46 PHILOSOPHICAL TRANSACTIONS. [aNNO 1672, 



themselves. So that there needs no such force to disjoin the quicksilver from 

 the glass, whatever there may be for disjoining its parts one from another. 



If therefore we should suppose the pressure of the grosser air downwards on 

 AB, (fig. 6, pi. ]) the surface of the stagnant quicksilver, and consequently by 

 means thereof, upwards at C, sufficient only to bear up that in the tube to the 

 height of I ; but the superadded weight or pressure of the purer air to hold it up 

 as high as D, 75 inches or more, while it is full, and the quicksilver well 

 cleansed; as if so long it could not enter at D; but in case it be not so cleansed, 

 or be already sunk to H, this purer air would enter at D, and thrust it down to 

 I, counterbalancing the pressure (at C) of the purer, but not of the grosser air, 

 which I take to be the sum of the cause assigned by M. Huygens : I am yet to 

 seek, why it may not as well penetrate D at first to begin the descent, as after- 

 wards to pursue it; and why not as well begin the descent when the quicksilver 

 is well cleansed of air, as when it is not so ; and why also, if the pure air do 

 freely enter at D, it does not presently fall ; or, if not freely, why, when it does 

 fall, it falls suddenly and not leisurely from D to I; especially since so small a 

 weight as DH of pure air (for the grosser cannot enter,) is very inconsidei-able ; 

 if not at all, or not freely pressed by that incumbent on D; and the adhesion 

 not considerably less, by being separated only at the top, while it yet continues 

 to touch the sides. 



I am apt therefore, as heretofore, to ascribe the cause of this phenomenon to 

 the spring that is in air, and the w^ant thereof in quicksilver. For that in air 

 there is a spring or elasticity, is now undoubted; but in water cleansed of air, 

 though many experiments have been attempted to that purpose, it has not yet 

 been found that there is any : and I am apt to think the like of quicksilver ; 

 though i do not know that this has been yet so rigorously examined. Now 

 supposing that matter, being at rest, will so continue till it be put in motion by 

 some force ; this force may be either that of percussion from some body already 

 in motion, which is the case when the quicksilver falls by shaking or striking 

 the tube; or of pulsion, from a contiguous body beginning to move, as by the 

 expansion of some adjacent spring, which is the case, when the springy parts of 

 the air, either left in unpurged, or re-admitted in the quicksilver, by expand- 

 ing themselves, put the quicksilver in motion ; or some conatus or endeavour of 

 its own, such as is that of a spring; and therefore if water and quicksilver be 

 not such, they will not on this account put themselves in motion. 



Gravity or heaviness is, I know reputed to be such a conatus or proneness to 

 move downwards, and so to put itself in motion : and the wonder at present is, 

 why it does not so here. But if this which we call gravity, should chance to be 

 not a positive quality or conatus originally of itself, but only the effect of some 



