186 §=6©. T. S. Hunt on the Chemisiry of Natural Waters. 
nesia predominates. This salt also effloresces abundantly ina 
nearly pure form upon certain Jimestones, and is in some cases — 
due to the action of sulphates from decomposing pyrites po — 
magnesian carbonate or silicate. In by far the greater number — 
of cases, however, its appearance is unconnected with any such — 
process; and is, according to Mitscherlich, due to a reaction be — 
tween dolomite and dissolved gypsum. + 
§ 20. In support of this view, it was found by the chemist — 
just named that when a solution of sulphate of lime was male — 
to filter for some time through pulverized magnesian limestone 
it was decomposed with the formation of carbonate of lime and 2 
sulphate of magnesia. This reaction I have been unable tover | 
ify. A solution of gypsum in distilled water was made toper | 
colate slowly through a column of several inches of finely 
powered dolomite; and after ten filtrations, occupying as many 
ays, no perceptible amount of sulphate of magnesia had bet | 
formed. Solutions of gypsum were then digested for maly — 
months with pulverized dolomite, and also with crystalline — 
bonate of magnesia, but with similar negative results; nor 
the substitution of a solution of chlorid of calcium lead tole | 
formation of any soluble magnesian salt. Solutions of gypsi 
were then impregnated with carbonic acid, and allowed api 
in contact with pulverized dolomite and with magnesite 8™ — 
fore, during six months of the warm season, when only If — 
preciable traces of magnesia were taken into solution. 
experiments show that no decomposition of dissolved gyps™™ 
effected by native carbonate of magnesia or by the double ak 
bonate of lime and magnesia at ordinary temperatures. a 
21. I find however that hydrated carbonate of magne! 
readily completely decomposes a solution of gypsum when age 
tated with it, with formation of carbonate of Jime and sulphi® 
of magnesia, and the same result is produced with the native 
hydrate of magnesia when mingled with a solution + ie 
rbonic acid), ad ae 
Suffice to explain the results obtained by Mitscherlich, @ fie 
phate of magnesia eee” 
In the experiments above beso Nig 
y pure crystalline dolomites from the Guelph an 
formations were made use of. poratio?s 
_ 3 44. When sea-water is exposed to spontaneous hi te, ey? 
the lime which it contains separates in the form of sulphat and 
sum being but sparingly soluble in a concentrated. DHBA T” 
