ITS MATERIALS— THE LONDON CLAY, 25 



may be seen whenever a deep excavation or well-sinking pene- 

 trates to a considerable depth below the surface. 



The change of colour produced by the complete oxidation 

 of disseminated ferruginous matter is conspicuously seen when 

 bricks or tiles made of blue clay are burnt, red or reddish bricks 

 or tiles being the result. The iron matter in the unaffected 

 deep clay is, however, an oxide, but only a first oxide, that 

 is, it is a compound of iron with less oxygen than it is capable 

 of combining with, and is hence called a protoxide, but the 

 compound that gives the brown colour is one in which there is 

 a larger proportion of oxygen and it is hence commonly called a 

 per-oxide. It has, moreover, in addition to the oxygen, a certain 

 amount of water combined with it, and so is called a hydrated 

 per-oxide. But when blue-clay bricks are burnt, the iron is 

 not only thoroughly oxidized, but is rendered anhydrous or devoid 

 of water, and this it is that produces the bright red, not brown, 

 colour which is the delight of modern architects and now so 

 much in favour, and perhaps nowhere more so than at Hampstead, 

 where in Fitzjohn's Avenue and in the new buildings of the 

 High Street we may always see a conspicuous illustration of 

 this interesting and beautiful chemical phenomenon. 



The substance of the clay, as indeed of all clays, is a 

 compound of a very different character. It is none other than 

 a combination of the substance of sand, or silica, or quartz, the 

 same as rock crystal or ** pebbles," with an oxide of that very 

 light, silvery metal we know as aluminium. Thus it is that the 

 basis of the clay is alumina. Now, although aluminium is a 

 light, white metal, alumina is a mineral of the hardest and 

 most intractable character, the next hardest, indeed, to the 

 diamond. In its commoner form it is called corrundum, and 

 its powder emery, but, in its finest and brightest form, in its 

 apotheosis so to speak, it is none other than the ruby and the 



