[ 174 ] 
nitrous air in this method is, that 
more meafures of it are required 
before the faturation of the air to 
be examined is compleated 
The reafon of this will appear 
obvious, if we confider that it 1s 
only the true nitrous air which 1s 
capable of diminifhing refpirable 
airs, and that it performs this dimi- 
nution in the proportion to its 
ftrength, fo that weak nitrous aif 
will always diminifh common aif 
in the proportion of its own good — 
or bad quality. Now I will, fup- 
pofe that the nitrous air, to be added 
fo the two meafures of common aif 
is become fo weak, either by ftand- 
ing, or by the admiffion of any 
other air, as to poffefs only half the 
ftrength of good nitrous air. The 
confequence will be, that as much 
again cf it will be required to {a- 
turate 
