tasteless, and has barely any alkaline reaction, yet it is a very strong alkaline base; 20.7 parts of it neutralize as much sulphuric acid as 47 of potash. A salt is not always a crystallizable substance, and _vice versâ_. The progress of our chemical knowledge has therefore demanded a wider extension and application of the term _salt_, and it is not now confined merely to a combination of an acid and an alkali, but is conferred even on compounds consisting only of sulphur and a metal, which are termed _sulphur salts_. So also in combinations of chlorine, iodine, bromine, and fluorine, with metallic bodies, neither of which are acid or alkaline, the term _haloid salts_ has been applied by Berzelius, from the Greek ([Greek: _als_], sea salt, and [Greek: _eidos_] form), because they are analogous in constitution to sea salt; and the mention of sea salt again reminds us of the wide signification of the term salt, originally confined to this substance, but now extended into four great orders, as defined by Turner:-- ORDER I. _The oxy-salts._--This order includes no salt the acid or base of which is not an oxidised body (ex., nitrate of potash). [Page 75] ORDER II. _The hydro-salts._--This order includes no salt the acid or base of which does not contain hydrogen (ex., chloride of ammonium). ORDER III. _The sulphur salts._--This order includes no salt the electro-positive or negative ingredient of which is not a sulphuret (ex., hydrosulphuret of potassium). ORDER IV. _The haloid salts._--This order includes no salt the electro-positive or negative ingredient of which is not haloidal. (Exs., iodide of potassium and sea salt). To fix the idea of salt still better in the youthful mind, it should be remembered that alabaster, of which works of art are constructed, or marble, or lime-stone, or chalk, are all salts, because they consist of an acid and a base. In order to cause a substance to crystallize it is first necessary to endow the particles with freedom of motion. There are many methods of doing this chemically or by the application of heat, but we cannot by any mechanical process of concentration, compression, or division, persuade a substance to crystallize, unless perhaps we except that remarkable change in wrought or fibrous iron into crystalline or brittle iron, by constant vibration, as in the axles of a carriage, or by attaching a piece of fibrous iron to a tilt hammer.