2g Nitre-Beds. (Book VI, 



continually formed in inhabited places; it is found 

 in great quantities upon walls which are flickered 

 from the rain ; and the Bifliop of Llandaff collected a 

 confiderable portion from the decayed mortar of an old 

 barn. 



There appear to be three principal circumftances 

 that promote its formation ; the firft is, the prefence 

 of chalk, or any other calcareous earth, as appears 

 by its being collected from walls covered with plafter, 

 or from the ruins of ancient edifices. This fait is like- 

 yr'ik found perfectly pure in chalky earths. The late 

 Due de la Rochefoucault obtained it, in the proportion 

 of one ounce in the pound, from the chalk of Roche 

 Guyon. 



The fccond circurnflance neceffary for the pro- 

 duction of this fait, is the putrefaction, or fpontaneous, 

 decompofition of vegetable and animal matters. It 

 is a well known fact, that places which are moiflened 

 with animal liquids, or contain animal matters in a 

 ilate of putrefaction, fuch as dunghills, (tables, privies, 

 &c. produce much, nitre. This conftant obfervation 

 has been applied to the formation of artificial nitre- 

 beds. Dry ditches are dug and covered with Iheds 

 open at the fides j thefe are filled with animal fub- 

 ftances, fuch as dung, the excrements of quadrupeds, or 

 birds, with the remains of vegetables. Thefe matters 

 are from time to time watered, efpecially with water 

 charged with animal or vegetable matters capable of 

 putrefaction, and they are turned up to renew their 

 furfaces from time to time. When the putrefaction 

 is in an advanced ftage, a fmall portion of the rnattev 

 is taken up and lixiviated, to afcertain whether it con- 

 tains nitre ; and when it is found fufficiently charge^ 

 with that fait, the whole is lixiviated. 



The third circumftance which in the opinion of 



feme. 



