A continuation of new experiments physico-mechanical, touching the spring and weight of the air and their effects. The I. part whereto is annext a short discourse of the atmospheres of consistent bodies / written by way of letter to the right honourable the Lord Clifford and Dungarvan by the honourable Robert Boyle ...

EXPERIMENT X.

Containing two or three Tryals of the force of the Spring of our Air uncompress'd upon stable and even solid Bodies, (whereto tis external.)

IN prosecution of the Enquiry propos'd in the Title, we made (among others) the following Tryals.

The I. TRYAL.

1. WE took the Brass-hoop, mention'd in the 5^th Experi∣ment, (whose Diameter is somewhat above 3. Inches,) and having caus'd a Glazier to cut some Plates of Glass, such as are used for making the Quarrels of Windows, till he had brought them to a Size, & a roundness fit to serve for Covers to that brass-hoop, we carefully fasten'd one of them with Cement to the up∣per Orifice of the Hoop or Ring, and then cementing the lower Orifice to the Engine, so that the Vessel, compos'd of the Me∣tal and Glass, serv'd for a small Receiver; we whelm'd over it a large and strong Receiver, which we also fasten'd on to the En∣gine with Cement after the usual manner. By which Contri∣vance it was necessary, that when the Pump was set on work, the included Receiver (of Brass and Glass) should have its Air with∣drawn, and yet the Air in the larger Receiver should not be

pump'd out but by breaking through the Glass, so that the inter∣nal Air of the Metalline Receiver (as we may call it for distincti∣ons sake) being pump'd out, the Glass Plate, that made part of that Receiver, must lye expos'd to the pressure of the Ambient Air shut up in the other Receiver, without having the former as∣sistance of the now withdrawn Air to resist the Pressure; where∣fore, as we expected, at the first or second Exuction of the Air, included in the small metalline Receiver, the Glass-plate was, by the Pressure of the incumbent Air, contain'd in the great Recei∣ver, broken into an 100 pieces, which were beaten inwards into the Cavity of the Hoop.

The II. Tryal.

2. This done, to shew that there needed not the Spring of so great a quantity of included Air to break such Glasses, we took a∣nother Roundish one, which, though wide enough at the Orifice to cover the Brass Ring & the new Glass-plate that we had cemen∣ted on it, was yet so low, that we estimated it to hold but a 6^th part of what the large Receiver, formerly imploy'd, is able to contain; and having whelm'd this smaller vessel, which was shap'd like those Cups they call Tumblers, over the Metalline Recei∣ver, and well fasten'd it to the Engine with Cement, we found that though this External Receiver had a great part of its Cavity fill'd by the included one, yet when this Internal one was exhau∣sted by an Exuction or two, the Spring of the little Air that re∣main'd, was able to break the Plate into a multitude of frag∣ments.

The III. Tryal.

3. Because the Glass-Plates hitherto mention'd seem'd not so thick, but that the Pressure of the included Air might be able to give considerabler Instances of its Force; in stead of the Metalline Receivers hitherto employed, we took a square Bottle of Glass, which we judg'd to be able to contain about a Pint (or Pound) of Water, and which had been provided to keep subtle Chymical Liquors in, for which use we are not wont to choose weak ones.

This we inverted, and apply'd to the Engine as a Receiver, over which we whelm'd the large Receiver formerly mention'd; and having cemented it on, as in the foregoing Experiments, we set the Pump on work to empty the internal Receiver, (or square Bottle,) by which means the withdrawing of the Air, and the fi∣gure of the vessel (which was inconvenient for resisting) suffer'd the Pressure of the Air included in the external Receiver to crush the viol into a great number of pieces.

And to vary this Experiment, as we did that of breaking the metalline Receivers, we took another Glass of the shape and about the bigness of the former, and having apply'd it to the Engine as before, and cover'd it with a Receiver that was little higher than it self, we found, that upon the exhaustion of the Air the second square Glass was likewise broken into many fragments, some of which were of so great a thickness, as mov'd some wonder, that the bare Pressure of the Air was able to break such a vessel, though probably the Cracks, that reacht to them, were begun in much weaker parts of the Glass.

NB. 1. The bottoms and the necks of both these square Bottles were entire enough; by which it seem'd probable, that the vessels had been broken by the Pressure of the Air against the Sides, which were not onely thinner than the parts above named, but expos'd a larger Superficies to the lateral Pressure of the Air, than to the perpendicular.

2. We observ'd in one of the two last Experiments, that the Vessel did not break presently upon the last Exuction that was made of the included Air, but a considerable time after, which it seems was requisite to allow the comprest parts of the Glass time to change their places; and this Phaenomenon I therefore mention, because the same thing that here happen'd in the breaking a Glass inwards by the Spring of the Air, I elsewhere observ'd to have happen'd in breaking a Glass outwards by the same Spring.

3. To confirm, that it is the Spring of the External Receivers Air that is the Agent in those Fractures of Glasses, and to prevent

or remove some scruples, we thought fit to make this variation in the Experiment. We applyed a Plate of Glass, just like those formerly mentioned, to the Brass-hoop; but in the cementing of it on, we plac'd in the thickness of the Cement a small Pipe of Glass of about an Inch long, whose Cavity was not so big as that of a Straw, and which being left open at both the ends might serve for a little Channel, through which the Air might pass from the External Receiver to the Internal; over This we whelm'd one of the small Receivers above mentioned, & then, though we set the Pump on work much longer then would have needed if this litle Pipe had not been made use of, we found, as we expected, that the Internal Receiver continued entire, because the Air, whose Spring should have broken it, having liberty to pass through the Pipe, and consequently to expand it self into the place deserted by the Air pump'd out, did by that Expansion weaken its Spring too much, to retain strength enough to break the Metalline (or Inter∣nal) Receiver.

But here tis to be noted, that either the Pipe must be made bigger than that lately mentioned, or the Exuction of the Air must not be made by the Pump as nimbly as we can, or other∣wise the Plate of Glass may be broken notwithstanding the Pipe; because the Air contain'd in the External Receiver, having a force much greater than is necessary to break such a Plate, it may well happen (as I have sometimes found it do) that if the Air be hastily drawn out of the Internal Receiver, that Air, which should succeed in its room, cannot get fast enough out of that external Receiver through so small a Pipe, and the Air remaining in that external Receiver will yet retain a Spring strong enough to break the Glass. To illustrate which, I shall propose this Experiment, That sometimes, when I have at the flame of a Lamp caus'd Glass Bubbles to be blown with exceeding slender Stems, if they were nimbly remov'd out of the flame whilst they were ignited, they would according to my conjecture be either broken, if they cool'd too fast; or compress'd inward, if they long enough retain'd the

Softness they had given them by Fusion. For the Air in the Bubble being exceedingly rarified and expanded, whilst the Glass is kept in the flame, and coming to cool hastily when remov'd from thence, looses upon refrigeration the Spring the heat had given it, and so, if the External Air cannot press in fast enough through the too slender Pipe, there will not get in Air enough to resist the Pressure of the Atmosphere, and therefore if this Pres∣sure find the Bubble yet soft, it will press it a little inwards, and either flatten it, or make a dimple in it, though the Orifice of the Pipe be left open.