12 
PHYSICAL PROPERTIES. 
dividing the absolute weight 390, by the difference 
of its weight 80, in air and water. Thus, 390 divi- 
ded by 80 is 4.876,* the specific gravity of the min- 
eral. 
Impenetrability is that property of matter by 
which two bodies cannot at the same time occupy the 
same space. This property has been called the 
mainspring of all mechanical motion, as it affords to 
the oars of boats, to the wheels of mills, and various 
kinds of machinery, and the sails of vessels, a resist- 
ance in water and air which propels them onward ; 
and without such a property of matter, human ge- 
nius could be of no avail. 
Divisibility, — Matter has been said to be infinitely 
divisible, but it would be more correct to say that 
it is divisible to an extreme degree of minuteness. 
For example) a grain of gold may be so extended 
by hammering as to cover 50 square inches of sur- 
face, and contain two millions of visible points; and 
the gold which covers the silver wire used in ma- 
king gold lace, is spread over a surface twelve times 
as great. A grain of iron, dissolved in nitro-muri- 
atic acid, and mixed with 3137 pints of water, will 
be diffused through the whole mass ; for, by using 
some chemical test, a portion of iron may be de- 
tected in every part of the liquid. This proves that 
the iron has been divided into more than 24 mill- 
ions of parts ; and if the same quantity were still 
farther diluted, the diffusion of the iron through the 
whole mass might be proved, by concentrating any 
portion of it by evaporation, and employing its ap- 
propriate tests. Newton succeeded in determining 
the thickness of very thin laminae of transparent 
substances by observing the colours which they re- 
flect. A soap-bubble is a thin shell of water, and 
* A very convenient instrument for ascertaining the specific 
gravity of bodies is Nicholson's Portable Balance, which may 
be obtained at any of the shops where philosophical instruments 
are kept for sale, with directions for its use. 
