6 SOURCES OF THE NITROGEN OF VEGETATION 



nitrogen from atmospheric som-ces only. Attempts had also 

 been made to grow plants in artificial media with a known 

 supply of nitrogen, which could be compared with the amount 

 of nitrogen found later in the fully grown plant. Boussingault, 

 to whom the first experiments of this nature were due, soon 

 found that very elaborate precautions must be taken to obviate 

 the influx of nitrogen either in dust or as ammonia in the 

 atmosjDhere and in the water employed, hence in all his later 

 experiments the plants were grown in closed cases fed with air 

 from which all ammonia had been withdrawn by acid. Bous- 

 singault's conclusions were against the fixation of any nitrogen, 

 but they were not accepted universally ; in particular, Ville 

 brought forward other similar experiments, in which the plant 

 showed a distinct gain of combined nitrogen. In 1857 the 

 subject was taken up at Rothamsted, and a most elaborate 

 series of experiments were carried out by Dr Evan Pugh, 

 at that time working in the Rothamsted laboratory. The 

 experimental plants were grown under glass shades, and 

 every precaution was taken to ensure the freedom from 

 ammonia of the air entering the shades, and also of the other 

 materials — the burnt earth, the pots, the water, the manures — 

 employed in the experiment. 



The experiments were made with wheat, barley, oats, clover, 

 beans, peas, and buckwheat, and the trials were repeated, in the 

 one case with no manm^e in the pots, and in the other with the 

 supply of a small quantity of sulphate of ammonia. The soils 

 employed were made up from either ignited pumice or ignited 

 soil, and the glass shades under which the plants were grown 

 rested in the groove of a stoneware vessel, mercury being used 

 as a lute. The air, previously passed through sulphuric acid 

 and sodium carbonate solution and washed, was forced into 

 the apparatus, so as to always maintain a greater pressure 

 inside than out, thus minimising all danger of unwashed air 

 leaking in ; carbonic acid was also introduced as required. 

 Under these rigorous conditions the following results were 

 obtained : — 



