446 4bbe fontana’s Account of Airs 
mult mention, that, after ufing all the cautions I am mafter 
of; the greateft error in the diminutions is not greater than 
one fixtieth of the common air introduced in the tube : 
fo that, after having made five or fix experiments fuccef- 
fively, the probable error is fo fmall that it may be fafely 
negledted. But, if one chofe to operate upon a quantity 
of air nine or ten times greater than that I commonly 
ufe, the error could not be 1 oooth part of the quan- 
tity of the air, which quantity would not be more than 
a few cubic inches. In the defcription of my method I 
fhall aifo mention the means by which I obtain nitrous 
air of a nearly equal and con ft ant goodnefs, and in what 
manner I can refer my experiments to l'ome conftant 
ftandard. For want of this method it is that we are not 
certain of the obfervations made about the falubrity of 
f 
common air in different places, and that no tables of its 
changes have been made. 
I have not the leaft hefitation in aflerting, that the ex- 
periments made to afcertain the falubrity of the atmo- 
fpherical air in various places, in different countries and 
fituations, mentioned by feveral authors, are not to be 
depended upon ; becaufe the method they ufed was far 
from being exadt, the elements or ingredients for the ex- 
periment were unknown and uncertain, and the refults 
very different from one another. 
Wnen 
