''* CHEMISTRY. 



the salt is heated, causes it to burst with a crackling noise: this is 

 termed decre2ntaiion. 



Atmospheric pressure exerts an influence on the crystallization of 

 salts ; il for mstance, a hot saturated solution of sulphate of soda 

 be corked up m a bottle while the latter is full of vapour, the solution 

 will cool down to the temperature of the air without crystallizing ; 

 but on admiitmg the air, crystallization commonly commences, and 

 the whole becomes solid in a i^vf seconds. 



Crystals are of various forms ; they are divided by crystallo^raphers 

 mto ^-impU and compound. By cleavage is meant that property of 

 a crystal by which it admits of being split in certain directions. 



Bodies are said to be isomorphous when they have the same crys. 

 talline structure, but a different chemical composition ; thus arseniate 

 of soda IS isomorphous with phosphate of soda. On the other hand, 

 the same substance may have, under different circumstances, two 

 crystalhne forms ; in which case it is said to be dimcrriohous : sul- 

 phur and carbon are examples. The instrument employed to ascer- 

 •tain the angles of crystals is termed a goniometer. 



SECTION I. 



OXYSALTS. 



This class of salts includes those, of which both the acid and base 

 are oxides. It will be convenient to classify them according to their 

 different acids. ^ 



SULPHATES. 



Their solutions may always be recognised by yielding a white 

 precipitate, (sulphate of baryta) with a solution of a salt of baryta. 

 Heated m contact with charcoal or hydrogen they are converted into 

 sulphurets, which, if moistened, yield the peculiar odour of sulphu- 

 retted hydrogen. They are almost all insoluble in alcohol. The 

 sulphates of baryta, tin, antimony, lead and bismuth, are quite inso- 

 luble in water. Those of lime, strontia, mercury, silver, and a few 

 others, are nearly insoluble ; while all the other sulphates are so- 

 luble. ^ 



^ The most important sulphates are those of potassa, soda, magne- 

 sia, hme, baryta, iron, copper, zinc, and mercury. 



The most important double sul2^hal€s are the different alums. 

 Common alum is a double sulphate of alumina and potassa. It may 

 be prepared from native alum stone, in which the materials exist 

 ready formed, or from a direct union of its constituents. 

 ^ There are other varieties of alum, in which the sulphate of potassa 

 is replaced by sulphate of soda, or sulphate of ammonia ;— also iron 



