SODA, LIME AND MAGNESIA. 17 



Soda. We do not often see this substance by itself, 

 but almost always in combination with other bodies. 

 a. Some of the more common of these are carbonate of 

 soda, that is, the common washing soda of the shops; 

 and chloride of sodium, that is, common salt. Both 

 of these compounds contain a large proportion of soda. 

 6. It is wmite, and when pure has the same attraction 

 for water, the same caustic and burning taste, as pot- 

 ash; in fact the two are much alike in many of their 

 properties, and also in the purposes which they seem 

 to serve in plants. 



Lime is a very common substance, and is well 

 known in all its usual forms, a. As quick or caustic 

 lime, it is of a white color, having a strong burning 

 taste, and powerful caustic properties. It absorbs large 

 quantities of water, and at the same time becomes hot, 

 falling into a fine powder. Fresh burned lime, when 

 exposed to the air, does not remain long in this caustic 

 state, but drinks in moisture and crumbles gradually 

 away. b. In nature it is always found combined with 

 some other body, as, for instance, the common lime- 

 stone (carbonate of lime), or the sulphate of lime 

 (gypsum, or plaster of Paris), which are both most 

 abundant rocks. Common limestone or marble, when 

 burned, becomes quickl ime. The phenomena of slaking 

 quicklime are easily shown and explained. Every ton 

 of quicklime, during slaking, absorbs one-fourth of a 

 ton of water, which becomes a part of the stone itself. 



Magnesia is not so well known as lime, although it 

 is abundant on the earth's surface and in many rocks. 



a. The most common and easily obtained form is the 

 calcined magnesia of the shops. This is a light, 

 white, tasteless substance, familiar to all who use much 

 medicine. Epsom salts, so much in vogue as a me- 

 dical prescription, is another compound of magnesia. 



b, When burned, magnesia has something of the caustir* 



2* 



