26 CHEMICAL PHYSICS. 



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. Steel 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 particles 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. Any quantity of gas in a closed vessel will 

 fill it completely ; the smallest particles show the highest degree of 

 mobility and move freely in every direction. Cohesion is entirely sus- 

 pended in gases ; indeed, there is no attraction between the particles, 

 but they are in rapid motion and tend to spread out in all directions ; 

 hence must be retained in a closed vessel. The motion of the par- 

 ticles causes bombardment on the sides of the vessel, and thus pro- 

 duces pressure. This characteristic property, possessed by all gases, 

 is known as elasticity, or, better, as tension, and is so unvarying that 

 a law 1 has been established in relation to it. This law is known 

 as the Law of Boyle, who discovered it in 1661 ; sometimes it is 

 referred to as the Law of Mariotte. It 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. 



1 In science, a law or generalization is a brief statement which describes some constant mode 

 of behavior, or sums up the constant features of a set of phenomena of a like kind. 



