404 On the Reaction of Sulphate of Magnesia, S^c. [Dec. 



sition. Presuming upon this property of the, ^>y,o,^ajtp^/a| phy- 

 sician prescribed several years ago to M. de Sojnra^riy^^ apnjj^- 

 ture of powdered sulphate of magnesia and saturated caijbonal^ 

 of soda. He gave alternately either this mixture alone or bicar- 

 bonate of soda. Being charged with the preparation of both 

 these medicines for a journey of three months, which M. de 

 Sommariva made annually to Italy, I always had the precaution 

 of placing the mixture in a very dry state, and divided into par- 

 cels in tin canisters, to preserve it from humidity. I used the 

 same precaution with regard to the carbonate of s6dai->il 

 observed that the sulphate of magnesia was free of %iBh^OGklo- 

 rate. ' '^ >^^> ?.-A^o'^T 



During three years M. de S. a man very careful of his health, 

 and besides a good observer, never perceived that cold water 

 became turbid when he dissolved the two salts together in it; 

 but in 1822, having been obliged to prolong his journey beyond 

 the usual time, he laid up a store for a year. Tow;ar4 ,the end 

 of the fifth month, M. de S. remarked that the same water 

 which he ordinarily used became slightly milky, and that the 

 change which he rightly attributed, though without being able 

 to explain the cause, to the alteration of the powder, went on 

 increasing as the time advanced. At length, by the seventh 

 montli, the precipitate which formed in the water became so 

 considerable, that M. de S. deemed it proper to intermit the iise 

 of the powder, and sent for some more, promising to inform nie, 

 on his return from Italy, of what, according to his expression, 

 had happened. M. de S. returned at the end of six months, 

 and sent me back the powder in question, which I submitted to 

 the following experiments. 



1. This powder put into a quantity of cold water, double that 

 which is necessary for dissolving the two salts, rendered it 

 milky. 



2. Dissolved in a large quantity of water it deposited a white 

 powder, which, on being washed several times and dried, was 

 found to be subcarbonate of magnesia. 



The liquor in which this deposit was formed was limpid after 

 being filtered, and was not rendered turbid, either cold or hot, 

 by the soluble alkaline subcarbonates. All the acids stronger 

 than the carbonic disengaged this latter from it. Lastly, when 

 suitably evaporated, sulphate and carbonate of soda were 

 obtained, part of the latter of which was in the form of subcar- 

 bonate. To explain here the presence of the carbonate of soda 

 it requires to be known that the quantity of bicarbonate mixed 

 with the sulphate of magnesia, was more than sufficient to 

 decompose this latter salt. 



There results from this observation that the sufficiently pro- 

 longed contact of sulphate of magnesia and bicarbonate of soda 



