472 NITRE. 



as well as of other plants, it is sometimes found crystallized in nee 

 dies. It exists in tobacco ; and sometimes the stalks of this plant 

 contain so much nitre that when dried, they will burn like a squib. 



The nitre in plants appears to be derived from the soil. 



How is nitre formed ? 



(/.) During the decomposition of bodies containing nitrogen, this 

 principle has been supposed to combine with the oxygen of the air, to 

 form this acid, and this unites with any proper base. 



Lime is often present, and forms in this manner nitrate of lime, 

 which by a substitution of the alkali, from weeds, ashes, &c. is chang- 

 ed into nitrate of potassa. 



(g.) There seems great reason to believe that the atmosphere is, to a 

 certain extent, converted by electrical agencies, into nitric acid, as 

 nothing more is necessary, than that the elements should unite in 

 nearly the reversed proportions in which they exist in the air. There 

 is a popular impression that thunder and lightning, and also clear 

 frosty weather, are favorable to the production of nitre. That its 

 production depends upon atmospherical phenomena, seems to be 

 proved from the fact, that the lixiviated saltpetre earth becomes im- 

 pregnated again in a year or or two, by exposure to the air. 



(h.) Jlrtificial nitre beds. Most of the nitre used on the continent 

 of Europe, is produced from composts, formed of garden mould, lime 

 rubbish, ashes, and marly earths, and animal and vegetable sub- 

 stances, of every description. The bed is screened by a thatched 

 roof, through which the air has access, although it does not circulate 

 very freely. The heap is frequently stirred, and moistened from 

 time to time with the drainings of the barn yards, and of the kitchen. 

 To favor the process, situations are sometimes chosen on the de- 

 clivities of hills. Moderate light, moderate moisture, a temperature 

 from 65 to 90, and (as asserted,) additions of common salt, pro- 

 mote the production of nitre. 



8. EXTRACTION. The nitrous earths, mixed with quick lime and 

 ashes, are placed in large vats or barrels, with perforated bottoms, cov- 

 ered with straw, and sometimes there is a second bottom, below the 

 first, with a stop cock between. Water dissolves the nitrates, the 

 ashes decompose the nitrates of lime and magnesia, and the nitrates 

 of potash, and other soluble salts, are drawn off below. In the re- 

 fining of nitre, eggs, milk, soap, and twigs of euphorbia, are used. 

 The solution is then concentrated by heat, and suffered to crystallize. 

 It is at first a dirty mass with many impurities, particularly common 

 salt. From these it is purified by successive solutions, evaporations, 

 and crystallizations. The earthy bases are precipitated by potassa, 

 or ashes. Such salts as are less soluble than nitre, are separated 

 during the evaporation, and such as are more soluble, are drawn off 

 with the mother water. These operations are repeated three or four 



