combine into a most stable -system with 711.,*': and tin- double salt 

 IUHV ' >e eonsit lered as this crystallo-hydrate. in which one molecule of 

 watri 1 i.s replaced bv the molecule K ,S< ,. s 



'Ih' /""''./' "/ /"/'///'//'/ /HIS/I- sii/f* IN ;i verv remarkable peculiarity 

 'f magnesia and other leeble bases, and especially of those corre- 

 -pondini;' with polwalent nietal.s. The very powerful bases corre- 

 -pondinu' \vitli unixalent metals like potassium ami sodium do not 

 font i basic salts, and, indeed, are more prune in ^ive acid salts, Avliil^t 

 inau'iiesiitin easilv and frequently forms basic >alts, especially Avith 

 feeble acids, although tliere are oxides as, for example, copper and 

 load oxides which Mill more frequently u'ive lia>ic sails. If a cold 

 solution <f mau'ne-ium sulphate be mixed \\ith a .solution of sodium 

 carbonate there is formed a gelatinous precipitate of a basic salt, 

 .Mir- I l< > i ,. j M u -< '< . ; , ( ,il L< ) : but all the magnesia is not precipitated in 

 thi- ca>e. as a portion n f it remains in solution as an acid double salt. 

 If sodium carbonate be added to a boiling solution of magnesium 

 sulphate there is formed a precipitate of a Mill more h;ic salt, 

 1 M .-si >_, + -! N*n,C< ).. ( + HI J ' = i Xa,S< ) , + ( '< )., + .M -< oil ),.: > ,.M r(."( ). 

 'ij I ,* '. This basu.' salt forms the ordinary medicine, t/>>"//c.^t>t (nt<t</- 

 tt'-xiit </Hi<t). in the toi'm ot li^'ht porous lumps, <)ihei' basic salts are 

 fiirmeil under certain modifications of temperature and conditions of 

 decomposition. l>ut //"- normal >"//. I\I ^C( ).,. \\'hich occurs in nature 

 ;is inau'iiesite in the form of rhoinl )ohedra of s]>. u'r. .')(.'(), cannot bt^ 

 obiained bv sueli a method of pi-ecipitation. In fact the formation of 

 the dillerent basic salt> slio\\sthe power of \vater to decompose the 

 noi'inal -ah. It is possible, however, to obiain this salt by artificial 

 mean-, both in an anhydrous and hvd rated state. .V solution of 

 mau'iiesmm carbonate in water containing carbonic acid is taken for 

 this purpo-e. 'I lie j'eason (it this is readily understood carbonic 

 anhx'di'ide is one (jf the products of the decomposition of magnesium 



