20 INTRODUCTION. 



Brittleness is that property of solids which causes them to be broken easily 

 when external force is applied to them. Glass, sulphur, coal, etc., are brittle. 



Tenacity is that property in virtue of which solids resist attempts to pull 

 their particles asunder. Iron is one of the most tenacious substances. 



Malleability, possessed by some solids, is the property in virtue of which they 

 may be hammered or rolled into sheets. Gold is so malleable that it may be 

 beaten into sheets so thin that it would require about 300,000 laid upon one 

 another to measure one inch. 



Ductility is the property in virtue of which some solids may be drawn into 

 wire or thin sheets as, for instance, copper, iron, and platinum. 



Liquid state. The characteristic features of liquids are, that they 

 have no self-subsistent figure; that they consequently require some 

 vessel to hold them ; and that they present a horizontal surface. While 

 in a solid substance the smallest parties are held together by cohe- 

 sion to such an extent that they cannot change their relative position 

 without force, in a liquid this cohesion acts with much less energy 

 and permits of a comparatively free motion of the particles; the 

 repellant and attractive forces nearly balance each other in a liquid. 

 That cohesion is not altogether suspended in a liquid is shown by the 

 formation of drops or round globules, which, of course, consist of a 

 large number of smallest particles. If there were no cohesion at all 

 between these particles of a liquid, drops could not be formed. 



The terms semi-solid and semi-liquid substances are used for bodies occupy- 

 ing a position intermediate between true solids and fluids; butter, asphalt, 

 amorphous sulphur, are instances of this kind. 



Gaseous state. Matter in the gaseous state has absolutely no 

 self-subsistent figure. Gases fill any vessel or room entirely ; the 

 smallest particles show the highest degree of mobility and move 

 freely in every direction. Cojiesioji. js entirely suspended in gases ; 

 indeed, the smallest particles exhibit toward each other an infinite 

 repulsion, so that force is necessary to restrain them within any given 

 bounds whatever. It, therefore, follows that gases set up and main- 

 tain a pressure against the walls of vessels enclosing them. This 

 characteristic property, possessed by all gases, is known as elasticity, 

 or, better, as tension, and is so unvarying that a law has been estab- 

 lished in relation to it. This law is known as the Law of Mariotte 

 (though really discovered by Boyle, of England, in 1661), and may 

 be expressed thus: {'The volume of a gas is inversely as the pressure ; 

 the density and elastic force are directly as the pressure and inversely 

 as the volume. J For instance : If a vessel contains one cubic foot of 

 a gas under a pressure of ten pounds, the volume will be reduced to 



