EXTENSION OR FIGURE. 19 



capable of assuming two or more different forms of crystals are 

 said to be polymorphous. When substances of different kinds 

 crystallize in exactly the same form we call them isomorphous 

 (sulphate of magnesia and sulphate of zinc). 



Liquid state. The characteristic features of liquids are, that 

 they have no self-subsistent figure ; that they consequently re- 

 quire some vessel to hold them; and that they present a hori- 

 zontal surface. Whilst in a solid substance the smallest particles 

 are held together by cohesion 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 compara- 

 tively free motion of the particles. That cohesion is not alto- 

 gether 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. 



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. Cohesion is entirely suspended 

 in gases. The most characteristic feature of gases is their great 

 elasticity or tension. Elasticity is that property of matter, by 

 virtue of which it requires force to change its bulk or shape, 

 and a continued application of that force to maintain the change, 

 as it returns to its former bulk or shape when the force is 

 removed. In gases this elasticity is so complete, that a law has 

 been established 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. For in- 

 stance: If a vessel contains one cubic foot of a 'gas under a 

 pressure of ten pounds, the volume will be reduced to one-half, 

 one-tenth, or one-hundredth of one cubic foot, if the pressure be 

 increased to 20, 100, or 1000 pounds respectively. On the con- 

 trary, the gas will expand to 2, 10, or 100 cubic feet, if the 

 pressure is reduced to 5, 1, or one-tenth pound respectively. 

 Vapors, produced by evaporation of liquids or solids, have the 

 same properties as gases. 



