80 PRINCIPLES OF GARDENING. [CH. II. 



nating in idleness, the cultivator alloAving his plants 

 to be overdosed with moistui'e, in order that the 

 trouble of cleansing their leaves ^ith the syringe and 

 sponge may be avoided. With these implements the 

 cleansing is much more effectual than when the rain 

 is allowed to be the agent; and it is a process pro- 

 moting the plant's health, if repeated once a week, 

 during a morning, when the weather is mild. 



Returning to the consideration of the food obtained 

 by a plant from the soil by the agency of its roots, 

 we find that silica, or the pure substance of flint, is 

 present in all soils ; is soluble in water, requiring 

 one thousand times its weight of this liquid to dis- 

 solve it [Kirwans Mineralogy, vol. i. p. 10); is found 

 in many plants, and in all the grasses that have been 

 analysed. Alumina, or the basis of clay, present in all 

 soils, is so soluble in water as to be inseparable by 

 the filter, and is much more so when any of the acids 

 are present {Sennehier's Physiolog. Veget. vol. iii. 

 p. 18); it is found in plants in minute quantities, 

 especially in the grain of barley, oats, wheat, &c. 

 (Schrceder, in GehlensJourn. vol. iii. p. 5*25.) Lime 

 is found in almost all soils ; it is easily soluble in 

 water, and there is but one plant that is not known to 

 contain some of it as a constituent, the Salsola Soda. 

 [Ann. de Chimie, vol. xviii. p. 76.) Magnesia, gene- 

 rally present in soils, is soluble in water, and is 



