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stone. Feldspar, for instance, contains siliceous, al- 

 uminous, calcareous earths, fixed alkali, and oxide of 

 iron, which exist in one compound, in consequence of 

 their chemical attractions for each other. Let this 

 stone be ground into impalpable powder, it then be- 

 comes a substance like clay: if the powder be heated 

 very strongly it fuses, and on cooling forms a coher- 

 ent mass similar to the original stone; the parts separ- 

 ated by mechanical division adhere again in conse- 

 quence of chemical attraction, If the powder is heat- 

 ed less strongly the particles only superficially com* 

 bine with each other, and form a gritty mass, which, 

 xvhen broken in to pieces, has the characters of sand. 



If the power of the powdered feldspar to absorb 

 water from the atmosphere before, and after the ap- 

 plication of the heat, be compared, it is found much 

 less in the last case. 



The same effect takes place when the powder of 

 other siliceous or aluminous stones is made the sub- 

 ject of experiment. 



I found that two equal portions of basalt ground 

 into impalpable powder, of which one had been strong- 

 ly ignited, and the other exposed only to a temperature 

 equal to that of boiling water, gained very different 

 weights in the same time when exposed to air. In 

 four hours the one had gained only two grains, whilst 

 the other had gained seven grains. 



When clay or tenacious soils are burnt, the effect 

 is of the same kind; they are brought nearer to a state 

 analogous to that of sands. 



