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remain top full ^ even to the height of 75 inches^ 
M. Hugens^ to render a probable caufe of this ftrange ef- 
conceivethj That, befides the preffiire ofthe Air^which 
keeps the Mercury fufpended at the height of about 27 inches, 
and of the truth of which we areconviuced by a great uum- 
ber oi other cftefts diat we fee ^ there is yet another prefllire, 
ftronger than that, of a more fubtile matter than Air, which 
without difficulty penetrates gla{sj water, quickfilverj and 
all other bodies, which we find impenetrable to Air. This 
prefllire, befaith^ being added to that of the Air, is capable 
to fuftain the 75 inches of Mercury^and poffibly more, as long 
as it works only againft the lower furface, or againft that of 
the Mercury, in which ftandsthe opei] end of the tube: But 
as foon as \\ can work alfo on the other fide, (which happens 
when ftriking or hitting againft the tube^ or intromitting into 
it afmali bubble of Air, you give way to this matter to begiu 
to a^^t) thepreflurcof it becoms equal on both fides, fo that 
there is no more but the preffare of the Air, which fuftains the 
Mercury at the ordinary height of 27 inches. 
If you ask, why the Quickfilver in the Tube of this Experi- 
ment does not feel the preflure of thi$matter,even whilit that 
veffel is yet full J £mct lA.Hugens fuppofeth, that it pierceth 
without difScalty the Glafs as well as the Mercury, 6cc ? And 
why the particles of this matter do not joy n together and be- 
gin the preffure, in regard that they go and come ihorow the 
whole extent of the Mercury, and that the Glafs does not hin- 
der their communication with thofe that arc without: 
To remove this difficulty, which in M. ftgrn/ own opinion 
is very great, heanfwers, That though the parts of the mat- 
ter, by him fuppofed , do find paffage between thofe that 
compofe the glafs, quickfilver, &c 3 yet they there find not 
fufficiently large ones for many to pafs together, nor to move 
there with that force which is requifite to feparate the parts of 
the quickfilver, that have fome connexion together* And 
this very fame connexion, he faith^ \% the caufe, that though 
on the fide of the inner furface of the glafs, which touches 
the fufpended Mercury, many of its parts be preflfed by the 
particles of this matter; yet there being alfba great numbef 
of them, tftatfeclno prcfTure by rcafon of the parts of the ! 
glafs,! 
