348 On the aqneous solution of Carbonate of Magnesia. 



who have examined this compound ; the variation or want of 

 perfect accordance, probably chiefly depends on the degree 

 of dryness of the preparation examined, or on the quantity 

 of water retained in the powder admitting of expulsion at 

 212, which water being hygrometrical, at least in part, must 

 vary with the degree of dryness of the atmosphere to which 

 it is exposed. 



The method by which these two compounds of magnesia 

 and carbonic acid were analysed was a simple one, admitting 

 of considerable accuracy. 



The quantity of water expelled at a temperature of 212 

 was determined by exposure of an hour or more to the heat 

 of a steam-bath ; the quantity of carbonic acid, by acting on 

 the compounds, very carefully weighed, by muriatic acid, 

 saturated with carbonic acid, over mercury in a graduated 

 tube; and the quantities of magnesia and of water expelled at 

 a higher temperature than 212 by the action of a red heat, 

 continued for two or three hours, till no further loss of weight 

 was produced by a continuance of the high temperature. In 

 estimating the proportion of carbonic acid, the calculation was 

 made on the ground that 1 00 cubic inches of this gas weigh 

 47'262 grains. 



A few words relative to the properties of the first -men- 

 tioned carbonate. Its tendency to crystallize is remarkable : 

 however obtained, even when rapidly separated by the expul- 

 sion of the excess of carbonic acid by heat, it has been de- 

 posited in a crystalline form. This form is not obvious to the 

 naked eye ; but when the powder is examined by the micro- 

 scope, each particle is found to be a distinct prismatic crystal. 

 And the persistence of this form is no less remarkable ; it is 

 not destroyed by decomposition ; the powder after ignition, 

 after the expulsion of the whole of the water and carbonic 

 acid, under the microscope shows no alteration ; each particle 

 is still prismatic, and when moistened with water is trans- 

 parent. 



It is asserted that this carbonate readily loses the water 

 with which it is combined. In a dry atmosphere it loses a 

 portion of the water, which perhaps may be considered as 

 hygrometrical, and at the same time loses its transparency ; 

 but I find, as has been already remarked, that a temperature 

 of 212 expels only one portion, and that a high temperature 

 is requisite to expel the three remaining proportions, and 

 which are probably the strictly chemically combined water. 



It is also said that this compound is altered by the action 

 of cold water, and by that of boiling water ; that in one 



