of cork be pushed down into water, little air-bubbles are seen 
sticking to it. When a lump of sugar is dropped into a cup of tea, the atmo- 
sphere of air which surrounds the particles does not quit them till they 
are dissolved ; bubbles are seen rising till all the sugar has disappeared. 
6. Porosity.—Looking at a piece of cork or of sponge, we see that it is 
full of little holes; these holes are called pores, and the substances ° 
having these holes are said to be porous. In ordinary language, it is 
only such substances as cork and very soft wocds, in which these little 
holes are visible, that are said to have pores; but, in reality, all substances 
are more or less porous. When a piece of bone is examined with a micro- 
scope, it appears almost like a pile of empty boxes; and a piece of wood 
appears like a bundle of pipes. Even the densest solids, as gold and 
silver, have been proved to be porous. Thus, when a hollow sphere of 
silver was filled with water, and squeezed with great force, the water oozed 
through the silver, and stood in drops on the outside of the sphere. 
7. Density.—This property is very closely connected with porosity: the 
two are, in fact, the converse of each other, because the more porous a 
body is, it is the less dense. If we squeeze a body to half its former 
size, there is of course no less matter in it, but we say we have doubled 
its density, while we have reduced it to half its original volume; so 
that there are in the body after being compressed more atoms (that 
is, more matter) in the same space than there were before. In com- 
paring the density of different substances, the density of water (distilled) 
is taken as a standard, and called 1. Measured by this standard, the 
density of other bodies is called their Specific Gravity: thus, the 
specific gravity of a body whose density is double that of water is said 
to be 2. Density is a very variable property: it is often increased owing 
to the OCompressibility of bodies, and as often lessened by their Dilata- 
bility. An iron rod when heated becomes both thicker and longer, and 
contracts again with cold. Se much is this the case, that when measure- 
ments are being made with an iron rod or chain, if the chain be exposed to 
great heat or cold, allowance must be made for difference of length. The 
iron rims of wheels could not be made to fit so tightly, were it not that 
they are put on when hot. Being a perfect fit when hot, when the iron. 
cools and contracts, the rim binds the wheel very closely. Gases are also 
highly dilatable. Ifa bladder be filled almost full of cold air, so as to 
shew the bladder loose and wrinkled, when heated it is seen to swell out 
and become quite tight, from the air expanding: on the other hand, 
many cubic feet of air may be compressed into a single inch, 
