274 EARTHS. 



The carbonate is most commonly used, but the pure earth, sold un- 

 der the name of calcined magnesia, is sometimes preferred, because 

 no gas is extricated from it in the stomach. Magnesia sometimes 

 forms large and dangerous accumulations in the bowels, of several 

 pounds weight, particularly when its use has been long persevered 

 in, and the earth has not been duly evacuated, by acids, forming 

 with it saline combinations. It sometimes enters into the clays, and 

 other materials which go to form porcelain, in the fabrication of 

 which, on account of its infusibility, it serves a valuable purpose. It 

 is one of the four earths which form a large part of the crust of this 

 planet. Soapstone owes its peculiar properties to magnesia, particu- 

 larly its infusibility : magnesian stones, such as soapstone and talc, 

 are much employed, not only to resist fire, but because they are so 

 easily wrought by tools into any desired form.* They are used in 

 building- 



MAGNESIUM. 



1 . Obtained in the same way as the other metals of the earths. 



2. A white and brilliant solid ; (a little mercury still remaining in 

 combination with it.) 



3. Sinks rapidly in water, although surrounded by bubbles of gas. 



4. Both in air and water reproduces magnesia ; in air gains 

 weight, as the balance proves, both with respect to this and other 

 earths. 



5. POLARITY. Magnesium goes to the negative pole, and is there- 

 fore electro-positive. 



6. The combining weight is estimated by Dr. Thomson at 12, and 

 this, with 1 proportion of oxygen, forms magnesia, which is the only 

 known oxide of magnesium, whose equivalent is of course, 26. 

 There can be no doubt that magnesia is a metallic oxide. Hitherto 

 chemists have been unable to make it absorb more oxygen. 



SEC. V. SILICA. 



1. NAME. Sttev is the Latin for flinty which is composed of this 

 earth, nearly pure ; limpid rock crystal is almost pure silica, and sev- 

 eral other siliceous minerals, as chalcedony, carnelian, opal,, agate, 

 &c. consist principally of this earth. The purest white sand contains 

 little else : in the form of quartz it constitutes mountain masses, and 

 ki that of sandstone vast strata. 



* Savage nations are acquainted with these uses : many of their containing ves- 

 sels, especially vessels for cookery, are made of these minerals. After the abori 4 - 

 gines of this country became acquainted with the Europeans, they made bullet 

 moulds of soapstone ; they were ingeniously arranged in halves, with a regular mouth , 

 and were tied together by withes ; I have such a specimen. Soap stone is also used 

 fte diminish friction in machinery. Am* Jour. Vol. XIV, p. 376. 



