The Atmosphere. 29 



The problem concerning the assimilation of free nitrogen was 

 finally settled by an exhaustive study made in 1857 to 1858 by 

 Lawes, Gilbert and Pugh at the Rothamsted Experiment Station 

 in England. These investigators completed 27 experiments with 

 cereals, legumes and buckwheat. The plants were grown under 

 glass jars inverted in mercury to isolate them from the air. A 

 supply of air, freed from nitrogen compounds and mixed with 

 carbonic acid was forced through the apparatus and all nitrogen 

 compounds were carefully excluded from the soil and water used 

 in these experiments. The results fully confirmed the conclu- 

 sions of Boussingault. 



In the course of other experiments it was observed that while 

 supplies of nitrogen compounds in the soil stimulated the growth 

 of cereals, they were without appreciable effect upon legumes. 

 It remained for the German bacteriologist, Hellriegel, to dem- 

 onstrate that leachings from soils cropped to legumes stimulated 

 the growth of these crops on infertile soils, but failed to affect 

 cereals. Then followed the discovery of a remarkable affiliation 

 of bacteria and leguminous plants by which the plants obtain 

 supplies of nitrogen from the atmosphere. This discovery finds 

 a practical application in the growth of leguminous crops in 

 rotations for the purpose of maintaining the supply of nitrogen 

 in the soil. In field experiments the soil supply of nitrogen has 

 been maintained by growing clover in rotation with cereal crops. 

 A small amount of nitrogen compounds also accumulates in the 

 soil by the growth of bacteria which thrive independently of 

 higher plants. 



Oxygen. This constituent of the air is prominent among the 

 chemical elements because of its extreme activity. It combines 

 with the waste products from plant or animal life in the process 

 of combustion or decay and makes possible their destruction and 

 removal. This process is frequently accompanied by perceptible 

 heat, as in the rapid combustion of fuels, or the less active com- 

 bustion of manures and silage. It is the source of heat in the 

 animal body. The hardening of so-called " drying -oils " is also 



