Carbonate of Magnesia with excess of Carbonic Acid. 347 



posure to the air in an open vessel, exposure to a tempera- 

 ture of 100 Fahrenheit, in a vessel loosely corked, and to the 

 action of the air-pump under an exhausted receiver. 



The result in each instance was very similar; carbonic 

 acid gas escaped, or was expelled, and a salt was deposited 

 in the form of minute prismatic crystals. 



This separation of the magnesia in a solid form, on the 

 disengagement of the excess of carbonic acid, was no more 

 than might have been expected from the known nature of 

 the compound, and the artificial manner in which it is form- 

 ed by the condensation of the gaseous acid ; and must be 

 considered as quite incompatible with the declaration of its 

 " fluid state " in the stomach and bowels, and sufficient 

 ground to call in question the propriety of placing confidence 

 in the preparation as a medicine, in preference to common 

 carbonate of magnesia or calcined magnesia, than either of 

 which it is so much more costly an article. 



The prismatic salt deposited on the escape of the excess of 

 carbonic acid, has been examined by several chemists ; rest- 

 ing chiefly on the results of the experiments of Berzelius, 

 and the late Dr. Henry, it has been considered as a hydrated 

 carbonate of magnesia, composed of one proportion of mag- 

 nesia, one of carbonic acid, and three of water. 



From the experiments which I have made on it 5 it appears 

 to be composed as follows ; viz. 



29-61 Magnesia. 



32*22 Carbonic acid. 



10-27 Water expelled at 212 Fahrenheit. 



27*90 Water expelled by a higher temperature, as 



by ignition. 



100-00 



or of one proportion and half of magnesia, and carbonic acid, 

 one of water expelled at 212, and three proportions of 

 water expelled by a higher temperature. Compared with the 

 common carbonate of magnesia, from the results which I have 

 obtained operating on the latter, this appears to differ chiefly 

 from the former in possessing half a proportion more of mag- 

 nesia, and one proportion less of water, being composed of 



4-1-52 Magnesia. 



33-31 Carbonic acid. 



17-47 Water expelled at 212. 

 7*70 Water expelled at a higher temperature. 



100-00 

 These results accord tolerably with those of other inquirers 



