120 PRIZE ESSAY: 



whether nitrogen was directly absorbed from the atmosphere and 

 assimilated, M. Boussingault observed the growth of minute 

 green cryptogamia on the outside of the flower-pots, which had 

 been exposed to the air, but he failed to detect any vegetable 

 growth on those from which fresh air had been carefully excluded. 



215. The sudden growth of varieties of fungi during misty 

 weather has often been noticed, and their appearance may be ac- 

 celerated by the introduction of a small quantity of vapour of 

 ammonia into any confined space where they are observed. I 

 am not aware that any extensive experiments have been made 

 upon the growth of fungi in an atmosphere rich in ammonia, 

 such as certain fogs. I have, however, remarked with surprise 

 their absence in an atmosphere from which ammoniacal vapours 

 were probably abstracted by powdered charcoal, without, how- 

 ever, drawing any conclusions from the observation until attracted 

 by the curious discovery of M. Boussingault, that fogs are emi- 

 nently rich in ammonia. 



216. The presence of a large quantity of this important plant 

 food in certain fogs is not difficult to account for. Not only 

 does the gradually increasing quantity of aqueous vapour in the 

 atmosphere before the positive appearance of mist in any locality, 

 collect and condense rare and widely diffused ammoniacal vapours, 

 but the exhalations from the soil produced by decomposing ve- 

 getable matter, are arrested and accumulate. The period of the 

 year when fogs rich in ammonia may be expected depends natu- 

 rally upon the frequency of the fall of rain upon the moisture 

 of the atmosphere, and upon the winds. In Canada it appears 

 reasonable to suppose that we may expect to find fogs rich in 

 ammonia during the hot months of July and August, when the 

 rain fall is not so great as in September. During these months 

 mists frequently hang over the fields, particularly in low situations. 

 The exhalation of vapour of water from the leaves of plants be- 



