242 CHEMISTRY. 



becoming ferric hydrate. The second oxide, called sesqui- 

 oxide, forms one of the abundant ores of iron. It is some- 

 times crystallized, as in iron-glance; or compact, as in red 

 iron-stone ; or radiated, as in red hematite ; or earthy, as in 

 red ochre. When mixed with clay it is the clay iron-stone. 

 It is that which gives the red color to so many stones and 

 bodies of earth. The red chalk, so called, used in making: 

 red pencils, is one form of this oxide. This sesquioxide 

 may be prepared artificially by heating ferrous sulphate to 

 redness, or by igniting ferric hydrate, obtained by precip- 

 itating a ferric solution with an alkaline hydrate. 



Ferric hydrate Fe 2 O 3 .3H 2 O also occurs in nature; in 

 large masses it is the brown iron ore, or limonite, from 

 which the metal can be profitably obtained. Mixed with 

 clay it forms the yellow day iron-stone, yellow ochre, etc. 

 The yellow or brown color of soils and of stones which 

 have been long exposed to the air is owing to ferric hy- 

 drate. The ochrey deposit which is seen always about the 

 edges of chalybeate springs is ferric hydrate, made in this 

 case chiefly from the carbonate, the carbonic acid passing 

 off and leaving the oxide to become a hydrated sesquioxide. 

 Observe the difference in color between the hydrated ses- 

 quioxide and that which is not hydrated ; the former is yel- 

 low, the latter red. The reason that bricks become red by 

 burning is that the water is expelled from the iron rust 

 which is in the clay, and it therefore becomes anhydrous. 

 This term, which is much used in chemistry, means dry, or 

 without water, the prefix an meaning without. Iron rust 

 is the ferric hydrate, 2Fe 2 O 3 .3H 2 O. This water is a part of 

 the dry solid, being combined intimately with its ingredi- 

 ents. It is really, therefore, water solidified without freez- 

 ing. In every hundred grammes of it there are about 

 fourteen and a half grammes of water, and nearly forty 

 grammes of oxygen. Both are condensed in uniting with 



