TJie Scottish Naturalist. 



13 



The small specimen presents us with iron in two different con- 

 ditions. (1.) We have the compact peroxide of iron, passing 

 from the amorphous or shapeless mass into kidney-shaped haema- 

 tite, fibrous in texture, an anhydrous ore, destitute of water or 

 analysis. (2.) We have a band of hydrated peroxide of iron, at 

 first supposed to be limonite, (so called from its yellow or lemon 

 colour), but which Prof. Heddle has ascertained to be Gotheite. 



This brings me to a very interesting part of my subject. Where 

 did this iron come from ? Is it a volcanic product ? or has it 

 existed in another form and been forced into this bed of lime- 

 stone ? If we bear in mind that not ten paces distant from its 

 place of origin there is a huge mass of trap rock, of true volcanic 

 nature, which contains iron, we naturally look to it in the search 

 for the origin of this lode of Haematite ; a substance we should 

 judge as a foreign element when found in a bed of sedimentary 

 rock, such as this limestone. 



There are agents in nature, however, quite as powerful to effect 

 changes in the crust of the earth as volcanic eruptions, though 

 more silent in their operations. I refer to air and water, simple 

 and innocuous as they may seem at first sight. 



Air (composed of oxygen and nitrogen gas, in the proportions 

 of, roughly speaking, 1 part oxygen and 4 parts nitrogen, with 

 watery vapour and a trace of carbonic acid gas), and water (com- 

 posed of 8 parts of oxygen to 1 part of hydrogen) derive their 

 powers of effecting important chemical changes from the tendency 

 of their constituent gases to unite with other substances or ele- 

 ments with which they meet, e.g., oxygen and carbon combine to 

 form carbonic acid gas, which is one of the most important agents 

 in nature, and is a product of the decomposition of organic sub- 

 stances, of combustion and of volcanic exhalations. 



Oxygen, as a gas, readily enters into combination with the ele- 

 ments in most minerals \ and amongst these its affinity for iron is 

 very great. What we call rust is simply the product of the com- 

 bination of oxygen with iron. The conditions necessary for this 

 union are moisture and heat. When the moisture of the atmo- 

 sphere comes into contact with iron, the moisture or water is 

 decomposed, and the oxygen of the water combines with the iron ; 

 the product of this chemical union being the oxide of iron or 

 rust. When oxygen and any element (e.g., iron, as we are dealing 

 with it) are united in the same equivalents, a protoxide is formed.. 



