258 DIFFERENCES BETWEEN GOOD AND BAD CLAYS. 



ence of the sun. I am myself, however, unfamiliar with the 

 physical characters of these clays on the large scale, though 

 very many samples have been sent to me from a distance, and 

 have been examined in my laboratory. As to their chemical 

 composition, it has not been made out by analysis in what 

 respect the good clays differ from the bad. This arises from 

 the circumstance, that it is difficult to obtain specimens which 

 can be ascertained, from practical men, to belong to each of the 

 classes, in order that a comparative examination, of such as 

 exhibit opposite effects in practice, might be simultaneously 

 made. 



Clays may differ not only in the proportions of alumina, 

 silica, lime, and alkaline and organic matters, -which they 

 respectively contain, . but also as regards the state of chemical 

 combination in which these substances exist in the clay. When 

 my attention was first drawn to this subject, I was inclined to 

 look upon the proportion of lime in a clay as likely to exercise 

 an important influence upon its comparative value for the pur- 

 pose of burning. On this supposition the Oxford clay, in which 



1 have found from 15 to 30 per cent of carbonate of lime, ought 

 to possess very superior advantages for burning. I am not 

 certain, however, that such is really the case, as clays have 

 been sent to me, which were described to be well adapted for 

 this process of improvement, and to be extensively employed 

 for the purpose, in which the proportion of lime did not exceed 



2 or 3 per cent. It is probable, however, that the relative pro- 

 portions of the several constituents of a clay do affect not only 

 the absolute value of a clay for burning, but also the method 

 by which it may best be burned, and the way in which it acts 

 when it is afterwards laid upon the land. 



The state of chemical combination in which its several con- 

 stituents exist in a clay has, however, quite as much to do 

 with the good effect of burning upon them, and with their 

 more useful action when afterwards spread upon the land. A 

 diversity in this respect may possibly also be a cause of dif- 

 ference in the fitness of clays for the burning process, though, 

 in reference to this point, chemical inquiries have not as yet 

 been made. 



