9> 



AGRICULTURAL CHEMISTRY. 41 



ash and soda; first, we find metallic calcium (Ca); 

 next, this, combined with oxygen only, lime (oxide of 

 calcium), (CaO), (whick is quick-lime, as it comes 

 fresh from the kiln) ; we have also carbonate of lime 

 (CaO, CO') — marble, limestone, chalk, and some 

 varieties of marl ; also the shells of insects and fish, 

 are different forms of carbonate of lime, more or less 

 impure. When lime combines with water, (consoli- 

 dates water in itself, so as to be still apparently dry), 

 it is called hydrate of lime. Such is the condition of 

 water-slacked lime. SiiBh also is the condition of 

 many iron ores and other minerals. They consolidate 

 in themselves large amounts of water, and yet are ap- 

 parently dry. Such are called hydrates^ as hydrate 

 of lime, hydrate of iron, and others. From some hy- 

 drates the water is separated by a gentle heat ; from 

 others it cannot be driven off but by a very high 

 heat. 



59. Magnesia (MgO). This is the oxide of mag- 

 nesium. It is known as calcined magnesia. Some 

 impure lime-stones, as those called dollomite in Berk- 

 shire county, Mass., contain large quantities of carbon- 

 ate of magnesia, in some cases not less than 40 per 

 cent. This is often called magnesian lime-stone. If 

 the carbonic acid be driven off by heat, a light, dry, 

 white powder remains. This is calcined magnesia. 



60. Alumina (Al'O''), as its name imports, is a 

 compound of aluminum and oxygen, two atoms of the 

 former to three of the latter. Alumina is a perfectly 

 white powder, and is the basis of all clay soils. Pure 



