RUST, SMtJT, ETC. 123 



until the heap has well taken, and then to be firmly closed for 

 the purpose of allowmg slow combustion to go on in the absence 

 of air. When cool the charcoal may be crushed in a stout can- 

 vass bag by a lever, not by blows, and when sifted, furnishes the 

 required material for sowing. 



220(a). If we assume with Fresenius that the quantity of 

 ammonia in the atmosphere amounts to less than one ten-mil- 

 lionth ; the amount it would contain would exceed 50,000,000 

 tons, while that of the carbonic acid in the atmosphere is 

 3,300,000,000,000 tons, the weight of the air itself being 

 .5,050,000,000,000,000 tons or five thousand and fifty billions. 



220(b). Water is absorbed by the roots of plants alone ; and 

 the same water may repeatedly pass through the same crops, for 

 the amount crops exhale during their growing season greatly ex- 

 ceeds the rainfall, hence they must derive much water from dew 

 which is absorbed by the soil, and taken up by the roots, to be 

 again exhaled and again deposited in the form of dew. The 

 amount of dew may be equal to one-half of rainfall during the 

 summer months. 



221. Whatever "specific" will cure mildew, will also arrest 

 rust.(i) Both are fungi, very nearly allied to one another, so 

 much so, indeed, that it has been supposed by very eminent bo- 

 tanists that rust is merely a state in the development of mildew, 

 and both species are produced under similar climatic conditions. 

 Cuthbert Johnson says, " Salt, if not a complete preventive, is 

 an effectual cure of the mildew." Mr. Chattertou, in the annals 

 of agriculture, tells us that " on the sea side the wheat is little 

 damaged by the mildew, yet within three miles inland the crops 

 are as much affected as those still further from the sea." " This 

 fact can be supported by the experience of most farmers whose 

 fields skirt our native shores." Not only does^the soil in such 



(1) Mildew and rust are often found together. 



