In the combined state nitrogen is fairly widely distributed, being found in nitre, Chile saltpetre, ammonium salts and in various animal and vegetable tissues and liquids.
It is mentioned in the De inventione veritatis ascribed to Geber, wherein it is obtained by calcining a mixture of nitre, alum and blue vitriol.
In John Houghton's Collections on Husbandry and Trade, a periodical work begun in 1681, there is one of the earliest notices of turnips being eaten by sheep:" Some in Essex have their fallow after turnips, which feed their sheep in winter, by which means the turnips are scooped, and so made capable to hold dews and rain water, which, by corrupting,; _ mbibes the nitre of the air, and when the shell breaks it runs about and fertilizes.
These are washed with ammonium chloride until the filtrate is colourless, ignited, fused with caustic potash and nitre, the melt dissolved in water and nitric acid added to the solution until the colour of potassium ruthenate disappears.
The residue is then fused with caustic potash and nitre, dissolved in water, saturated with chlorine and distilled on the water-bath in a current of chlorine.