ALTTMINE.

61

exposed to damp air, they soon become moist, and atlast we find, instead of the solid salt, a very strongsolution of it in water. It is to the presence of a smallquantity of these two salts in common sea-salt that itsdeliquescent properties before alluded to are due. (132.)

179. The phosphate of magnesia, like the carbonate,is seldom found alone, but usually mixed with thephosphate of lime: thus we find it associated with thatsubstance in small quantity in many animal and vege-table matters, and likewise in the soil. The uses ofmagnesia are limited, and the most important purposesto which its compounds are applied are as medicines.

180. Alumine, or pure clay, the last of the earthybases, is a very abundant and widely-diffused substance.It occurs native both pure and in combination, but it ismost usually met with mixed with another earth calledsilex, and combined with a quantity of water. It issometimes found pure and free from water or any acid,and constitutes the hard gems called the ruby andsapphire ; but these are very rare, and seldom met with.

181. The properties of alumine, in the ordinary statein which it occurs in the soil, are very different fromthose of the preceding bases; it is quite insoluble inwater, has no caustic powers, and does not absorb car-bonic acid from the air. It has a strong attraction forwater, and when thoroughly wet it appears in the formof a very tenacious paste, remarkable for its greatplasticity, and the ease with which it may be mouldedinto any form. All the varieties of clay derive their