SCIENCE OF AGRICULTURE. 131 



protoxide consists of iron 3^ parts, oxygen 1 part 

 by weight. The peroxide contains a larger portion 

 of oxygen. The protoxide is black, the peroxide is 

 red. By exposure to the oxygen of the atmosphere, 

 especially when heated, the protoxide imbibes the 

 larger dose of oxygen, and becomes a peroxide. 

 Sulphuret of iron is a compound of iron and sulphur. 

 It is of a golden metallic color, and where it exists 

 in massive rocks, often fills the imagination of the 

 discoverer with dreams of wealth, never to be real- 

 ized. By exposure to the oxygen of the atmosphere, 

 the sulphur undergoes a slow combustion and is 

 changed to sulphuric acid, which unites with the 

 iron and forms sulphate of iron, green vitriol, or 

 copperas. Understanding this, the following remarks 

 of Dr. Dana will be understood. 



" If we attempt to account for the action of clay, 

 independent of its amending a sandy soil, we should 

 bear in mind that our common clays contain more or 

 less sulphuret of iron. The conversion of this into 

 the persulphate of iron is the natural consequence 

 of exposure ; free sulphuric acid results, which acts 

 on any lime in the soil, forming sulphate of lime — 

 plaster. So, by spreading clay we spread plaster of 

 Paris. The iron in clay also plays its part thus : 

 The protoxide of iron, unchanged, would not be 

 beneficial in agriculture. It does not act on geine. 

 By exposure, the protoxide becomes peroxide ; and 

 then begins an action similar to that of lime. If the 

 free sulphuric acid, produced as supposed, finds not 

 lime enough, it will decompose all earthy geates 

 and thus a fresh portion of nutriment be set at lib- 

 erty. Both the effects of clay, the production of 

 plaster and the formation of the peroxide of iron, 

 are speedily produced by burning the clay, as is of- 

 ten practised. Some facts have lately come under 

 my eye, and have recalled others to mind, which I 

 have followed up experimentally, all tending to show 

 that if iron peroxidates itself in contact with vegeta- 

 ble fibre, the texture of the fibre is weakened and 



