Roy G. Neville Aerospace Corporation El Segundo, California
The Discovery of Boyle's Law, 1661-62
Superficial observations on the chemical and physical properties of air were made in remote antiquit,y. It was rccognized that, air was in some way necessary tto support fire and flame. The fact that air (or gas) possesses pressure was also clearly recognized, though not proved. The Aristotelian notion that "natnre abhors a vacuum" still persisted in the seventeenth century, and it, was this hypothesis that led to many heated discussions between the protagonists of the theory (the plenists) and the antagonists who believed that it was possible t,o create a vacuum. We shall very briefly outline the experiments which led to the overthrow of the Aristotelian hypothesis. I t was not until the mid-17th century that any systematic or scientific investigations of the air were carried out. In 1643, t,he Italian mathematician Evangelists Torricelli (160847) showed that if a glass tube approximately 3 ft long and roughly an inch in diameter, sealed at, one end, was filled with mercury then carefully inverted in a dish of mercury, the mercury in the tube (held vertically) fell to a height of about t,hirty inches above that in the dish. The space above t,he mercurv in the tube was termed the "Torricellian vacuum1' (i). Blaise Pascal (1623-62), the French mat,hematician, reasoned that if the pressure of the air on the mercury in the dish was responsible for supporting the mercury in the tube, then mercury should rise to a lesser height on a mountain top than it does at sea-level. In 1648, Pascal's brother-in-law Perier filled a Torricellian tube on top of the Puy-de-DZtme, a French mountain, and demonstrated that the mercury did indeed rise to a lesser height than it did at the base of the mountain. Although Pascal's book, in which this experiment is described, did not appear until 1663 (2) Boyle heard about Perier's experiment some time in the 1650's. Also, by way of Schott's book of 1657 (S), Boyle learned of Otto von Guericke's air pump and of his many experiments with it (4). Boyle had also heard about the different experiments carried ont in the "Torricellian vacuum" by the Accademia del Cimento in Italy,
although a full account of these crlehrated experiments did riot appear until 1667 ( 5 ) . All this work on the air great,ly interested Boyle. With his mechanically-miuded assistmit Robert Hookc (1635-3703) ( 6 ) , hr designed and constructed an air pump which was superior to that usrd by Gnrrirkc.
'
The pump end occerrorier used by Boyle (from o photograph of on engraving in the 1662 edition of Boyle's "New Experiments"].
Few students in elementary science courses escape being exposed to Boyle's Law. However, the fascinating story behind this great generalization is seldom told in classrooms. To be sure, this information is available in accessible but specialized books on the history of science. In the hope that the tercentenary (1062) of the announcement by Boyle adds further interest to this history, we here present abrief review. It complements another similar essay on the great "Sceptical Chymist" published in theve pages. See: NEVILLE,R. G. 38,106 (1961).
356
/ Journal of Chemical Education
This air pump Boylc termed his "pncumntical engine,'' and by its means he evacuated glass globes and bell jars, and showed that as the air pressurc was reduced abovethemercury in which a Torricellian tube wasstanding the mercury in thr tuhe fell. During the period ca. 16.55-1660, Boyle first proved that the air had wight, and that the height of a Torricellian harometrr
varied from day to day. Although he did not undcrst,aud t,he underlying reasons for these daily variations he concluded that
as well unknown to us, as the effectsare unheeded by us
Boyle and Hooke carried out many experiments using t,he new "pneumatical engine," but a description of these is beyond t.he scope of this article (7).
Experiments Leading to Boyle's Law
Boylc's epic work on the physical behavior of the air may be compared wit,h the advancement of science which followed from the invention of the X-ray tube in the 19th cent,ury, or the cyclotron and atomic pile in this ccnt,ury (7). lcor the 17th century Boyle and Hooke displayed remarkable and unprecedented expcrimcnt,al precision, t,hough it was of course crnde by onr modem standards, since little account was taken of thc t,cmpcrature. We shall now discuss the experiment,s whirh led directly to the discovery of Boyle's Law. In IMO, Boyle published his first scientific work, entit,lpd "Kew Experiments Physico-Mechanicall, Touching the Spring of the Air, and its Effects, . . . .,, (8). He explained the "spring" of the air hy drawing an analogy with a mass of fleece or wool which may he compressed or expanded according to the pressure applied t,o it. In addition t,o the Torricellian experiment in which the barometric height of the mercury was shown to depend on the exterual pressure, Boyle demonstrat,ed that air had pressnre by pumping out a bell-jar containing a partially deflated lamb's bladder which inflated as the bell-jar was evacuated. Boyle showed also that a complet,elyempty bladder did not inflate in a vacuum. Although the "New Experiments" was considered a valuablc publication by most of t,he scientific world, there were t,vo notablc except,ions. First,, the great, Thomas Hobbes of Malmesbury (1588-1679), a political philosopher, pseudo-scient,ist, and Arist,otelian, of cantankerous disposit,ion, wrot,e a work entkled "Dialogus Physicns," lli61 (9),in which he tried to refute several of Boyle's experiments by at,t,emptingto explain t,hem in Aristotelian or plenist terms. More important t,o the history of science was a book cnt,itled "De Corporum inseparabilitate" (lo), written by a Jesuit named Il'ranciscus Linus, which attacked Boyle's explanation of the Torricellian vacuum. Alt,hough Linus agreed wit,h Boyle t,hat t,he air possesses weight and "spring," he maintained t,hat the spring was not, adequat,e t,o support the column of mercury in a Torricellian tube. Linus proposed t,hat t,he mercury was held up by a "funirulus," or thread of mysterious tenuous substance which, when exposed to pressrlres below at,mospheric, strongly attracted all materials in its vicinity. I t mas this "funicuh~s"that was supposed to be attached to the top of the Torricelliau tube and supported the column of mercury! Linus thought, that, whnl t,hr top of t,hr tube is closed by the finger, the inward pull t,hat is felt is dne to t,hc "funicnh~s!" To thr attacks of Hobbrs and particularly of Lim~s,
Boyle felt compelled to reply. This hc did in 1662, in long appendixes to the second edit,ion of his "Xew experiment,^." Although his answer to Hobbes is beyond the scope of this article, it was in the "Defense Against Linus" that Boyle described the expcriment,~ which led to his description of the relationship between the pressnre and volume of the air, later called "Boyle's Law." Boyle mas by no means alone in pondering this relationship. Thus, Boyle himself mentioned that Richard Towncly had suggest,ed the theory "that supposes the pressures and expansions to be in reciprocal proportion" (11). When Boyle discussed t,his statement with a "certain person" (most probably Hooke) he was told that experiments had been conduct,ed in 1660 which were in substantial agreement with such a theory (12). l:urthermore, Boyle pointed out that Lord William Brouncker (1620?-84) (15) had carried out similar experiments on the air a t about the same period. The experimental verification of the relationship betmrrn pressure and volume of the. air was set down in the Register Book of the Royal Society on Sept,ember 11, 166I . Boyle descrihed his experiment in the following words: We took then a long glass tube which by a dexterous hand and the help of a lamp was in such s. manner crooked a t the bottom that the part turned up was almost pardlel to the rest of the tube, and the orifice of this shorter leg of the siphon (if I may so rall the shole instnonent) heing hermetically sealed, the length of it was divided into inches (each of which was suhdivided into eight parts) by a straight list of paper, which containing those divisions was cutrefully pasted all along it: then putting in as much quicksilver as served to fill the arch or hended part of the siphon, that the mercury standing in a level might roach in the one leg to the bottom of the divided paper and just to the same height or horizontal line in the other, we took care, by frequently inclining the tuhe, so that the air might freely pass from one leg into the other by the sides oi 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 t a pour quicksilver into the longer leg of the siphon which by its weight pressing up that in the shorter leg did hy degrees straighten the included air; and continuing t,his pouring in of quicksilver till the air in the short,er leg was hy condensation reduced to take up hut half the space it possessed (I say possessed, not filled) before; we cast our eyes upon the longer leg of the glass, on which was likewise oasted s. list of naner carefullv divided
prove, that the greater the weight is that leans upon the air, the more forcihle 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 tho hypothesis, that as aecording to i t the Air in that degree of density and correspondent measure of resistance to which the weight of the incumbent atmosphere had brought it, was able t o cannt,erhalanre and resist the pressure of a mercurial rylinder of nhout 2'1 inches, as we are taught by the Torrirellisn experiment: so here the same air being brought to a. degree of density ahout twice as grezt as that it had before, obtains a spring twice as strong as formerly. As may appear hv its heing able to sustain or resist a cylinder of 20 inches in t,he longer tuhe, together with the weight of the atmospheried cylinder, that lean'd upon those 21) inches of mercnrv, and, as we just now inferred from the Torricellian experiment, was equivalent to them.
Vnfortnnatrly, Boylc's tuhc was hrokrn aud hr was Volume 39, Number 7, July 1962
/
357
unable to continue the experiment for a while. Later, he obtained a much longer tube that was
.
So tall we could not conveniently make useof it in achamber.. (and). . .were fain to use it on a pair of stairs, which yet were very lightsom, the tube being for preservations sake by strings so sosnended. that it did not touch the box ~resentlvto he mentioned.
The lower end of the U-tube was placed in a box to catch the mercury in the event of accidental breakage. Measurements were made by one person standing on the st,airs pouring in the mercury, while the other person read off the differences in height between the mercury in the sealed portion of the U-tube and the longer arm. As Boyle poured more and more mercury into the longer arm, and the volume of air in the shorter arm contracted, he concluded that:
...Till further tryal hath more clearly informed me, I shall not venture to determine whether or no the intimated theory will hold universally and precisely, either in condensation of air, or rmefactiau; all that I shall now urge being, that however, the tryal already made sufficiently proves the main thing for which I here d e d g e it; since by it 'tis evident, that as common air when redue'd to half iit wonted extent, obtained near about twice as forcible a spring as it had before; so this thuscomprest air being further thmst into half this narrow room, obtained thereby a spring about as strong again aa that it last had, and consequently four times as strong aa that of the common air. And there is no cause to doubt, thst if we had been furnisht with a greater quantity of quicksilver and a very strong tube, we might by a further oompression of the included air have made it couuterbalmce the pressure of a far taller and heavier oylinder of mercury. For no man perhaps yet knows haw near to an infinite oompression the air may he capable of, if the wmpressing force be competently increast. Boyle's "Table of the Condensat,iou of the Air" Table 1.
A Table of the Condensation of the Air
is given in Table I, and it can readily be seen that he compressed the air in the shorter limb to almost four atmospheres. He also showed that, if the pressure above the mercury in the longer tube was diminished by sucking on the open end, the trapped air in the shorter arm of the U-tube expanded. Similar experiments were carried out to test the hypothesis at pressures below atmospheric and very good agreement was found (see Table 2). It will be observed that Boyle took no precautions to control the temperature at which his measurements were made. Nevertheless, his observations are in remarkably close agreement with the values predicted assuming Boyle's Law. Boyle continued his experiments on the air thoughout a great part of his life. These experiments are recounted in two "Continuations" to the "New Experiments," the first of which appeared in 1669, and the second in 1682 (14). Boyle dispenses with the funicular hypothesis of Linus in the following elegant manner: I must not now stand to propose the several reflections that may be made upon the foregoing observations touching the compression and expansion of air; partly because we could scarce avoid making the historical p a t somewhat prolix; and partly because I suppose we have already said enough to shew what was intended, namely, that to solve the phenomena there is not of our Adversaties hmothesis any need: the evincing of which will appear to be hf no smdl moment in our present controversie, to him that considers, that the two maine t h i s 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 phenomena; for which we must therefore have recourse to his Funiculua. Now aa 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 explication of the Tonicellim exprimenit and those of our engine obliges us to ascribe to it. Wherefore since besides the several difficulties that incumber the Hypothesis we oppose, and especially its being scarce, if a t all, intelligible, we can Table 2.
A Table of the Rarefaction of the Air
Subtracted from 2g3/r
leaves
Reproduced from Boyle's "New Experiments," 1662. A A . The number of eaual spaces in the shorter lee that con-
C. ~ 6 heirht e of the mercurial cvlider. that counterbalanced the pressure orthe atmosphere,
that supposes the' pressures and expansions %o be in ;eciprocai
358
/
Journal of Chemical Education
B. The height of the mercurial cylinder, that together with the spring of the included air counterbalanced the Dressure of the stmosphite. C. The pressure of the atmosphere. D. The complement of R to C, exhibiting the pressure SUBbinined by the included air. E. What thst pressure should be according to the hypothesis.
adde that i t is unnecessary; we dare expect that such readers as are not hyass'd by their reverence for Aristotle or the Poripatetick Schools, will hardly reject an Hypothesis which, besides that i t is very intelligible, is now prov'd to be sufficient, only to imhrace a Doctrine that supposes such a. rarefaction and condensation, as many famous Naturalists rejected far its not being comprehensible, even when they knew of no other way (that w a probable) of solving the phenomena mwnt to he explicated by it.
In his experiments on air, using the air pump, and in his ~xperimentson chemistry, we see Boyle as the true forenmuer of the modern physicist and chemist (15). Compared with his contemporaries, with the possible exception of Newton, Hooke, and Huygens, Boyle's outlook was surprisingly fresh and free from the erroneous beliefs of the time, and his experimcnti on the air stand as a truly great milestone in the development of modern science (16). Literature Cited (1) TORRICELLI'S famoua experiment was described along with his mathematical discoveries in the famous hook "Opera. Geometrica," Firenee, 1644. (2) PASCAL,BLAISE,"Traitea de 1'Equilihre des Liqueurs et do la Pesmteur de la Masse del'Air,"Paris, 1663. (3) SCHOTT,KASPAR, "Mechanics. Hydralllico-PneumatiCB," Herhipali, 1657. (4) OTTOVON GUERICKE'S own description of his air pump, the celebrated Magdeburg Hemispheres experiment, the manometer, and other experiments on the nature and properties of the air, did not appear until 1672, in the classic book "Experiments Nova (ut vocantur) Magdeburgica de Vacuo Spatio," Amstelodami, 1672. (5) "Accademia. del Cimento, Saggi di Natnrali Esperiensr," Firenze, 1667. A beautiful English edition appeared a few yeam later: "Essayes of Natural Experiments Made in the Academie del Cimento, Under the Protection of the Most Serene Prince Leopold of Tuscany. Written in Italian by the Secretary of that Academy. Englished by Richard Waller, F.R.S.," London, 1684. The history of the "Accademia,'hnd other early scientific societies, is described in Martha Omstein's "The Role of Scientific Soeidies in the Seventeenth Century," The University of Chicago Press, Chicago, 3rd ed., 1938. (6) For accounts of Hooke's relationship with Royle, and other matters, see: 'ESPINASSE,MARGARET, "Robert Hooke," William Heinemann Ltd., London, 1956; KEYNES, GEOFFREY,"A Bibliography of Dr. Robert Hooke," H. W., Clarendon Press, Oxford, 1960; and ROBINSON, AN,, ADAMB,WALTER,"The Diary of Robert Haoke 1672-80," London, 1935. (7) A summary of Boyle's experiments in pneumatics appears in Case 1 of the "Harvard Case Histories in Experimental Science," edited by James B. Conant, Harvard University P r e ~ Cambridge, , Mass., 1957,Vol. 1, pp. 1-63. (8) The full title runs as follows: "New Experiments PhysicoMechanicall, Touching The Spring of the Air, and ita Effects (Made, for the most part, in a. New Pneurn~tieal Engine) Written by way of Letter to the Right Honorable Charles Lord Viscount of Dungarvan, Eldest Son to the Earl of Corke," Oxford, 1660. An extensive analysis of $1 Boyle's published works may be found in the late JOHN F. FULTON'. excellent book, recently puhlished: "A Bibliography of The Honourahle Robert Boyle," Clsrendon Press, Oxford, 1961,Zndedition.
(9) HOBBES,THOMAS,"Di&logus Physicus, sive De Natura
Aeris Conjectura," London, 1661. For an account of GEORGE. Hobbes' philosophical works, see: SAMPSON, "The Concise Cambridge History of English Literature," University Press, Cambridge, England, 1946, pp. 380-92. (10) LINWS,FR~Ncrscus,"Tractatus de eorporum inseparabilitato; in quo experiments de vacuo, tam Torricelliana, quam Magdehurgiea, & Boyliana, examinantur," London, 1661. (11) Boyle speaks highly of Richard Townely, referring t o him as "that ingenious Gentleman," who, as the result of reading Boyle's "Physics-Mechanicall Experiments," and most prohably Experiment 17 therein, suggested the hypothesis regarding the relationship between the volume and prcssure of the air. Townely had oarried out some experiments of his own on the air which, as Bayle relates, "he had begun to set down what occurred to him to this purpose in a short disoourse, whereof he afterwards did me the favour to shew me the beginning, which gives me a just curiosity to see i t perfected." Unfortunately, an account of Townely's experiments on the air was never published, although he did contribute some meteorological papers to The Philosophical Transactions of The Royal Society. The present writer possesses a copy of Royle's "An Essay About The Origine 8: Virtues of Gems," Landan, 1672, which hears the book-plate of Richard Townely. Hitherto, the date of Townely's birth was unknown, but the book-plate is dated 1702, and states his age as 73. This would place Toanely's birthdate a t ahout 1629. A paper by Townely and W. Derham appeared in the "Philosophical Transactions" for 1705, though whether Townely was still living a t that time is not k n o ~ n . The fact that Boyle and Townely were on good terms far many years makes this obscure figure worthy of further study. (12) H o o a ~published a n account of his experiments on the air in his "Mierogra.phia," in which he concluded that "From these experiments, I think, we may safely conclude, that the Elater of the Air is reciprocal to its extension, or a t least very neer." See: "Mierogra.phia: Or Some Physiological Descriptions of Minute Bodies Made By Magnifying Glasses. With Observations and Inquiries thereupon," London, 1665, pp. 222-7. For a facsimile reprint of this classic of scionce, see: GUNTEER,R. T., "Early Science in Oxford. Vol. XIII. The Life and Work of Robert Hooke (Part V)," Oxford, 1938. ( l a ) Brouneker was a. founder member of The Royal Society, and was its first president (1662-77). (14) Fdl titles of these two "Continuations" are given by Fulton (ref. 8). (15) For a revien of BOYLE'Sclassic book "The Sceptical Chymist," 1661, see: NEVVILE,R. G., J. CHEM.EDUC., 38, 106 (1961). (16) On the European continent, and particularly in France, Boyle's Law is often referred to as Mariotte's Law. EDMEMARIOTTE (1620-84) can have little rlsim to the discovery of Boyle's Law for it is very probable that he already knew of Boyle's work on air, and his experiments were carried out many years after those of Royle. I n fact, Mariotte did not publish his hook "Essay de la natur de Pair," reporting his experiments, until 16ilI. The names of Hooke and Townoly certainly have more right to he associated with Boyle's Law than the name of Mariotte. For a discussion of Mariotte's work, see: HOEFER,FERDINAND, "Histoire de la. Physique et de la Chimie," Paris, 1872, pp. 43-50; also, MAGIE, W. F., "A Source Book in Physics," McGraw-Hill Book Co., Ine., Nnr. York and London. 1935,pp. 8&92.
Volume 39, Number 7 . July 7 962
/
359
