394 CARBONATES, 



(e.) Decomposed by all the acids, with effervescence. 



(f.) Also by all the alkaline bases, forming carbonates ; and in 

 turn decomposes their salts, by do'uble attraction. 



(g.) Lime water and carbonate of magnesia produce a mixed pre- 

 cipitate of the two carbonates. 



4. COMPOSITION. According to Bucholz, if precipitated with 

 heat, it contains magnesia 42, carbonic acid 35, water 23; if with- 

 out, magnesia 33, carbonic acid 32, water 35. Mr. Dalton states it 

 at magnesia 43, carbonic acid 40, water 17. Berzelius thinks that it 

 is a compound of carbonate of magnesia, 3 proportions, with one of 

 what is called quadro-hydrate of the same earth. Dr. Henry in- 

 clines to the same opinion, and states the composition thus ; 



3 equivalents of carbonate, 42x3 =126 69.2 



1 do. quadro-hydrate, 20+36= 56 30.8 



Its equivalent, 182 100.0 

 Or of magnesia in the carbonate, - - 32.93 ) . Qc > 



Do. in the hydrate, - - -11.00$- 



Carbonic acid, - - 36.32 



Water, - - - 19.75 



100.00 



Berzelius found the common magnesia of the shops, after being thor- 

 oughly washed in boiling hot water, to be composed of magnesia 

 44.58, carbonic acid 35.70, water 19.72 = 100.00. 



CRYSTALLIZED CARBONATE OF MAGNESIA. 



\ 



1. PROCESS. By diffusing magnesia in water, and passing a cur- 

 rent of carbonic acid gas through it to saturation. 



2. PROPERTIES. 



(a.) Much more soluble than the carbonate, requiring only 48 

 parts of cold water, and water impregnated with carbonic acid takes 

 up 13 grains to the ounce. 



(b.) When the solution is heated, although transparent before, it 

 becomes turbid, and again resumes its transparency on becoming cold.f 



(c.) It crystallizes, in transparent hexagonal prisms, terminated by 

 a hexagonal plane ; partly in groups and partly solitary ; length about 

 6 lines, and breadth 2. 



(d.) Effloresces in the air, and decrepitates in the fire ; it loses 

 about .75 of its weight, while the common carbonate loses only .50. 



* Thomson's First Principles, Vol. II, p. 303, and Henry, 10th ed. Vol. I, p. 617. 



t The heat must be discontinued just at the point where the solution becomes tur- 

 bid, or the carbonic acid will be driven off. The reason of this turbidness is sup- 

 posed to be the elasticity of the gas, tending to escape, and thereby beginning to 

 let go its hold on tho magnesia. Dr. Hope, Note Book. 



