134 lie Principles of Part III. 



THE mofl common materials out of 

 which nitre is made, and, therefore, called 

 the matrix of nitre, are the rubbifh and 

 earth of old houfes, and efpecially of dove- 

 cotes, ftables, and church-ides ; particular 

 fat earths ; the afhes of burnt vegetables ; 

 putrefied animal or vegetable fubftances $ 

 and fome particular kinds of ftones. Thefe 

 materials are expofed to the air for fome 

 months, particularly the winter months j 

 for, during that time, nitre is generated in 

 the greateft plenty. The place where the 

 materials are expofed, mould be acceffible 

 to the air and winds, but not to the rays 

 of the fun, or to the rains. The air is ne- 

 ceffary, as it generates the nitre j but the fun 

 is deftructive, as it exhales it. The rains 

 are prejudicial, becaufe they warn it away 

 when made. Drought is as deftrudive, as 

 that exhales the faline parts after they are 

 formed, and hinders the fermentation, ne- 

 cefTary to draw the nitre, and to open the 

 matrix to receive it. It is obierved, that 

 the north winds are particularly productive 



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