[ 18 3 



fully applied, and I have found in them no sensible 

 quantity of gypsum. In general, cultivated soils contain 

 sufficient of this .substance for the use of the grasses - ? 

 in such cases, its application cannot be advantageous. 

 For plants require only a certain quantity of manure ; 

 an excess may be detrimental, and cannot be useful. 



The theory of the operation of alkaline substan- 

 ces, is one of the parts of the chemistry of agriculture, 

 most simple and distinct. They are found in all plants 

 and therefore may be regarded as amongst their es- 

 sential ingredients. From their powers of combina- 

 tion likewise, they may be useful in introducing vari- 

 ous principles into the sap of vegetables, which may 

 be subservient to their nourishment. 



The fixed alkalies which were formerly regarded 

 as elementary bodies, it has been my good fortune to 

 decompose. They consist of pure air, united to high- 

 ly inflammable metallic substances ; but there is no 

 reason to suppose that they are reduced into their 

 elements in any of the processes of vegetation. 



In this part of the course I shall dwell at consi- 

 derable length on the important subject of Lime, and 

 I shall be able to offer some novel views. 



Slacked lime was used by the Romans for man- 

 uring the soil in which fruit trees grew. This we are 

 informed by Pliny. Marie had been employed by the 

 Britons and the Gauls from the earliest times, as a 

 top dressing for land. But the precise period in 

 which burnt lime first came into general use in the cul- 

 tivation of land, is, I believe, unknown. The origin 

 of the application from the early practices is sufficient- 



