RUST, SMUT, ETC. 121 



ing then checked, and their juices partially stagnating in an at- 

 mosphere often rich in ammoniacal vapours, all the conditions 

 for the appearance of the fungus called " Rust" on the stems 

 and leaves of the cereals appear to be fulfilled. 



217. It is commonly remarked that rust is most prevalent on 

 new land ; this is perhaps explained by the large amount of ve- 

 getable matter thrown into a state of decomposition by excess of 

 air and the consequent production of ammonia. There is no 

 doubt that much of the ammonia thus generated would combine 

 with vegetable acids, and be fixed by clay, &c. ; but some por- 

 tion could not fail to combine with carbonic acid and escape into 

 air in the form of the volatile carbonate, as is observed to a 

 greater degree on manure heaps even where gypsum or other 

 solid fixers of ammonia are employed to avoid it. "We must re- 

 gard new land as a storehouse of ammonia and other plant food, 

 which become liable to volatilize when liberated by too free an 

 exposure to air without proper precautions. 



218. If the supposition be correct that " Rust" is mainly oc- 

 casioned by the concurrence of mists or fogs in July and August, 

 rich in ammonia, stimulating the growth of the sporules in the 

 stagnated juices of the plants ; and that the active agent in in- 

 ducing the sudden appearance of that destructive parasite is 

 really ammoniacal vapours, we have a remedy at hand which 

 promises, when properly and carefully applied, if not entirely to 

 check, at least so far to arrest the growth of the parasite as to 

 claim a general trial, especially as its effects would probably prove 

 equally availing in arresting mildew. What we require is an 

 available absorbent of ammonia and its votatile compounds, not 

 an absorbent which will destroy this valuable plant food, but one 

 which possesses the property of inducing it to assume another 

 form, perhaps equally available as a fertilizer, although of much 

 slower action. Recent observations show that powdered charcoal 



i 



