506 On the Causes of the Efficacy of Burnt Clay. 



its mechanical condition. This has been done with a sample of 

 plastic clay from Coldash Common, near Newbury, which I ov>e 

 to the kindness of Mr. Robert Brown, of Cirencester. By 

 washing carefully clay with water, and collecting the deposit 

 which subsides from the muddy liquid at different intervals, a 

 tolerably s,'0od idea of the state of division of the clav can be 

 formed :— - 



Analysis of Plastic Clay from Coldash Common, near Neichury, 



Berkshire. 



1. IMeCHAJTICAL ExAMINATIOiN". 



100 parts of this clay contain — 

 Fine quartz sand, deposited after five minutes . . . 54*64 

 Clay, with a little very fine sand, deposited after ten minutes. 10-52 

 Fine clay, deposited after fifteen minutes . . . .2-57 

 Finest clay, remaining suspended in water after the lapse of 



a quarter of an hour ....... 32*27 



100-00 



2. Chemical ExAMi3srAT]0>". 

 a. General Analysis. 



"Water of combination, and a little organic matter . . 8*698 



Sand 54*640 



Pure clay ......... 25*462 



Oxides of iron and alumina, lim^e and other substances, 



soluble in acids 11*200 



100*000 



;b. Detailed Analysis. 

 "\Yater of combination, with a little organic matter . . 8*698 



Silica 73*736 



Soluble oxides of iron and alumina . . . 5*031 



Insoluble 7-107 r- 



Lime . . .2-980 



Magnesia 0-047 



Potash 2*077 



Soda 0-525 



Chlorine a trace 



100-201 



Not only the mechanical state of division, and the relative pro- 

 portions of the foreign admixtures which are found in agricultural 

 clays, influence their characters, but also the state of combination 

 in which the constituents of such clays occur, materially alters 

 their properties. In the subjoined analyses of three samples of 

 clays from Dumbelton, Gloucestershire, made , in my laboratory, 

 care has been taken to ascertain the state of combination in which 

 the constituents of these clays are likely to occur : — 



