CAUSES OF DECAY 389 



mon as not to need references. The proto-slllcates of iron, 

 also, are affected by oxidation and result in persllicates. In 

 general, the iron compounds may be said to rust through 

 the oxidizing agency of the atmosphere. Inasmuch as in 

 our buildings the stone is fully exposed to the action of the 

 rain-water, the effect of oxidation Is at a maximum, and 

 greater than in the ledges and out-crops, and also greater 

 than In the quarry.* 



The deoxidatlon Is exceptional and is rarely seen In walls 

 above ground. It may go on in foundations and In sub- 

 marine structures. 



Hydration occurs in the case of minerals which are 

 anhydrous, and they absorb water from the air. Brown 

 hematite is an example of a hydrated iron-oxide. The 

 production of this oxide of Iron, or ochre, is common In 

 the weathering of rocks. And Its occurrence in building 

 stone may be due in many instances, to the oxidation and 

 hydration of pyrlte, magnetite or other iron minerals. 

 Owing to the degree of change, there are stages In the 

 progress to complete hydration, and several compounds — 

 between the ferrous oxide and the hydrated ferric oxide. 



The action of these several agencies upon building stone 

 is not generally limited to a single one, nor separate from 

 those which act mechanically. The wind and rain, frost 

 and heat ; the dissolving, oxidizing and hydrating action of 

 the atmospheric air, are so Interlocked that it is Impossible 

 to determine and define the effect of each one. They com- 

 bine in the work of decomposition and disintegration, and 

 tend to level in the dust the structures which man raises 

 and which he fondly hopes can resist them. But change, 

 that law of our material world, Is stamped on what he 

 builds. 



'* For the stateliest building man can raise Is the ivy's 

 food at last." 



*Bischoff, Chem. and Phys. GeoL, Vol. iii, p. 60. 



