14 CHYMISTRY APPLIED TO AGRICULTURE. 



sphere exercises over vegetation and over the principal 

 qperations which are carried on in rural establishments, 

 such as fermentations, the preparation of various produc- 

 tions, and the decomposition of some substances, in order 

 to apply them to particular purposes ; we are astonished 

 at finding nowhere any of the simple and unexpensive in- 

 struments which announce its changes every moment. 



I do not propose that delicate or complicated instru- 

 ments should be provided ; but I wish to find on every 

 farm an hygrometer, to ascertain the humidity of the at- 

 mosphere, a thermometer to indicate the changes of tem- 

 perature, and a barometer to determine the weight of the 

 atmosphere. This last instrument would be particularly 

 valuable, as predicting the changes of the weather; the 

 rising of the mercury announces the return of dry weather, 

 and its sinking warns us of rain and storms. We can 

 regard these variations but as signs; but they are signs 

 much more certain than those which country people de- 

 rive from the changes of the moon. 



CHAPTER II. 



OF THE NATURE OF EARTHS, AND THEIR ACTION UPON 

 VEGETATION. 



Nearly all vegetables derive their support from the 

 earth. There are however some, the seeds of which, be- 

 ing deposited upon trees by birds or by the winds, germi- 

 nate and grow, appearing to be in the situation designed 

 for them by nature ; such are the mistletoe, the mosses, 

 &/C. There are others that float upon the water, or fasten 

 themselves upon dry rocks, upon slates, or tiles ; of the 

 last kind are the fleshy plants. As the earth furnishes 

 the greatest number of plants, and all those which are of 

 the most importance to man, its influence upon vegetation 

 is of the greatest consequence, and at the same time one 

 of the most difficult things of which we can treat. 



Plants are not, like animals, endowed with powers of lo- 

 comotion ; but are always fixed to a limited portion of the 

 soil. They depend upon the small space which they oc- 

 cupy for the supply of their wants ; they can place under 

 contribution only those portions of the surrounding air, 



