21 



shown by the above table, we are bound to look to the alkalies as 

 the chief fertilising substances in clays. The analyses of three 

 different kinds of clays, from Gloucestershire, given above, as well 

 as a great many others published by Professor Johnston, exhibit a 

 great difference in the relative proportion of potash and soda which 

 they contain. Whilst some of them contain considerable quantities 

 of potash and soda, others contain but mere traces. Now, if it be 

 true what has been advanced with regard to the fertilising substan- 

 ces in clays, and the effects produced in burning, we cannot hesi- 

 tate to pronounce all clays which contain no potash or soda in an 

 undecomposed form, or mere traces, as entirely unfit for burning. 

 Experience, I think, will prove that such clays, naturally unfertile, 

 will not be improved in the least by burning. On the other hand, 

 those clays which contain undecomposed insoluble silicate of 

 potash and soda, in the shape of fragments of felspar or any 

 other mineral, will be found the more useful after burning, I think, 

 the more of these alkalined silicates they originally contain. 



6. Can it be determined by chemical analysis whether a clay 

 will be efficacious when burnt or not? 



From the preceding remarks it follows that the fertilising 

 effects of clay mainly depend on the proportion of alkalies which it 

 contains ; and as any good analytical chemist may determine the 

 exact quantity of potash and soda which may be extracted from 

 a clay, we possess the means of deciding at once whether a clay 

 is likely to be efficacious when burnt or not. The advantages 

 which result from a previous analytical examination become most 

 conspicuous when we consider that the trifling expense for 

 analysis will guard the farmers against failure and loss attending 

 the investment of much money and labour in burning soils, which 

 cannot be rendered more fertile by this operation. Chemistry, 

 in this manner, I have no doubt, will be found to confer material 

 practical benefits to those who avail themselves of its aid. It 

 cannot be expected that every farmer should himself be a good 

 chemist, were it desirable or necessary ; but we may justly demand 

 of him that he should properly appreciate the labours of those 

 engaged in chemico-agricultural researches. Without a know- 

 ledge of the first principles of the science, however, the practical 

 man will never be able to appreciate properly the aid which 

 chemistry is capable of conferring on him, nor will he fully under- 

 stand the direct bearing which chemistry exercises on many 

 practical operations. We would therefore * recommend the study 

 of the principles of the science as the foundation of true agricul- 

 tural progress. 



7. What are the causes of the failure attending over-burning? 

 When clay is burnt too strong it becomes hard like stone, loses 



much in porosity, and does not crumble to powder on exposure to 

 the air. To these mechanical changes, no doubt, the failure of 



