FUNDAMENTAL PROPERTIES OF MATTER. 21 



themselves around certain points as nuclei according to the directions of great- 

 est cohesion, and thus crystals with regular faces and angles and definite 

 internal arrangement are built up. The size of the crystals obtained will 

 depend on several factors, but whatever the size may be, the angles between 

 the faces and the position of the faces will be the same for every individual 

 substance. Hence the shape of crystals is a valuable means of identifying 

 substances. If a concentrated hot solution of a substance be cooled quickly, 

 and especially if the liquid be disturbed, as by stirring, the crystals will be 

 small, sometimes almost microscopic in size. But this is often an advantage, 

 because large crystals are apt to enclose some of the liquid containing the 

 impurities between the layers. On the large scale, as in industries, enormous 

 crystals are obtained by the slow cooling of a great volume of solution, for 

 example, in the case of alum, potassium dichromate, etc. When a substance 

 is not much more soluble in a hot than in a cold liquid, for example, common 

 salt in water, the liquid must be removed by allowing it to evaporate, either at 

 ordinary or at elevated temperature, to obtain a good yield of the substance 

 in crystal form. 



Sometimes sticks, strings, wires, strips of lead, etc., are suspended in the 

 solutions, to offer starting-points for the formation of crystals and a ready 

 means for removing the crystals from the liquor. A familiar example is the 

 string in the center of a stick of rock-candy. 



A relatively few substances when heated pass from the solid to the gaseous 

 state, without undergoing intermediate liquefaction. When the vapor of such 

 substances comes in contact with cool surfaces, it is deposited in crystals which 

 sometimes attain to remarkable size and beauty. This process is known as 

 sublimation and is used in the case of several medicinal agents on the market, 

 for example, iodine, benzoic acid, ammonium chloride, etc. The words iodine 

 resublimed, found on labels, and the popular name for mercuric chloride, namely, 

 corrosive sublimate, refer to the process of sublimation employed in obtaining 

 these substances. 



If two or more (non-isomorphous) substances for instance, common salt 

 and Glauber's salt be dissolved together in water, and the solution be allowed 

 to crystallize, the attraction of like particles for one another will be readily 

 noticed by the formation of distinct crystals of common salt alongside of 

 crystals of Glauber's salt ; neither do the particles of common salt help to 

 build up a crystal of Glauber's salt, nor the particles of the latter a crystal of 

 common salt. Advantage is taken of this property in separating (by crystal- 

 lization) solids from each other, when they are contained in the same solution. 



Not all matter can form crystals ; some substances never have been 

 obtained in a crystallized state, such as starch, gum, glue, etc. A 

 solid substance showing no crystalline structure whatever is called 

 amorphous. 



Some substances capable of crystallization may be obtained also in 

 an amorphous state (carbon, sulphur). Other substances are capable 

 of assuming different crystalline shapes under different conditions. 

 Thus sulphur, when liquefied by heat, assumes, on cooling, a shape 

 different from the sulphur crystallized from a solution. One and the 



