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ground into impalpable powder, it then becomes 
a substance like clay : if the powder be heated 
very strongly it fuses, and on cooling forms a 
coherent mass similar to the original stone ; the 
parts separated by mechanical division adhere again 
in consequence of chemical attraction. If the pow- 
der is heated less strongly, the particles only super- 
ficially combine with each other, and form a gritty 
mass, which, when broken into pieces, has the 
characters of sand. 
If the power of the powdered feldspar to absorb 
water from the atmosphere before, and after the 
application 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 
subject of experiment. 
I found that two equal portions of basalt ground 
into impalpable powder, of which one had been 
strongly 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. 
In the manufacture of bricks the general prin- 
ciple is well illustrated ; if a piece of dry brick 
earth be applied to the tongue it will adhere to it 
very strongly in consequence of its power to 
absorb water ; but after it has been burnt there 
will be scarcely a sensible adhesion. 
