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 4.875,* 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 Nicliolson's Portable Balance, which may 

 be obtained at any of the shops where philosophical instruments 

 are kept for sale, with directions for its use. 



