HISTORICAL AND INTRODUCTORY 21 



periment under glass shades, but supplied with washed and 

 purified air and with pure water. In spite of the ample 

 supply of mineral food the plants languished and died : the 

 conclusion seemed irresistible that plants could not utilise 

 gaseous nitrogen. For all non-leguminous crops this con- 

 clusion agreed with the results of field trials. But there re- 

 mained the very troublesome fact that leguminous crops 

 required no nitrogenous manure and yet they contained large 

 quantities of nitrogen, and also enriched the soil considerably 

 in this element. Where had the nitrogen come from ? The 

 amount of combined nitrogen brought down by the rain was 

 found to be far too small to account for the result. For years 

 experiments were carried on, but the problem remained un- 

 solved. Looking back over the papers ^ one can see how very 

 close some of the older investigators were to the discovery of 

 the cause of the mystery: in particular Lachmann (158) in 

 1858 and Bretschneider (54) in 1861. Lachmann showed 

 that the nodules invariably present on the roots contained 

 " vibrionenartige " organisms, while Bretschneider showed 

 that the nitrogen fixation which occurred in normal soil did 

 not take place in ignited soil. But these papers were both 

 published in obscure journals and attracted little attention, 

 and once again an investigation in agricultural chemistry had 

 been brought to a standstill for want of new methods of 

 attack. 



The Beginnings of Soil Bacteriology. 



It had been a maxim with the older agricultural chemists 

 that "corruption is the mother of vegetation". Animal and 

 vegetable matter had long been known to decompose with 

 formation of nitrates : indeed nitre beds made up from such 

 decaying matter were the recognised source of nitrates for 

 the manufacture of gunpowder during the European Wars of 

 the seventeenth and eighteenth centuries.^ No satisfactory 



^ A good summary of the voluminous literature is contained in Lohnis 

 Handbuch der Landw. Bakteriologie, pp. 646 et seq. 



^ " Instructions sur I'^tablissement des nitrieres, public par les R^gisseurs 

 g^n^raux des Poudres et Salp^tre. Paris, 1777. 



