EXPERIMENTS WITH BURNED CLAY. 



263 



position ; and their value, when burned, will no doubt depend 

 much upon this composition. Some abound in lime, some are 

 richer in alkaline matter, some in magnesia, and some in alu- 

 mina; and, as I have shown, they vary also in the state of 

 chemical combination in which the constituents exist in them. 

 It may be asked by experiment, however, 



1. Are clays which, like the Oxford clay, abound in lime, 

 always beneficial, or are they good clays for burning ? If so, it 

 might be easy to mix lime or chalk, during the burning, with 

 clays which, like those of the lias and the coal measures, are 

 generally poor in lime. 



2. Are all the clays of the same geological formation usually 

 good or bad for burning ? 



3. Will the admixture of a little salt to a clay, to one rich 

 in lime or otherwise, during the slow burning, render it more 

 valuable ? 



4. Do good clays always yield a sensibly larger quantity of 

 soluble matter than bad clays after they have been burned in a 

 similar manner? Can we establish the proportions of saline 

 matter thus dissolved as a simple 'test of the relative values of 

 different clays? 



5. Do they especially improve root and other green crops, 

 as Mr Woodward states ? These require much saline matter ; 

 and that they should be benefited most by burned clay, is con- 

 sistent with the explanation of its mode of action which I have 

 given in the preceding section. The action of different burned 

 clays on different crops should nevertheless be carefully observed 

 and recorded. For example, 



a With one clay against another similarly burned. Their 

 respective physical characters and chemical composition ought 

 at the same time to be noted. 



b With a bad clay burned alone, and with an addition of 

 lime. 



c With both against a third portion to which a quantity of 

 common salt has been added, and with a fourth which has been 

 burned with an addition of both lime and salt. 



