68 Nitre what , and how obtained * 
Tetre may conffantly be obtained from the 
Mortar and Rubbilh of old Walls, which 
have been long expofed to the Air ; and that 
the fame Materials, after being diverted of it 
by Art, will afford a farther Quantity by 
lying a few Years together. The extreme 
Hardnefs of the Mortar in fome old Build¬ 
ings, where it is found more difficult to be 
broken than even the Bricks or Stones it 
ferved to cement, feems likewile occartoned 
by its being fo fully impregnated with this 
Spirit * or Principle, that it becomes in a 
manner petrified: and the alkalious Salts 
wherewith the Excrements of Animals 
abound, ferve probably, in the like Manner, 
to fix and embody the fame Spirit, and 
conftitute Salt-Petre. Some have under¬ 
taken to make it likewife from Lime, Pot- 
Ailaes, '&c. but howfoever it be produced, 
it may always be dirtinguifhed by its hex¬ 
agonal Shoots. 
Let us now confider what other Pro¬ 
ductions of Nature appear in this hexago¬ 
nal Figure; and we fhall find that all 
Cryftal, unlefs either rtraitened for Room 
* May not what we call a volatile Spirit confifl of Parti¬ 
cles exceedingly minute, and of fome determinate Figure ; 
Whofe Property it is to repel each other, from certain Points, 
in certain Directions, and to certain Diftances; whereby 
they are unable to combine together, unlefs by penetrating, 
mixing, and becoming embodied with other Matter, vvhofe 
Pores they can ilrike into ? and may not their repulfive 
Power be increafed by Heat or Motion f 
9t 
