16 AGRICULTURAL TEST-BOOK. 



U&amp;gt; tho way i,n which it disappears again from the organic sub 

 stances. 



50. Plants receive water from (a,) the dew, (b,) from ruin, 

 (c+) from the soil by evaporation; in case of irrigation, and in 

 certain localities, directly (d,) from streams or ponds, and fa) 

 from vapor held in suspension in the atmosphere. 



Jl. A cubic inch of water at 62 Fahr, Barometer 30 inches, weigh* 

 in aJr 252,458 grains. The imperial gallon Las been defined to contain 

 10 pounds avoirdupois (70,000 grains) of distilled water at that temper- 

 ntnre and pressure. Its capacity is, therefore, 2 7 7. 19 cubic inches. Th 

 specific gravity of water at 60 Fahr. is 1, being tho unit to which thw 

 densities of all other liquids and solids are conveniently referred ; it is 

 8t5 times heavier than air at that temperature* 



Water readily combines with or absorbs a great variety of 

 sriid or gasous bodies ; and it is by means of water that most 

 of tho materials of the structure of plants are introduced. Con 

 sequently the composition of the water which cultivated planfe 

 may receive in any given locality is of great importance to tl* 

 Farmer. 



53. We have already seen (28-37) that rain, washing the 

 air, receives a variety of substances from the atmosphere which 

 it conveys to the soil and plant; but the amount of these substan 

 ces varies in different positions. Thus Dr. Madden has calculated 

 that the quantity of rain which falls at Penicuick, in Scotland, 

 in a year, brings down upon each acre of land in tliat neighbor 

 hood more than 600 ibs. weight of common salt, (Johnston.) 

 while in countries remote from the sea it is believed that no 

 salt is held in suspension in the atmosphere. And thus, M 

 nan may supply or withhold an important element of vegeta 

 tion. 



54. Rain water is purer than spring, river, or sea watei. 

 Dew is believed to be purer than ordinary rain water. 



Dew is caused by the cooling of the earth, plants, (fee., by noctur 

 nal rqdialion. The appearance of dew always follows, never precede* 



