[ xi} 
- that the difficulty of the undertak= 
ing furpaffes their abilities. 
WHEN this ‘book was entirely 
printed, and nothing but the latter 
end of the pretace unfinithed, “I 
was informed by my friend the 
Abbé Fontana, that he difcovercd a 
gunpowder, and the difcovery of a 
new and powerful explofive air), 
in which I fay, that the rapid pro- 
greis our modern philofophers daily 
occ ; 3 make 
