( 5«^x ) 
By Aif^t^ind^MtMdbej would fametimes(as in Dialogue 
Thy^c(0 p. 4^ ) have us uoderftand a fure Mther ^ or (as his 
words 2iXt;)Aerem ^ib omni Terr£ Aqu^q^efflnv^is fHrnm.qHdis 
mtntur ejfe Mther-^ to which, 1 fuppolej a Dowers the Materia 
jubtHis oi Des Cartes, md MovifMBgem h\s morejubtik Mat- 
ter than Air. On the other haod ^ M^tlngens here , by Air^ 
leeilis tounderftaud thai feculent matter arifiog from thofe 
the Earths and Waters Effuma^whtch are lotermiDgled with 
this fukile Matter, Wq meza by Air ^ the Aggregate of both 
iheje^ or whatever clfe makes up that Heterogeneous Fluid 
wherein we breath, comnianly called Air 3 the purer part 
of which is Mt.Hobbes's Air 5 and the feculent of it is Mon- 
fieur Hugens's Air. 
And therefore, where I fpeakof Facuity csLUs'^d by the 
Torricellian Experiment, or fuch other ways, I do exprefly 
caution (De Motu 0.14. and Hobb, Heam:f. 152. and elfe- 
where^j not to be underftood as affirming Abfolnte vacuity 
(which whether or no there be^ or can be in nature , I lift 
not to difpute 5) but at leaft an Abfcnce of thn Heteroge- 
neous mixture tfkich we call Air^ fuch as that is wherein we 
breath'-^ without difpudng againft the "purus Mther of Mr. 
tidhbes ^ or the Materiajubtilis of Des Cartes or M.Bugens^ 
as not neceffary to the Inquiries in hand. 
To the Prcflure of this Purer matter ( which they fup- 
poie fofubtilej asto penetrate the Mercury^ Marble, and 
Glafs it felf, ) they adicrifae the Sufpeofion of the Qtijck- 
filver to fo great an height. And my Lord Brounck§r io par- 
ticular, while that Pieceof mine was under the Prefsj had 
a defign ( as He theo fignifiedto me) of profecutiog the 
Experiment (as Monfieur Hugens does now advifejj to fee if 
he could bring It to feme Determination (of which I might 
there have given an account 5 if it could have been dif- 
patcht time enough^) what were the utmoft height at which 
it mi^ht be thus made to ftand 5 thereby to determine the 
prcflure of this Purer matter ^ asthatof the Common Air 
is determined by the Torricellian Experiment. But his lei- 
flre not then fervingji only gave that brief account of his 
Notionj^ 
