CAUSES OF THE ATTACKS OF EUST, &c. 107 



acid about five parts to the million. Dew always contains 

 ammonia, and mists have prevailed so rich in this sub- 

 stance that the water had an alkaline reaction. Barral 

 analysed the water collected in the rain-gauge of the Ob- 

 servatory at Paris. He found that in one year 1074 Ibs. 

 of ammonia fell with the rain, and 10 '7 Ibs. of nitric acid. 

 In July he found the amount of the ammonia to be the 

 greatest ; in September the amount of nitric acid to be the 

 greatest. The ammonia was least in March, and increased 

 gradually to July. In August it diminished suddenly, and 

 continued to diminish until October, attaining its second 

 maximum in February. These observations, although 

 very interesting, are not satisfactory, because they were 

 made in the neighbourhood of a great city. Hence we 

 find that Boussingault discovered much less ammonia in 

 the air far away from towns a gallon of rain water con- 

 taining only one twenty-fifth of a grain of ammonia. As 

 a general fact, however, the water collected during fogs was 

 extraordinarily rich in ammonia, containing on an average 

 one-third of a grain to the gallon ; but an instance has 

 been known before referred to of a gallon of water from 

 a fog containing not less than four grains of ammonia. 

 The constant presence of this substance in the atmosphere 

 is not only now fully established, but its influence upon 

 vegetable growth in this gaseous form is of the highest in- 

 terest, and possibly of the highest importance. The ex- 

 periments of M. Ville upon the effects of ammonia in air 

 upon vegetation, show how rapidly and remarkably its in- 

 fluence is felt. If ammonia be artificially introduced into 

 air in the same proportional average as carbonic acid is 

 found to be constantly present, namely, about one part in 

 2,500 parts of air, its influence soon shows itself upon the 

 leaves, which continually acquire a deeper and deeper tint. 

 The presence of such ammoniacal vapours not only stimu- 

 lates the vegetation, but changes the growth of the plant, 

 and causes the development and enlargement of particular 

 organs. In prosecuting a series of experiments on the 

 phenomena of vegetation, with a view to ascertain whether 



