Chap. 6.J Natural lliftory of Nitre. -29 



fome, is fuppofed to favour the production of nitre, 

 is the accefs of air ; this is the caufe of the formation 

 of the nitre found on walls ; and for this reafon it 

 is that nitre beds require to be frequently turned over, 

 in order that the air may touch them in all points. 

 The neceffity of the accefs of air is laid to be ftill more 

 evidently fhewn by the nitre contained in chalk, as in 

 is never found below a certain depth. When the three 

 circumftances here treated of are united, the production 

 of falt-petre is very abundant. Nitre-beds ought aj- 

 xvays to be conftructed on thefe principles. 



The theory of the formation of nitre has not been 

 long known. Glauber, and many other chemifts 

 fmce his time, fuppofed nitre to exift ready formed 

 in vegetables, from which they imagined it to pafs into 

 animal fubftances, and to become difengaged by putre- 

 faction j but it was foon perceived, that vegetables do 

 not contain a fufficient quantity of nitre to account for 

 what is obtained from nitre beds. M. Thauvenei, 

 whofe didertation on the formation of nitre was ho- 

 noured with the prize of the academy, has made many 

 experiments to difcover its origin : he found, that the 

 nitrous acid is formed by the combination of an elaitic 

 fluid, difengaged from animal matters in a ftate of 

 putrefaction, and pure air. He has likewife difco- 

 vered, that the nitrous acid, once formed, combines 

 with calcareous earth, when any animal matters arc 

 made ufe of; and that the remains of vegetables are 

 ufeful to afford the fixed alkali, which is the bafis of 

 common nitre. But M. Thouvenel did not determine 

 the nature of the gas, which is difengaged from putre- 

 fying animal matters. It is to Mr. Cavendim that 

 we are indebted for the proof that it is the fame gas, 

 which conftitutes one of the principles of the atmo- 

 fphere, under the name of azote, or phlogifticated air, 



or 



