By Air^ I Rnd^t.Hokbef would, foiHe£laies(as iti Diahgw 
Thyftc^ p* 4* ) have m uoderftand a pure JSiher , or (as his 
words zve) Aerem ab omni Terr£ AqH£q^(ffiHviis pHrum^qHalk 
futatur ejje ASfher'^ to whfchj I fuppolej aolwers the Materia 
fuhiilis of Des Cartes^ and Mon^Bugens his morejubtile Mat- 
ter than ^i/'. On the other haod , M.Uugens- here ^ by y^/V, 
lecms tounderftaod that feculent matter arifing from thofe 
iht Earths and Waters Effluviajwhich are intermingled with 
this fuhtile Matter. We mean by Air , the Aggregate of both 
theje^ or whatever elfe makes up that Heterogsneom Fluid - 
wherein wc^reath, commonly called Air *^ the purer part 
of which is Mr.Hobbes's Air y and the feculent of it is Moa- 
fieur HHgens\ Air. 
And therefore, where I fpeakof Vacuity cmid by the 
'Torricellian Experiment, or fuch other ways, I do exprefly 
caution (De Motu C,i^, znd Hobb. Heaut. p. I'yi, and elfe- 
wherej not to be underftood as affirmiog Abfohite vacuity 
(which whether or no there be^ or can be in nature ^ I Jift 
not todifpute 5) but at leaft an Abfence of that Hefercge- 
neo^ mixture vphichwe cali Air ^ Juch 0s that is wherein we 
breath '-y without difputiog againft the purus Mther of Mh 
Hobbes 3 or the Materia fubtilis oi Bes- Cartes or M.Bugensy 
as not neceflary to the Inquiries in hand. 
To the Preffure of this Purer matter ( whicli they fup- 
pofe fo fubtile 3 as to penetrate the Mercury, Marble, and 
Glafs it fclfj ) they adfcribe the Sufpenfion of the Qn/ck- 
filvcr to fo great an height. And my Lord Brouncker in par* 
ticular, while that Piece of mine was under the Prefs, had 
a defign ( as He then fignified to me) of profecuting the 
Experiment (as Monfieur Hugens does now ad vifej/ - o fee if 
he could bring it to fome Derermioatiori (of which I might 
there have given an account j if it could have been dif- 
pateht time enoughj,) what were the utmoft height at which 
n m!.^hc be thus made to ftand ^ thereby to determine the 
preffure of this Purer matter 3 as that of the Common Air 
is determin'd by the Torricellian Experiment. But his lei- 
ftre not then ferviog^ I only gave that brief account of hi^ 
Notion^ , 
