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BEcause the Air being invisible, it is not always easie to know whether it be sufficiently pump'd out of the Receiver that was to be exhausted; we thought it would be very convenient to have some Instrument within the Receiver, that might serve
for a Gage, or Standard; whereby to judge whether or not it were sufficiently exhausted.
To this purpose divers Expedients were thought on, and some of them put in practise; which, though not equally commodious, may yet all of them be usefully imploy'd, one on this occasion, and another on that.
The First (if I misremember not) that I propos'd, was a Blad∣der, (which may be greater or less, according to the Size of the Vessel it is to serve for) to be very strongly tied at the neck, after having had onely so much Air left in the folds of it, as may serve to blow up the Bladder to its full dimensions, when the Receiver is very well exhausted, and not before. But though Your Lord-ship will hereafter find that I yet make use of small Bladders on certain occasions, in which they are peculiarly convenient, yet in many cases they do, when the Glasses are well exhausted, take up too much room in them, and hinder the Objects, included in the Receiver, from being observ'd from all the sides of it.
Another sort of Gage was made with Quick-silver, pour'd into a very short Pipe, which was afterwards inverted into a litle Glass of stagnant Quick-silver, according to the manner of the Torri∣cellian Experiment. For this Pipe being but a very few inches long, the Mercury in it would not begin to descend, till a very Great proportion of Air was pump'd out of the Receiver; because till then, the Spring of the remaining Air would be strong e∣nough to be able to keep up so short a Cylinder of Mercury. And this kind of Gage is no bad one. But because, to omit some o∣ther litle inconveniences, it cannot easily be suspended, (which in divers Experiments 'tis fit the Gage should be,) and the Mercury in it is apt to be too much shaken by the motion of the Engine, there was another kind of Gage by some Ingenious man (who e∣ver he were) substituted in its place, consisting of a kind of Si∣phon, whose shorter leg hath belonging to it a large Bubble of Glass, most commonly made use of at an Illustrious meeting of Virtuosi; where Your Lordship having seen it, I shall not need to describe it more particularly.
But none of the Gages I had formerly us'd, nor even this last, having the conveniences that some of my Experiments require; I was fain to devise another, which is That I most make use of, as having advantages, some or other of which each of the Gages al∣ready mentioned wants; for even that with Spirit of Wine, not to mention lesser disadvantages, hath a Bubble too Great to let it be useful in vessels so slender, as for some purposes I divers times imploy; and this short Cylinder of so light a Liquor as spirit of Wine, makes the subsidence of the Liquor be indeed a good sign that the Receiver is well exhausted, but gives us not an account what Quantity of Air may be in the Receiver, 'till it be arriv'd at that great measure of Rarefaction; and the same Liquor, being upon a very small leak (such as would not be prejudicial to many Experiments) impell'd up to the top of the Gage, we cannot af∣terwards by this Instrument take any measure of the Air that gets in at the Leak. But now there are divers Experiments where I desire to see the Phaenomena that will happen, not onely (or per∣haps not at all) upon the uttermost Exhaustion of the Air, but when the Pressure of it is withdrawn to such or such a measure, and also when the Air is gradually readmitted.
To make the Gage we are speaking of, take a very slender and Cylindrical Pipe of Glass, of 6, 8, 10, or more Inches in length, and not so big as a Goose-quill, (but such as we imploy for the Stems of seal'd Weather Glasses,) and having at the flame of a Lamp melted it, but not too near the middle, to make of it by bending it a Siphon, whose two Legs are to be not onely parallel to one another, but as litle distant any where from one another as conveniently may be. In one (which is usually the longer) of these Legs, there is to be left at the top, either half an inch, or a whole inch, or more or less than either, (according to the length of the Gage, or the scope of the Experimenter) of Air in its natu∣ral state, neither rarefied, nor condens'd; the rest of the longer leg, and as great a part of the shorter as shall be thought fit, being to be fill'd with Quick-silver. This done, there may be Marks
plac'd at the outside of the longer (or sealed) leg, whereby to mea∣sure the Expansion of the Air included in the same leg, and these marks may be either litle Glass Knubs, about the bigness of Pins heads, fasten'd by the help of a Lamp at certain distances to the longer leg of the Siphon, or else the divisions of an Inch made on a list of Paper, and pasted on either to the Siphon it self, or to the slender Frame, which on some occasions we fasten the Gage to.
This Instrument being convey'd into a Receiver, (which for expedition sake we choose as small as will serve the turn,) the Air is to be very diligently pump'd out, and then notice is to be ta∣ken to what part of the Gage the Mercury is deprest, that we may know, when we shall afterwards see the Mercury driven so far, that the Receiver, the Gage is plac'd in, is well exhausted. And if it be much desired to know more accurately (for one may arrive pretty near the truth by Guess) what stations of the Mer∣cury in the Gage are answerable to the degrees of the Rarefaction of the Air in the Receiver; that may be compassed either by Calculation, (which is not so easie, and supposes some Hypotheses,) or (though not without some trouble) by letting in the water as often as is necessary, into a Receiver, whose intire capacity is first measured, and in which there may be Marks made to shew when the water to be let in shall fill a fourth part, or half, or three quarters &c. of the Cavity. For if (for instance) when the Quick-silver in the Gage is deprest to such a Mark, you let in the water, and that Liquor appears to fill a fourth part of the Receiver, you may conclude, that about a 4th part of the Air was pump'd out, or that a 4th part of the Spring, that the whole included Air had, was lost by the Exhaustion, when the Quick silver in the Gage was at the Mark above mentioned; & if the admitted water do con∣siderably either fall short of, or exceed the quantity you expe∣cted, you may the next time let in the water either after the Mer∣cury has a litle past the former Mark, or a litle before it is arriv'd at it. And when once you have this way obtain'd one pretty
long and accurate Gage, you will not need to take so much pains to make others, since you may divide them by the help of that one; for this being plac'd with any other in a small Receiver, when the Mercury in the Standard-Gage (if I may so call it) is deprest to any of the determinate divisions obtain'd by observation, you may thence conclude how much the Air in the Receiver is rare∣fied, and consequently by taking notice of the place where the Mercury rests in the other Gage, you may determine what degree of Exhaustion in a Receiver is denoted by that station of the Mer∣cury in this Gage.
Perhaps I need not tell your Lordship that the Ground of this contrivance was, that whereas in divers other Gages, when the Pump came to be obstinately ply'd, the Expansion of the inclu∣ded Air would be so great, that it would either drive out the Li∣quor, especially if it were light, or in part make an escape through it: I judg'd that in such an Instrument, as that newly describ'd, those inconveniences would be avoided, because that the more the Air should come to be dilated, the greater weight of Quick-silver it would in the shorter Leg have to raise, which would suf∣ficiently hinder it from making that heavy liquor run over; and the same ponderousness of the Liquor, together with the slender∣ness of the Pipe, would likewise hinder the included Air from get∣ting through in Bubbles.
NB. 1. For most Experiments, where exact measures are not required, it will not be so necessary to mark the Gage at any o∣ther station of the Quick-silver then that which tis brought to by the Exhaustion of the Receiver, for by that alone we may know when the Air is well pump'd out of the Receiver, wherein the Gage is included: and when one is a litle us'd to some particular Gage, one may by the subsidence of the Mercury guess at the de∣gree of the Airs rarefaction, so near as may serve the turn in such Experiments. But when this Instrument is to be us'd about nice Tryals, where it may be thought requisite to have it divided ac∣cording to one of the ways formerly proposed, it will on divers
occasions be more secure (in case the maker of the Gage has skill to do it,) to put to the Divisions rather by litle Knubs of Glass, than by Paper; because this will on such occasions be in danger either to be rubb'd off, or wetted. And if Glass-marks be us'd, it will be convenient that every fifth, or tenth, or such Ordinal number as shall be judg'd fit, be made of Glass of a differing co∣lour, for distinction sake, & the more easie reckoning. We some∣times for a need apply, in stead of these Glass-knubs, little marks of hard sealing Wax, which will not be injur'd by moisture, as those Papers will that are pasted on; but these of Wax, though in many cases useful, are not comparable to the other in all, since if they be very small, they are easily rubb'd off, and if large, they make not the Division exact enough, and often hide the true place of the Quick-silver.
I shall here about the Mercurial Gages add onely this Hint, that what I propos'd to my self in that Contrivance, was not one∣ly to estimate the Air pump'd out of the Receiver, or that remai∣ning in it; but also, by the help of this Instrument (as elsewhere by another Experiment) to measure (somewhat near) the strength of the Spring of rarefied Air, according to its several degrees of Rarefaction; and by this Observation, in concurrence with other things, I hoped we might (according to what I have elsewhere in∣sinuated) be assisted to estimate, by the Cylinder of Mercury rais'd in the open leg, the Expansion of the Air included in the sealed leg: but of these things I design'd in this place to give but an Intimation.
3. That leg of the Gage that includes the Air, may be seal'd up either at the beginning, before the Pipe be bent into a Syphon, or (which is much better) after the following manner. Before you bend the Pipe, draw out the end of it, which you mean to seal, to a short and very slender Thread; then having made the Pipe a Siphon, pour into the leg, which is to remain open, as much Quick-silver as you shall judg convenient, which will rise to an equal height in the other leg; out of which by gently inclining
the Siphon, you may pour out the superfluous Mercury, (if there be any,) and when you see that there is an inch, or half an inch (or what part you design'd to leave for Air) unfill'd with Mercury, next to the end that is to be clos'd; and that the rest of that leg, and as much (as you think fit) of the other is full of Quick-silver, you may, by keeping the Siphon in the same posture, and warily applying the slender Apex above mentioned to the upper part of the flame of a Lamp, blown Horizontal, easily seal up that A∣pex without cracking, or prejudicing the open leg, or considera∣bly injuring the Air hole, that was to be seal'd up in the other. And this sealing of one leg must (as tis evident) keep the Mercu∣ry suspended in it, though it be higher by divers inches than that in the open leg, till the withdrawing of the external Air enable the included, by expanding it self to depress the Mercury in the seal'd leg, and raise it in the open.
4. How the length of these Mercurial Gages is to be varied, according to the Bigness and Shape of the slender Receivers they are to be imploy'd in, and how they may easily be made either to stand upright at the bottom of the Receiver, or be kept hanging in the middle, or near the top of it (as occasion may require,) and how the open end may be made to secure the Mercury, in cases where that is needful, belongs not so properly to this Treatise, as to the Second part of the Continuation; where, if ever I trou∣ble Your Lordship with it, the Usefulness of this sort of Gages, and the Circumstances that may advantage them, will best ap∣pear.
5. There being some Experiments, wherein it is not desir'd that the Receiver should be neer exhausted, but rather that the degrees of the Airs rarefaction, which ought not to be very great, should be well measur'd; we may in such cases make use of Gages shap'd like those hitherto describ'd, but made as long as the Re∣ceiver will well admit, and furnish'd in stead of Quick-silver either with spirit of Wine coloured with Cocheneel, or else with the tincture of red Rose-leaves, drawn onely with common Water,
made shap by a litle either of the Oyl, or the spirit of Vitriol, or of common Salt. For the lightness of these Liquors in compari∣son of Quick-silver will allow the Expansins of the Air included in the Gage to be very manifest, and notable enough, though not half, or perhaps a quarter of the Air be pump'd out of the Receiver.
6. You may also in such cases as these, where the Receiver is large enough, and is not to be quite exhausted, make use of a Mercurial Gage, differing from those above describ'd onely in this, that the shorter leg need not be above an inch, or half an inch long, before it expand it self into a Bubble of about half an inch, or an inch in Diameter; and having at the upper part a very short and slender unseal'd Pipe, at which the Air may get in and out: by which Contrivance you may have this Convenience, that You need not include so much Air, as otherwise would be requi∣site, at the top of the longer Leg, because the Mercury in the shor∣ter cannot, by reason of the breadth of the Bubble, whereinto the Expansion of the Air drives it, be considerably rais'd: Upon which account it becomes more easie to estimate by the Eye the degrees of the included Airs Rarefaction, which may be done almost as easily, as if there were water in stead of Mercury: provided it be remembred, that Quick-silver by reason of its ponderousness, does far more assist the dilatation of the Air, then so much Water would do.
See plate the Figure the