PLANT FOOD, 175 



necessarily commences also ; this is the passage of the mole- 

 cules of the dissolved substance through the water without a 

 movement of the latter. Thus in perfectly quiescent water 

 a substance may diffuse itself between the molecules of the 

 latter to considerable distances, and this may take place in 

 any direction, even when the substance is heavier than water ; 

 thus common salt placed in the bottom of a tall vessel of 

 water will dissolve and gradually diffuse throughout the 

 whole. 



3d. The rapidity of diffusion varies for different sub- 

 stances ; thus the diffusion rate of sugar is more than three 

 times that of common salt (exactly as 365 to 116). 



4th. Two or more diffusions may take place at the same 

 time in the same fluid, and they may move in the same or in 

 opposite directions. 



5th. Diffusion continues until all parts of the solution 

 contain equal quantities of the dissolved substance. 



6th. If at any point in a solution the dissolved substance 

 be removed in some way, as, for example, by the formation 

 of a new salt by chemical reaction, there will be, as a conse- 

 quence, a continued diffusion toward that point ; and if the 

 new salt be a soluble one it must diffuse in every direction 

 from the point of its formation. Thus the molecular move- 

 ments may become quite complex. 



§ III. PLAifT Food. 



228. — The most important elements which are used in 

 the nutrition of plants, or which, in other words, enter into 

 their food, are Carbon, Hydrogen, Oxygen, Nitrogen, Sul- 

 phur, Iron, and Potassium. These all appear to be necessary 

 to the life and growth of the plant, and if any of them are 

 wanting in the water, soil, or air from which the plant de- 

 rives its nourishment, death from starvation will soon follow. 

 There are other elements which are made use of by plants, 

 but as life may be prolonged without them, they are regarded 

 as of secondary importance. In this list are Phosphorus, 

 Calcium, Sodium, Magnesium, Chlorine, and Silicon. 



