( ^028 3 
I remain top fulf^ even to the height of 75 incbei* 
M.Hug&nsy to render a probable caufe of this ftrange ef- 
fed, conceivethj That, befides the preffureofthe Air5which 
Iteeps the Mercory fufpended at the height of about 27 inches, 
and of the truth of which we are conviiiced by a great num- 
ber ot other effefts chat wc fee 5 there is yet another preflure, 
ftroiiger than that, of a more fubtile matter than Airj which 
without difficulty penetrates glafs, water^ quickfilver^ and 
all other bodies^ which we fitid impenetrable to Air. This 
preflure, he faiih^ being added to that of the Air, is capable 
to fuftain the 75 inches of Mercury^ and poflibly more^ as long 
as it works only againft the lower furface, or agaiuft that of 
the Mercury, in which ftands the open end of the tube : Biit 
as loon as \\ can work alfo on the other fide^ (which happens 
when ftriking or hitting againft the tube, or intromitting into 
it a fmall bubble of Air^ you give way to this matter to begin 
to a6t) the prelTurc of it becoms equal on Both fides, fo that 
there is no more but the preflure of the Air, which fuftains the 
K^ercury at the ordinary height of 27 inches. 
If you ask, why the Quickfilver in the Tube of this Experi^ 
ment does not feel the prefTure of this matter,eyen whilftihac 
veffel is yet full 5 finceM. //i^^/?/ fuppdfeth, that it pierceth 
without difficulty the Glafs as well as the Mercury, &c ? And 
why the particles of this matter do not joy n together and be-^ 
gin th© preffure, in regard that they go.rfnd come thorow tt J 
whole extent of the Mercury, and that the Glafs does not hin- 
der their communication with thofe that are without: 
To remove thisdifficulty, which in M. H/^^^wV own opinion 
is very great, he anlwers. That though the parts of the mat- 
ter^ by him fuppofed, do find paflage between thpfe that 
compbfe the glafs, quickfilver, Sccj yet they there find not 
fijfEciently large ones for many to pa(i 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 faitb^ is the caufe, that though 
00 the fide of the inner furface of the glafs, which touches 
the fufpended Mercury^ many of its parts be prelTed by the 
particles of this matter s yet there being alfo a great number 
of them^ that feci no preCure by reafoix of the parts of the 
