extracted from different Kinds of Waters. 45 1 
air in fome particular places ; but only fay, that in gene- 
ral the air is good every where, and that the fmall differ- 
ences are noc to be feared fo much as fome people would 
make us believe. Nor do I mean to fpeak here of thofe 
vapours and other bodies which are accidentally joined 
to the common air in particular places, but do not change 
its nature and intrinfical property. This ftate of the air 
cannot be known by the teft of nitrous air, and thofe 
vapours are to be confidered in the fame manner as 
v * Ihould confider fo many particles of arfenic fwim- 
nung in the atmofphere. In this cafe it is the arfenic, 
and not the degenerated air, that would kill the animals 
who ventured to breathe it. 
In this place, therefore, I do not mean to fpeak of 
thofe changes which do not immediately alter the nature 
of the air itfelf. The other Hates of that fluid are of 
another kind, and they are not to be examined by 
means of nitrous or inflammable air (the ufes of 
which laft, I fhall Ihew on another occafion). The 
fame thing may be faid of thofe vapours or particles 
which may be good for refpiration, and do not change 
the nature of the air. Some vegetables, for ijfftance, , 
can diffufe through the air fuch exhalations as may 
be of real ufe to the animal oecondmy when they 
are„- 
