HOW PLANTS GROW. 591 



of the sun, it ascends ; as it ascends and meets with cooler cur- 

 rents, the invisible particles contained in it will condense into 

 larger particles ; after they become larger particles and satu- 

 rated, the force of the ascending currents fails to support them, 

 and they fall in the form of rain. 



The air, however, has in it more or less of other gases, or 

 other invisible constituents ; strange to say, all the other ele- 

 ments or substances do not have the same affinity for each 

 other, and will not unite or enter combinations. Copper and 

 iron cannot be welded together, unless a solder is used that 

 possesses an affinity for both. Water, however, has a great love 

 for other substances, or they for it ; among these substances 

 may be mentioned ammonia and carbonic acid, which promote 

 the growth and enter into the composition of all plants. A 

 careful observation and analysis of the rain, "both in this country 

 and in Europe, shows that these substances are brought down 

 in very considerable quantities by it. In France, about eight 

 gallons of carbonic acid are brought down each year, per acre, 

 and ammonia in varying amounts. In Kansas, where this sub- 

 ject has received attention, it appears that three and one-half 

 pounds was the average amount of nitrogen brought down per 

 acre, when the rainfall averaged 29 inches. If we suppose an 

 average dressing of saltpetre, to about 200 pounds per acre, 

 then this rainfall is about equivalent to one -ninth of this 

 amount. As we go south, the amount of nitrogen in the rain 

 increases, as is also proven in Kansas, at Manhattan, where the 

 amount of nitrogen brought down by the rain was only about 

 one pound, whereas 20 south of that point it was found to 

 be over six pounds ; in Maryland, the annual rainfall is about 

 42 inches, therefore we can safely calculate that about five 

 pounds per acre of ammonia are conveyed to the soil through 

 the rains. 



The Soil. We have seen that air and water are each com- 

 posed of two gases, chemically combined. The soil, or earth, is 

 a much more complicated substance, and varies greatly in its 

 nature in different situations. While air is a fluid, and water a 

 liquid, the earth is a solid. That is, it is solid in a cold state, 

 for all solid things can be made liquid if they are subjected to 

 a sufficiently high degree of heat. All the metals can be made 



