51.0 Oil the Causes of the Efficacy of Burnt Clay. 



exact quantity of potash which may be extracted from a clay, we 

 possess the means of deciding at once whether a clay is likely to 

 he efficacious or not. The advantages which result from a pre- 

 vious analytical examination become most conspicuous when we 

 consider that the trifling expense for analysis will guard the 

 farmers against failures 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 benefits on those 

 who avail themselves of its aid. 



5. What are the reasons of the failures attending overhurning ? 



Professor Johnston has already shown that, in overhurning the 

 constituents of clay, they are rendered less soluble than when pro- 

 perly burnt. My own experiments fully confirm the Professor's 

 observations. Overburnt clay, I have found further^ does not 

 absorb so much ammonia from the atmosphere as properly burnt 

 clay, which is easily explained by the diminished porosity, and 

 consequently diminished absorptive pov/er of such clays. 



A portion of overburnt clay from Huntstile was exposed to the 

 atmosphere, moistened with water, for two months and thirteen 

 days. The amount of ammonia absorbed by the clay was then 

 determined by combustion with soda-lime. 219 grains of air- 

 dry clay gave 0*155 grains of bichloride of platinum and ammo- 

 nium, or 100 parts furnished only 0*008 per cent, of ammonia. 

 Moderately burnt clay will absorb double the quantity of ammo- 

 nia from the atmosphere which will be absorbed by overburnt 

 clay under precisely the same circumstances. The causes of the 

 failures attending overhurning I am, therefore, inclined to ascribe 

 — 1. To the mechanical changes clay undergoes in burning. 

 2. To the chemical changes which render such clays less soluble. 

 "6. To the diminished power of absorbing ammonia from the 

 atmosphere. 



6. What is the reason that Burnt Clays improve especially 

 Root- crops ? 



All root-crops — such as turnips, carrots, swedes, mangolds, 

 potatoes, (Sec. — require much potash as a necessary article of 

 food. The ashes of these plants contain about half their weight 

 of potash. Mr. Woodward's observation that root-crops are par- 

 ticularly benefited by burnt clay thus receives an easy explanation 

 from the mode of its action, which 1 have endeavoured to explain 

 in the preceding pages. 



In conclusion, I may be allowed to recapitulate briefly the 

 principal and most practical facts which the foregoing investiga- 

 tion has taught me : — ^, 



