Pa Mr. Wat's Thoughts on ibe conflituent : 
15. I fhall not make, at prefent, any further deduétions’ 
from what I myfelf confider ftill in the light of a conjectural’ 
hypothefis, which I have perhaps dwelt upon too long already. 
I fhall only beg your attention to fome general reafoning on the 
fubject ; wens however, may poilibly ferve more to fhew the 
uncertainty of other fyftems on the conftituent parts of air, than 
the certainty of this. Some of thofe fyftems tuppofe dephlo- 
gillicated air to be coempofed of an acid and fedachhiaie elfe, 
fois fay phlogifton. If an acid enters into the compofition of 
it, why does not that acid appear again when the air 1s decom= 
pofed, by means of inflammable air and heat?) And why is 
the water which is the product of this procefs pure water? 
And if an acid forms one of its conftituent parts, why has no+ 
body been able to detect any ditterence in the dephlogifticated 
air, made by the help of different acids, when compared with 
one another, or with the air extruded by vegetables? Thefe 
airs, of fuch different origins, appear to be exactly the fame. 
And if phlogifton be a conftituent part of air, why does it at- 
tract phlogifton with fuch avidity ? Some have, on the other 
hand, contended that air is compofed of earth, united to acids. 
or phlogifton, or to both, or to fome other matter. Here we: 
muft aik, what earth it is which is one of the component 
parts of air? All earths which will unite with the nitrous or 
vitriolic acids, and with fome others, fuch as the phofphoric 
and the arfenical acids, will ferve as bafes for the formation 
of air, and the air produced from all of them appears by every 
teit to be the fame, when freed from accidental impurities.. 
To this argument it is anfwered, that it is not any particular 
{pecies of earth which is the bafis of air, but elementary or fim- 
ple earth, which is contained in all of them. ‘If this were’the 
/ ? matter 
