304 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



ACTION OF BACTERIA AND OF THE AIR IN CONNECTION WITH 

 DECOMPOSITION AND PLANT GROWTH. 



Bacteria, or microbes in general, of an immense number of different 

 kinds are almost ubiquitous on the whole surface of the earth and on 

 all exposed solids. The favorite habitat of most kinds is the moist 

 surface of some substance of organic origin undergoing decomposition. 

 But some sorts appear to flourish on almost any kind of solid exposed 

 to the air. Thus panes of glass, rocks, metals, tiles, and sand will 

 furnish a crop, the richer, no doubt, for any slight deposit from organic 

 liquids or gases. The chief work, and a very vast one, of microorgan- 

 isms is the transformation of dead organic matter into "inorganic" 

 substances. All the dead vegetable and animal substance lying 

 exposed or where air has access is being transformed into mineral 

 matter by this agency. Decomposition generally consists of oxidation 

 by a class of microbes which take their oxygen from the air, and then 

 the transformation and use of the oxygenized products which sink 

 deeper into the earth by another class of microbes, the anaerobic, 

 which not only themselves detach oxygen from its new compounds, 

 but allow of its being united with products which are formed by 

 chemical changes as a result of their activity. The whole process 

 converts the nitrogenous elements into ammonia, nitrous and nitric 

 acids, carbonic acid and water, and produces also phosphoric acid. It 

 takes place most readily in porous, somewhat moist earth and at a 

 high temperature. It is a necessary preparation of the soil for the 

 life of plants. The active bacteria of this decomposition, nitrification, 

 or mineralization do not extend to any great depth, generally not so 

 deep as 12 feet, below which the ground is sterile. The rapid oxida- 

 tion going on near the surface leaves little free oxygen for the use of 

 bacteria even at the depth of a few feet. The decomposition effected 

 chiefly by the aerobic bacteria in the upper layers enables plants to 

 draw nutriment from the new products, and thus the presence of air 

 and bacteria in the mold are necessary conditions for the growth of 

 vegetation. These newly discovered facts must have a very important 

 bearing upon agriculture. The relation of air supply, soil, tempera- 

 ture, and moisture to the microbic life in the earth, and consequently 

 to growing crops, will become a fruitful subject of research to chemists, 

 bacteriologists, and scientific farmers. 



Most of the diseases of plants are dependent to a very great extent 

 on conditions of weather, and many are transported by the air to new 

 situations where they spread as from a center. Thus they differ from 

 the spreading diseases of animals, which are not, on the whole, mainly 

 affected by the character of a season, and are not carried so far through 

 the atmosphere. The number of plant diseases of an infectious kind, 

 depending on fungi or microbes, is very great. The vine alone is 

 attacked by more than a hundred species. Some species live in alter- 

 nate generations on different plants; thus the rust of wheat requires 



