New experiments physico-mechanical, touching the air

CHAP. V. Two new Experiments touching the measure of the Force of the Spring of Air compress'd and dilated.

THE other thing that I would have considered touching our Adversaries Hypothesis is, That it is needless. For whereas he denies not that the Air has some Weight and Spring, but affirms that it is very insufficient to perform * 1.1 such great matters as the counterpoising of a Mercurial Cy∣linder of 29. Inches, as we teach that it may: We shall now endeavour to manifest by Experiments purposely made, that the Spring of the Air is capable of doing far more than 'tis necessary for us to ascribe to it, to salve the Phaenomena of the Torricellian Experiment.

We took then a long Glass-Tube, which by a dexterous hand and the help of a Lamp was in such a manner crooked at the bottom, that the part turned up was almost parallel to the rest

of the Tube, and the Orifice of this shorter leg of the Siphon (if I may so call the whole Instrument) being Hermetically seal'd, the length of it was divided into Inches, (each of which was subdivided into eight parts) by a streight list of paper, which containing those Divisions was carefully pasted all along it: then putting in as much Quicksilver as served to fill the Arch or bended part of the Siphon, that the Mercury standing in a level might reach in the one leg to the bottom of the divided paper, and just to the same height or Horizontal line in the o∣ther; we took care, by frequently inclining the Tube, so that the Air might freely pass from one leg into the other by the sides of the Mercury, (we took (I say) care) that the Air at last included in the shorter Cylinder should be of the same laxity with the rest of the Air about it. This done, we began to pour Quicksilver into the longer leg of the Siphon, which by its weight pressing up that in the shorter leg, did by degrees streighten the included Air: and continuing this pouring in of Quicksilver till the Air in the shorter leg was by condensation reduced to take up but half the space it possess'd (I say, possess'd, not fill'd) before; we cast our eyes upon the longer leg of the Glass, on which was likewise pasted a list of paper carefully divided into Inches and parts, and we observed, not without delight and satisfaction, that the Quicksilver in that longer part of the Tube was 29. Inches higher than the other. Now that this Observation does both very well agree with and confirm our Hypothesis, will be easily discerned by him that takes notice what we teach, and Monsieur Paschal and our English friends Experiments prove, that the greater the weight is that leans up∣on the Air, the more forcible is its endeavour of Dilatation, and consequently its power of resistance, (as other Springs are stronger when bent by greater weights.) For this being considered, it will appear to agree rarely-well with the Hypothesis, that as according to it the Air in that degree of density and correspon∣dent measure of resistance to which the weight of the incum∣bent Atmosphere had brought it, was able to counterbalance and resist the pressure of a Mercurial Cylinder of about 29.

Inches, as we are taught by the Torricellian Experiment; so here the same Air being brought to a degree of density about twice as great as that it had before, obtains a Spring twice as strong as formerly. As may appear by its being able to sustain or resist a Cylinder of 29 Inches in the longer Tube, together with the weight of the Atmospherical Cylinder, that lean'd upon those 29 Inches of Mercury; and, as we just now inferr'd from the Torricellian Experiment, was equiva∣lent to them.

We were hindered from prosecuting the trial at that time by the casual breaking of the Tube. But because an accurate Experiment of this nature would be of great importance to the Doctrine of the Spring of the Air, and has not yet been made (that I know) by any man; and because also it is more uneasie to be made than one would think, in regard of the difficulty as well of procuring crooked Tubes fit for the pur∣pose, as of making a just estimate of the true place of the Pro∣tuberant Mercury's surface; I suppose it will not be unwelcome to the Reader, to be informed that after some other trials, one of which we made in a Tube whose longer leg was per∣pendicular, and the other, that contained the Air, parallel to the Horizon, we at last procured a Tube of the Fi∣gure exprest in the Scheme; which Tube, though * 1.2 of a pretty bigness, was so long, that the Cylinder whereof the shorter leg of it consisted admitted a list of Pa∣per, which had before been divided into 12 Inches and their quarters, and the longer leg admitted another list of Paper of divers foot in length, and divided after the same manner: then Quicksilver being poured in to fill up the bended part of the Glass, that the surface of it in either leg might rest in the same Horizontal line, as we lately taught, there was more and more Quicksilver poured into the longer Tube; and notice being watchfully taken how far the Mercury was risen in that longer Tube, when it appeared to have ascended to any of the divi∣sions in the shorter Tube, the several Observations that were thus successively made, and as they were made set down, af∣forded us the ensuing Table.

A Table of the Condensation of the Air

A	A	B	C	D	E
48	12	00	Added to 29⅛ makes	29 2/16	29 2/16
46	11½	01 7/16	30 9/16	30 6/16
44	11	02 13/16	31 15/16	31 12/16
42	10½	04 6/16	33 8/16	33 1/7
40	10	06 3/16	35 5/16	35--
38	9½	07 14/16	37--	36 15/19
36	9	10 2/16	39 5/16	38⅞
34	8½	12 8/16	41 10/16	41 2/17
32	8	15 1/16	44 3/16	43 11/16
30	7½	17 15/16	47 1/16	46⅗
28	7	21 3/16	50 5/16	50--
26	6½	25 3/16	54 5/16	53 10/13
24	6	29 11/16	58 13/16	58 2/8
23	5 ¾	32 3/16	61 5/16	60 18/23
22	5½	34 15/16	64 1/16	63 6/11
21	5 ¼	37 15/16	67 1/16	66 4/7
20	5	41 9/16	70 11/16	70--
19	4 ¾	45--	74 2/16	73 11/19
18	4½	48 12/16	77 14/16	77 ⅔
17	4 ¼	53 11/16	82 12/16	82 4/17
16	4	58 2/16	87 14/16	87 ⅜
15	3 ¾	63 15/16	93 1/16	93 ⅕
14	3 ½	71 5/16	100 7/16	99 6/7
13	3 ¼	78 11/16	107 13/16	107 7/13
12	3	88 7/16	117 9/16	116 4/8

AA. The number of equal spaces in the shorter leg, that contained the same parcel of Air diversly extended.

B. The height of the Mercurial Cylin∣der in the longer leg, that compress'd the Air into those dimensions.

C. The height of a Mercurial Cylinder that counterbalanc'd the pressure of the Atmosphere.

D. The Aggregate of the two last Co∣lumns B and C, exhibiting the pressure sustained by the included Air.

E. What that pressure should be accor∣ding to the Hypothesis, that supposes the pressures and expansions to be in reciprocal proportion.

For the better understanding of this Experiment it may not be amiss to take notice of the following particulars:

1. That the Tube being so tall that we could not conveni∣ently make use of it in a Chamber, we were fain to use it on a pair of Stairs, which yet were very lightsome, the Tube be∣ing for preservations sake by strings so suspended, that it did scarce touch the Box presently to be mentioned.

2. The lower and crooked part of the Pipe was placed in a square wooden Box, of a good largness and depth, to prevent

the loss of the Quicksilver that might fall aside in the transfu∣sion from the Vessel into the Pipe, and to receive the whole Quicksilver in case the Tube should break.

3. That we were two to make the Observation together, the one to take notice at the bottom how the Quicksilver rose in the shorter Cylinder, and the other to pour in at the top of the longer, it being very hard and troublesome for one man alone to do both accurately.

4. That the Quicksilver was poured in but by little and lit∣tle, according to the direction of him that observed below, it being, far easier to pour in more, than to take out any in case too much at once had been poured in.

5. That at the beginning of the Operation, that we might the more truly discern where the Quicksilver rested from time to time, we made use of a small Looking-glass, held in a convenient posture to reflect to the eye what we desired to discern.

6. That when the Air was so compress'd, as to be crouded into less than a quarter of the space it possess'd before, we try∣ed whether the cold of a Linen Cloth dipp'd in Water would then condense it. And it sometimes seemed a little to shrink, but not so manifestly as that we dare build any thing upon it. We then tried likewise whether heat would notwithstanding so forcible a compressure dilate it, and approaching the flame of a Candle to that part where the Air was pent up, the heat had a more sensible operation than the cold had before; so that we scarce doubted but that the expansion of the Air would, notwithstanding the weight that opprest it, have been made con∣spicuous, if the fear of unseasonably breaking the Glass had not kept us from increasing the heat.

Now although we deny not but that in our Table some par∣ticulars do not so exactly answer to what our formerly-intima∣ted Hypothesis might perchance invite the Reader to expect; yet the Variations are not so considerable, but that they may probably enough be ascribed to some such want of exactness as in such nice Experiments is scarce avoidable. But for all

that, till further trial hath more clearly informed me, I shall not venture to determine whether or no the intimated Theo∣ry will hold universally and precisely, either in Condensation of Air, or Rarefaction: All that I shall now urge being, That however, the trial already made sufficiently proves the main thing for which I here alledge it; since by it 'tis evident, that as common Air when reduc'd to half its wonted extent, obtain∣ed near about twice as forcible a Spring as it had before; so this thus comprest Air being further thrust into half this nar∣row room, obtained thereby a Spring about as strong again as that it last had, and consequently four times as strong as that of the common Air. And there is no cause to doubt, that if we had been here furnisht with a greater quantity of Quick∣silver and a very strong Tube, we might by a further compres∣sion of the included Air have made it counterbalance the pres∣sure of a far taller and heavier Cylinder of Mercury. For no man perhaps yet knows how near to an infinite compression the Air may be capable of, if the compressing force be com∣petently increas'd. So that here our Adversary may plainly see that the Spring of the Air, which he makes so light of, may not only be able to resist the weight of 29 Inches, but in some cases of above an hundred Inches of Quicksilver, and that without the assistance of his Funiculus, which in our present case has nothing to do. And to let you see that we did not (a little above) inconsiderately mention the weight of the in∣cumbent Atmospherical Cylinder as a part of the weight re∣sisted by the imprisoned Air, we will here annex, that we took care, when the Mercurial Cylinder in the longer leg of the Pipe was about an hundred Inches high, to cause one to suck at the open Orifice; whereupon (as we expected) the Mercu∣ry in the Tube did notably ascend. Which considerable Phae∣nomenon cannot be ascribed to our Examiners Funiculus, since by his own confession that cannot pull up the Mercury, if the Mercurial Cylinder be above 29 or 30 Inches of Mercury. And therefore we shall render this reason of it, That the pressure of the incumbent Air being in part taken off by its expanding it¦self

into the Suckers dilated Chest; the' imprison'd Air was thereby enabled to dilate it self manifestly, and repel the Mercury that comprest it, till there was an equality of force betwixt the strong Spring of that comprest Air on the one part, and the tall Mercurial Cylinder, together with the contigu∣ous dilated Air, on the other part.

Now, if to what we have thus delivered concerning the com∣pression of Air we add some Observations concerning its spon∣taneous Expansion, it will the better appear how much the Phaenomena of these Mercurial Experiments depend upon the differing measures of strength to be met with in the Air's Spring, according to its various degrees of compression and Laxity. But, before I enter upon this subject, I shall readily acknowledge that I had not reduc'd the trials I had made about measuring the Expansion of the Air to any certain Hypothesis, when that ingenious Gentleman Mr. Richard Townley was plea∣sed to inform me, that having by the perusal of my Physico-Mechanical Experiments been satisfied that the Spring of the Air was the cause of it, he had endeavoured (and I wish in such attempts other ingenious men would follow his example) to supply what I had omitted concerning the reducing to a pre∣cise estimate how much Air dilated of it self loses of its Elasti∣cal force, according to the measures of its Dilatation. He added, that he had begun to set down what occurred to him to this purpose in a short Discourse, whereof he afterwards did me the favour to shew me the beginning, which gives me a just Curiosity to see it perfected. But, because I neither know, nor (by reason of the great distance betwixt our places of re∣sidence) have at present the opportunity to enquire, whether he will think fit to annex his Discourse to our Appendix, or to publish it by it self, or at all; and because he hath not yet, for ought I know, met with fit Glasses to make an any-thing-ac∣curate Table of the Decrement of the force of dilated Air; our present design invites us to present the Reader with that which follows, wherein I had the assistance of the same person that I took notice of in the former Chapter, as having writ∣ten

something about Rarefaction: whom I the rather make mention of on this occasion, because when he first heard me speak of Mr. Townley's suppositions about the proportion where∣in Air loses of its Spring by Dilatation, he told me he had the year before (and not long after the publication of my Pneuma∣tical Treatise) made Observations to the same purpose, which he acknowledged to agree well enough with Mr. Townley's Theory: And so did (as their Author was pleased to tell me) some Trials made about the same time by that Noble Virtuoso and eminent Mathematician the Lord Brouncker, from whose further Enquiries into this matter, if his occasions will allow him to make them, the Curious may well hope for something very accurate.

A Table of the Rarefaction of the Air.

A	B	C	D	E
1	00 0/0	Subtracted from 29¾ leaves.	29¾	29¾
1 ½	10 ⅝	19 ⅛	19 ⅚
2	15 ⅜	14 ⅜	14 ⅞
3	20 2/8	9 4/8	9 15/12
4	22 ⅝	7 ⅛	7 7/16
5	24 ⅛	5 ⅝	5 19/20
6	24 ⅞	4 ⅞	4 23/24
7	25 4/8	4 2/8	4 ¼
8	26 0/0	3 6/8	3 23/32
9	26 ⅜	3 ⅜	3 11/36
10	26 6/8	3 0/0	2 39/40
12	27 ⅛	2 ⅝	2 23/48
14	27 4/8	2 2/8	2 ⅛
16	27 6/8	2 0/0	1 55/64
18	27 ⅞	1 ⅞	1 47/72
20	28••	1 6/8	1 9/80
24	28 2/8	1 4/8	1 23/96
28	28 ⅜	1 ⅜	1 1/16
32	28 4/8	1 2/8	0 119/128

A. The number of equal spaces at the top of the Tube, that contained the same parcel of Air.

B. The height of the Mercurial Cylinder, that together with the Spring of the included Air counterbalanced the pressure of the At∣mosphere.

C. The pressure of the Atmosphere.

D. The Complement of B to C, exhibiting the pressure sustained by the included Air.

E. What that pressure should be according to the Hypothesis.

To make the Experiment of the debilitated force of expan∣ded Air the plainer, 'twill not be amiss to note some particu∣lars,

especially touching the manner of making the Trial; which (for the reasons lately mention'd) we made on a light∣some pair of Stairs, and with a Box also lin'd with Paper to receive the Mercury that might be spilt. And in regard it would require a vast and in few places procurable quantity of Quick∣silver, to imploy vessels of such kind as are ordinary in the Tor∣ricellian Experiment, we made use of a Glass-Tube of about six foot long, for that being Hermetically sealed at one end, serv'd out turn as well as if we could have made the Experi∣ment in a Tub or Pond of seventy Inches deep.

Secondly, We also provided a slender Glass-Pipe of about the bigness of a Swans Quill, and open at both ends: All along which was pasted a narrow list of Paper divided into Inches and half quarters.

Thirdly, This slender Pipe being thrust down into the grea∣ter Tube almost fill'd with Quicksilver, the Glass helpt to make it swell to the top of the Tube, and the Quicksilver getting in at the lower orifice of the Pipe, fill'd it up till the Mercury in∣cluded in that was near about a level with the surface of the surrounding Mercury in the Tube.

Fourthly, there being, as near as we could guess, little more than an Inch of the slender Pipe left above the surface of the re∣stagnant Mercury, and consequently unfill'd therewith, the prominent orifice was carefully clos'd with sealing Wax melted; after which the Pipe was let alone for a while, that the Air di∣lated a little by the heat of the Wax, might upon refrigeration be reduc'd to its wonted density. And then we observ'd by the help of the above-mentioned list of Paper, whether we had not included somewhat more or somewhat less than an Inch of Air, and in either case we were fain to rectifie the error by a small hole made (with a heated Pin) in the Wax, and af∣terwards clos'd up again.

Fifthly, Having thus included a just Inch of Air, we lifted up the slender Pipe by degrees, till the Air was dilated to an Inch, an Inch and an half, two Inches, &c. and observed in Inches and Eighths, the length of the Mercurial Cylinder,

which at each degree of the Air's expansion was impell'd above the surface of the restagnant Mercury in the Tube.

Sixthly, The Observations being ended, we presently made the Torricellian Experiment with the above-mention'd great Tube of six foot long, that we might know the height of the Mercurial Cylinder, for that particular day and hour; which height we found to be 29¾ Inches.

Seventhly, Our Observations made after this manner fur∣nish'd us with the preceding Table, in which there would not probably have been found the difference here set down betwixt the force of the Air when expanded to double its former dimen∣sions, and what that force should have been precisely accord∣ing to the Theory, but that the included Inch of Air receiv'd some little accession during the Trial; which this newly-men∣tion'd difference making us suspect, we found by replunging the Pipe into the Quicksilver, that the included Air had gain'd about half an eighth, which we guest to have come from some little Aëreal bubbles in the Quicksilver, contain'd in the Pipe (so easie is it in such nice Experiments to miss of exactness.) We try'd also with 12 Inches of Air shut up to be dilated; but being then hindred by some unwelcome avocations to prose∣cute those Experiments, we shall elsewhere, out of other Notes and Trials (God permitting) set down some other accurate Ta∣bles concerning this matter. By which possibly we may be assisted to resolve whether the Atmosphere should be look'd upon (as it usually is) as a limited and bounded Portion of the Air; or whether we should in a stricter sense than we did before, use the Atmosphere and Aëreal part of the World for almost equi∣valent terms; or else whether we should allow the word At∣mosphere some other notion in relation to its Extent and Li∣mits; (for as to its Spring and Weight, these Experiments do not question, but evince them.) But we are willing, as we said, to refer these matters to our Appendix, and till then to retain our wonted manner of speaking of the Air and Atmo∣sphere. In the mean time (to return to our last-mention'd Ex∣periments) besides that so little a variation may be in great

part imputed to the difficulty of making Experiments of this nature exactly, and perhaps a good part of it to something of inequality in the cavity of the Pipe, or even in the thickness of the Glass; besides this, I say, the proportion betwixt the seve∣ral pressures of the included Air undilated and expanded, especi∣ally when the Dilatation was great (for when the Air swell'd but to four times its first extent, the Mercurial Cylinder, though of near 23 Inches, differ'd not a quarter of an Inch from what it should have been according to Mathematical exactness) the proportion, I say, was sutable enough to what might be expected, to allow us to make this reflexion upon the whole, That whether or no the intimated Theory will hold exactly, (for about that, as I said above, I dare determine nothing reso∣lutely till I have further considered the matter) yet since the Inch of Air when it was first included was shut up with no other pressure than that which it had from the weight of the incumbent Air, and was no more comprest than the rest of the Air we breathed and moved in; and since also this Inch of Air, when expanded to twice its former dimensions, was able with the help of a Mercurial Cylinder of about 15 Inches to coun∣terpoise the weight of the Atmosphere, which the weight of the external Air gravitating upon the restagnant Mercury was able to impell up into the Pipe, and sustain above twenty eight Inches of Mercury when the internal Air by its great expan∣sion had its Spring too far debilitated to make any considerable (I say considerable, for it was not yet so dilated as not to make some) resistance: Since, I say, these things are so, the free Air here below appears to be almost as strongly comprest by the weight of the incumbent Air as it would be by the Weight of a Mercurial Cylinder of twenty eight or thirty Inches; and consequently is not in such a state of laxity and freedom as men are wont to imagine; and acts like some mechanical Agent, the decrement of whose force holds a stricter pro∣portion to its increase of dimension, than has been hitherto taken notice of.

I must not now stand to propose the several reflexions that may be made upon the foregoing Observations touching the Compression and Expansion of Air; partly because we could scarce avoid making the Historical part somewhat prolix; and partly because I suppose we have already said enough to shew what was intended, namely, that to solve the Phaenomena there is not of our Adversaries Hypothesis any need: the evincing of which will appear to be of no small moment in our present Controversie, to him that considers, that the two main things that induced the Learned Examiner to reject our Hypothesis are, that Nature abhors a Vacuum, and that though the Air have some Weight and Spring, yet these are insufficient to make out the known Phaenomena; for which we must therefore have re∣course to his Funiculus. Now as we have formerly seen, that he has not so satisfactorily disproved as resolutely rejected a Vacuum, so we have now manifested that the Spring of the Air may suffice to perform greater things than what our Expli∣cation of the Torricellian Experiments and those of our Engine obliges us to ascribe to it. Wherefore since besides the seve∣ral difficulties that incumber the Hypothesis we oppose, and especially its being scarce, if at all, intelligible, we can add that it is unnecessary; we dare expect that such Readers as are not byass'd by their reverence for Aristotle or the Peripatetick Schools, will hardly reject an Hypothesis which, besides that it is very intelligible, is now prov'd to be sufficient, only to imbrace a Doctrine that supposes such a rarefaction and conden∣sation, as many famous Naturalists rejected for its not being comprehensible, even when they knew of no other way (that was probable) of solving the Phaenomena wont to be expli∣cated by it.

Notes

• * 1.1

  Pag. 11.
• * 1.2

  See the 5. Figure.