Chemistry for Everyone
Boerhaave on Fire Damon Diemente Trinity School, 101 West 91st Street, New York, NY 10024;
[email protected] “If I have seen further than you and Descartes,” Newton wrote to Robert Hooke, “it is by standing upon the shoulders of giants” (1). We all appreciate the truth in Newton’s modesty. Chemists recognize their indebtedness to a long line of predecessors, whose names are well known and often applied descriptively: Boyle’s law, Avogadro’s hypothesis, Crookes’s tube, the Bohr atom. But many great names of the past have not been remembered through the years. Consider Herman Boerhaave, who today virtually is forgotten,1 but once was much esteemed. Lavoisier recalls him in the opening sentence of his Elements of Chemistry (2): That every body, whether solid or fluid, is augmented in all its dimensions by any increase of its sensible heat, was long ago fully established as a physical axiom, or universal proposition, by the celebrated Boerhaave.
Today, we recognize that this “universal proposition” admits a few exceptions, but to Lavoisier it was an established fact, stated at the very start of his magnum opus with the support of an honored name. Who was this man? Herman Boerhaave (1668–1738) (3) was a Dutchman, a professor first of medicine, then also of botany, and finally of chemistry at the University of Leiden. He began his career in 1703 and through the brilliance of his lectures and demonstrations he was soon known throughout Europe as a great teacher. His classroom was always crowded; wealthy students sent servants ahead to secure seats. In 1724 a spurious edition of his chemical lectures, compiled from students’ notes and full of errors, appeared in Latin, the language he taught in. Translations into modern languages quickly followed. He was concerned about the wide circulation of these corrupt editions, and especially disturbed when students attended his lectures with one of them in tow (4 ). But protesting that he was a lecturer and no author, he required strong urging from friends before consenting to labor on an authorized version. Finally, in 1732 the authorized Latin text was published under the title Elementa Chemicæ. It was an immediate hit and remained for decades the best textbook of chemistry in print, though to be truthful it is rather loosely organized—Boerhaave really was a better lecturer than writer. The genuine edition too was widely translated. The English version, A New Method of Chemistry (5), came out in 1741. The book opens along time-honored lines with discussions of earth, water, air, and fire. My purpose in this article is to offer to teachers for classroom use a selection of passages from Boerhaave’s chapter on fire. Now, today’s teacher of chemistry is apt to feel that little of significance to the modern classroom can be gleaned from a two-and-a-half-centuries-old text, and especially from a topic as old-fashioned as fire. But this view is decidedly shortsighted. Boerhaave was a dedicated empiricist. He describes dozens of demonstrations and experiments that can be instructively performed today, often including quantitative data that can be checked against modern equations. And he was among the great systematizers of his time, search42
ing for patterns, testing hypotheses, drawing attention to anomalies and unexplained results. All of this can be profitably assessed in the classroom in light of modern chemical theory. In the readings presented here I have found material for discussion in class, for investigation in the laboratory, and for a few homework assignments. Modern students are well able to comprehend and paraphrase Boerhaave, to check his results, appreciate his insights, and identify his shortfalls. To read from his chapter on fire is to stand witness to the birth and infancy of thermochemistry as conceived in the mind of a great chemist from the age when coherent chemical theory was just beginning to emerge. Let’s read a little Boerhaave and see what he has to say about fire in the year 1732, when there was no concept of energy in the modern sense, nor of its conservation, nor of its thermal and radiant equivalency, nor even any clear distinction between energy and temperature.2 The Hallmark of Fire Boerhaave’s discussion of fire does not begin, as one might expect, with a definition, for fire is elusive to recognize, let alone define. He points out that the traditional equation of fire with light or heat is inadequate, for these are not the same and can show themselves separately. There can be great heat without light: [13] [Some] think [that] light may be used as an infallible proof of the presence of fire,… and as the brightness of light decreases, fire is held to be diminished in equal proportion.… But… [i]f iron is taken out of the fire before it is red-hot, but when very near [red-hot],… in a dark place, it will emit no light, yet… lay it on dry wood, and it will raise sparks and flame, so much fire may there be without any light.
Likewise, there can be light without heat: [148] There is no necessary connection between a body’s shining much and being very hot. For the light of a winter’s sun… affects the eye so strongly as to blind it for some time, though its heat under that same circumstances is not sufficient to melt the thin icicles suspended in the air.… The image of the sun reflected from polished gold… or glass, yields a glitter which the eye can by no means bear, yet affords no heat perceivable either by sense or the thermometer, from which again we conclude that there is a great difference between… light and heat, or between lustre and fire.
In these passages, and in several that follow, Boerhaave’s meaning is clear, but alert students will recognize that he makes no sharp distinction between fire, heat, and temperature. If fire is not necessarily associated with light or with heat, what property is it associated with? Boerhaave’s answer is suggested by the excerpt above from Lavoisier’s Elements. The infallible sign of fire is the expansion it causes in any material
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Chemistry for Everyone
absorbing it. Upon hearing this, a few students will no doubt object that the addition of “fire” to melting ice gives a contraction: ice floats on water. Indeed, Boerhaave ought to have given consideration to this obvious counterexample, a classic ugly fact that puts an entire theory in peril, but he nowhere mentions it.3 It is upon the more general behavior, the expansion of matter with temperature, that Boerhaave constructs his theory of fire, and for his crucial evidence he appeals to the universal experience of all observers: [14] [O]n a careful inquiry, I do not find any substance to which we may not apply that which all men call fire, whether solar, culinary, or subterraneous. And all such bodies to which fire is thus applied, without one exception, are thereby rendered bigger, swell, and rarify, yet without any observable difference in their weight. This equally and uniformly obtains in solid bodies and fluid, hard and soft, light and ponderous.
Two claims are made here: first, fire causes expansion, and second, fire causes no increase in weight. The claim of expansion is supported by a lecture demonstration still seen in classrooms today: [18, 20] I take two cylindrical iron rods of equal length,… and likewise of the same thickness… [so] that both of them will just pass through the same iron ring. I now heat… the extremity of… [one] rod, and endeavor to pass it through the ring as before, but am not able with all my force, [it] having become much thicker than when cold. Yet wait but a little, till it has returned to its coldness, and you will see it pass the ring as before.
The claim that fire does not cause an increase in the weight of substances is not supported in detail, but Boerhaave does discuss the technique of mass measurement, and he warns that heated objects cannot be reliably weighed until they have cooled back to room temperature: [226] They who make… experiments will be apt.… to imagine… the heated body lighter than the cold one, the cause of which on further examination will be found owing to this, that the strings of the balance by which the scales are fastened to the beam are liable to be moist, and to dry… by the heat of the metal, which when put on the scale to be weighed, raises some of the watery part into vapor and makes that scale lighter. To prevent this it may be expedient to make use of metal chains instead of strings.
Today we attribute the seeming lightness of warm samples to rising convection currents in the air over them. Whatever its source, Boerhaave noticed the error and warned against it. I discuss this passage when I caution my students against weighing samples in warm crucibles or evaporating dishes. Theory of Fire Now, weightlessness is a property of fire and expansion is the evidence of its presence, but these do not explain what fire actually is. Does fire have corporeal substance? If it does, why is it weightless? Does fire comprise particles? If so, what are they like? If not, what is its physical basis? Boerhaave doesn’t answer all these questions, but he addresses them, and
he offers a survey of properties that any theory must explain. First of all, fire must have something to do with motion. This is shown by its ready production through friction, even in deep cold: [263] Flint and steel in frosty weather, however cold when at rest, yet by a single collision instantly produce a vehement fire in the cold atmosphere.
Second, unlike any other substance, fire penetrates all forms of matter. This, together with its weightlessness, could have been a strong argument for the immateriality of fire, but that case is not made: [4] Another difficulty in the way of philosophers who study the nature of fire is the excessive minuteness of its constituent parts. [These] not only surpass all other known bodies [in minuteness], but even penetrate… [into the most] solid and… [smallest] particles that we can… find.
Clearly, Boerhaave believes that fire is material, though enormously subtle. He therefore wonders why hot things cool faster when immersed in water than when exposed to air or put in a vacuum: [245] What… shall we say is the cause why fire passes so much sooner… into another grosser, than into a lighter body, or even into empty space, which it might penetrate with so much more seeming facility?
Third, fire is always everywhere—it pervades all nature; and we must not be misled by the arbitrary zero of the thermometer: [114, 115, 117] [F]ire may be immediately produced by the friction of any kind of body at all times, in every degree of cold, and [in] every place. [I]n the highest degree of cold… there is always some fire remaining, though it be usually mistakenly supposed that there is no fire left when the thermometer sinks to zero. [T]here is [no] space in nature destitute of fire… though [it is] not always discoverable by us.
In several passages like these, Boerhaave seems to assert that friction exposes fire hidden in the cold materials, rather than creating fresh fire where there was none before. The translation with the verb “produced” may therefore be inaccurate. Fourth, when bodies are at rest, that is, when there is no friction or chemical reaction between them, fire is evenly distributed in nature. If we interpret “fire” (and below also “heat”) to mean temperature, Boerhaave’s words are close to the Clausius statement of the second law. It would be an anachronism to say that he was on the trail of entropy, but he was perceptive enough to find the basic observation worthy of remark: [263, 117] Among all the bodies hitherto observed, there is none which spontaneously and when left to itself becomes hotter than all others. Not only is fire only contained in all space, but found equally distributed in all bodies, the rarest as well as the most solid. This I have never found anybody could believe at first, but the truth and certainty of it is [made clear] by the following experiment. In a sharp winter’s cold I placed… an exhausted
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Chemistry for Everyone bulb,… alcohol,… oil, water, solutions of various salts,… mercury, feathers, dust of metals, [and] sand in the open air, and found the same degree of heat… in them all, without the least difference. I have not therefore… been able to discover any body [able] to attract… fire… equably distributed, and fix it to itself, so as to have more than its share. In fine, I do not find that there is in nature any such thing as a magnet of fire.
He recognizes that the senses can be deceptive, though he was not able to explain these deceptions, since he lacked the concepts of specific heat and of thermal conductivity: [117] Here some will be apt to cry out that I am… alleging things not only vain and false but contrary to common sense…, which manifestly shows that iron, in winter, is colder than feathers, and mercury than alcohol. But let it be considered I am not here speaking of fire appearing to the senses by its degree of heat and cold, but of fire discovered by that characteristic above settled [upon] of expanding bodies.
My students, on reading this passage, know what Boerhaave’s critics mean—we sleep under warm feather quilts in the winter, not under sacks of cold iron turnings—but it is not necessarily easy for them to explain why in terms of modern concepts. Fifth, and finally, since fire is everywhere, the cohesive attraction that keeps matter together, at least in solids and liquids, and the expansive effect of fire are always in contrary operation: [118] [A]ll things… are divided into fire, which expands them, and matter which is continually striving against the separation of its elements. Thus the two principles, the one expansive, the other contractile, prevail through all bodies.
The attractive or contractile principle that operates between particles is not developed in anything like the modern electrostatic sense. It is rather obscurely described as a motion of the particles toward the center of the body. Perhaps he had gravity in mind, as many students still do when I poll them about the source of the attraction in a chemical bond. Note that Boerhaave calls the smallest particles of bodies “atoms”, and note his strikingly modern conception of the absolute zero: [42] [T]he mere absence of fire produces a motion in all solid bodies… by virtue of which all the atoms of the body tend towards its center,… draw nearer and cohere together. If, therefore, cold were a mere privation of fire, the power which contracts the particles of a solid body would be innate… in the nature of matter itself, while the power that expands would depend upon the fire and consequently be something extrinsic… to bodies. Accordingly, bodies would then [in absolute cold] endeavor to shrink into close masses till they arrived at the minimum, when they would remain perfectly solid and quiescent. Whereas, on the contrary, they are now [at normal temperatures] continually shaken by fire, and never arrive at the state of rest. So… the end of cold is a state of absolute rest between coherent particles, and the end of fire a perpetual agitation of particles.
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From all the passages in this section, an AP class recently concluded that Boerhaave did not believe heat or fire to be simply atoms in motion. They thought he saw fire as weightless particles readily absorbed by other substances and inducing incessant motions among the atoms of these other bodies. The fire particles and the motions they induce expand bodies and give rise to perceptions of heat. Lacking the concept of kinetic energy, he was not able to develop the idea of heat as motion without particles of fire to initiate the motion. Thus he had difficulty explaining the weightlessness of fire, its ubiquity in nature, its power to penetrate all matter, and its fleet departure from a vacuum. Expansion, Melting, and Boiling In addition to simple expansion, fire may cause fusion or vaporization. Boerhaave explains why these are related: [27] [F]ire… expands all the parts of the hardest body it is applied to, and removes them from their mutual contact on all sides.… [T]his expansion… is successively increased till at last the whole mass, if it is fusible, comes to melt: so that in the whole course of the growing heat the several parts of the heating body are perpetually receding from [one another].
So the expansion of a phase and the change between one phase and another are the different effects of a single cause. Melting is the natural limit of expansion in a solid, as boiling is in a liquid. The phase changes themselves are accompanied by expansion because they require fire to bring them about, and expansion is the unfailing sign of the addition of fire. In the absence of any concept of kinetic and potential energy, Boerhaave has composed a simple and satisfying model. But the model does not imply that melting or boiling should take place at constant temperature, and the discovery that this is so comes as a great surprise. In this passage “heat” means temperature of the water while “fire” seems to refer to the temperature of the flame: [87a] And here the candor, which I trust will ever accompany me, obliges me to acknowledge [an] extraordinary experiment… which you may consult in the original in the memoirs of the Academy of Sciences.… [W]ater heated to… boiling will not conceive any further heat how much soever the fire be increased.
Modern students take this result for granted, and are likely to wonder why Boerhaave was so impressed by it. Class discussion about it, however, soon comes to talk about dipoles, hydrogen bonds, the geometry of crystals, and his surprise becomes easier to understand. The phenomenon of boiling is investigated in some detail. Experiments were sufficiently thorough to discover that the boiling point is sensitive to pressure: [87a] Yet this excellent discovery [of the constant temperature of the boiling point] may receive a considerable improvement…, viz., that the heat [i.e., the temperature] of the same boiling water is always regularly greater by how much the weight of the atmosphere is greater which presses on its surface, and again that the same heat of the
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Chemistry for Everyone boiling water diminishes as the weight of the incumbent atmosphere grows less. Hence, in marking the degree of heat of boiling water, it will be necessary to note the weight of the atmosphere at the same time by the barometer, otherwise no certain measure will be expressed.
A lecture demonstration still popular today drives this point home: [87a] Under a glass receiver of an air-pump place a phial full of water, warm in 96 degrees, extract the air gradually, and as the atmosphere diminishes you will find a sensible ebullition raised in the water, which will be suppressed again as soon as the air is re-admitted into the vessel. Hence again we may note at what degree warm water begins to boil according to… the weight of the atmosphere.
This principle is applied to the steam digester, today known as the pressure cooker. Cooking in this device reduces hard bone to edible meal by processes not understood before Boerhaave’s time. He now provides an explanation, namely, elevated temperature: [87a] If water and air be enclosed in [a] digester, out of which nothing can escape, and thus made to boil, the [air] will be expanded… 1/3. Consequently the water will be pressed as if it had ten inches more than the common atmosphere [ca. 30 inches] upon it, so that boiling water in this engine will be thirty degrees hotter than ordinary, on this single account.
Boerhaave claims here that under a pressure of 11⁄3 atm, water boils 30 °F (17 °C) above the normal boiling point. His accuracy was recently checked by an AP class with the Clausius–Clapeyron equation: ln( p2/p1) = (∆Hvap/R)(1/T1 – 1/T2)
Thermometers: Measurement of the Degree of Fire Now, if expansion is the infallible evidence of fire, it follows that expansion can be used to measure degrees of fire, or temperatures. Of course, this is exactly what many thermometers do: they measure temperature by recording the expansion of a liquid, often mercury. Boerhaave was aware that no quantitative statements can be made about fire without reliable and accurately calibrated thermometers: [83, 84] Alcohol under the greatest natural cold hitherto known expands itself by the vital warmth of a human body to the 20th part of its bulk. In the meanwhile it must be observed that in this experiment the inner cavity of the thermometer is supposed to have continued the same, whereas in reality this also must have been dilated. If the proportion of the capacity of the instrument in severest cold to the capacity of the same under a degree of vital warmth could be accurately known, we might… learn the true ratio of the increase of the bulk of the liquor.
One of my classes investigated the expansion of 95% ethanol (the azeotrope received from distillation) over the interval from ᎑30 °C (a salt–ice–water mixture widely believed in the early 18th century to be the greatest attainable cold) to 37 °C (= 98.6 °F) to verify the number given here. A typical result was that 8.6 mL expanded to 9.5 mL, an increase of about 10%, or twice as much as claimed. The class used graduated cylinders capped with aluminum foil to contain the alcohol and measure its volume, with no attempt made to correct for contraction or expansion of the glass (they knew that Pyrex has a very low coefficient of expansion). They were not impressed with Boerhaave’s accuracy this time. Boerhaave recognized that the volumes of gases are particularly sensitive to temperature: [56] [T]he heat of boiling water rarifies air one third part beyond its former bulk.
where p1 = 1.00 atm, T1 = 373 K, ∆Hvap = 9720 cal/mol, and R = 1.99 cal/mol-K. They found that under 1.33 atm, water boils at 108 °C, only 8 °C (14 °F) hotter than ordinary. To boil at 117 °C, water must be pressed by 1.77 atm, or as if it had 23 inches more than the common atmosphere upon it. The class felt that Boerhaave’s results were not bad, taking into account the difficulties he must have encountered measuring conditions inside a pressure cooker. The cooling effect of evaporation and the protection from heat afforded by hydroplaning are also recorded, though not at all understood, since he had no concept of potential energy or latent heat:
Assuming “former bulk” to mean volume at room temperature (20 °C), Charles’s law gives: V2/V1 = T2/T1 = 373/293 = 1.27 ≈ 4/3. The result is not bad. He points out that the sensitivity of the bulk of air to fire (of its volume to temperature) could be put to good account in the construction of a gas thermometer, although there is no evidence that he or any of his contemporaries ever actually constructed one:
[225] Having a quantity of the purest alcohol in a phial, and pouring slowly and carefully the minutest drop thereof on ignited iron, one would naturally expect that it should instantly be kindled, whereas on the contrary it no sooner falls upon the concave surface of the iron that it gathers into a… globule like mercury and runs like the same over the metal without any sign of flame, and after in its progress it has arrived at a colder part of the iron, presently flies off in fume without raising any fire. This appears strange since sulfur, gunpowder, wood, and other bodies presently kindle when laid on the same iron, while alcohol, which when gently heated kindles the quickest of almost all bodies, will endure this fire without kindling at all: a problem worthy some pains to solve.
The Limits of Heat and Cold Boerhaave reports on investigations into the belief that the coldest possible temperatures were given by salt–ice–water mixtures. In today’s terms, these are freezing-point depression experiments, and easily tried by students, though perhaps using potassium nitrate solution instead of nitric acid for safety’s sake:
[64] The greatest contraction produced in any [solid or liquid] body by the strongest cold is less than the contraction observable in the air upon the least decrease of heat… cognizable by our senses, and consequently air on this account… is fit for discovering the quantity of fire.
[75, 76, 77] The hard winter of the year 1729 afforded an opportunity of making experiments for producing degrees of cold.… Seven ounces of dilute nitric acid reduced
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Chemistry for Everyone to the degree of coldness of the atmosphere, which was then 16 degrees, being poured on some ice finely ground, the thermometer immediately sank 30 degrees,… from 16 degrees above zero to 14 degrees below.… The liquid… was poured off and new acid poured on the ice that remained undissolved, upon which the thermometer immediately sank to 29 degrees below zero.… Pouring the acid on the refrigerated ice four times, and as often carefully clearing it from the… fluid, the thermometer was at last found to sink 40 degrees below zero.
These experiments are among those that led him to speculations on whether there is any natural limit to cold: [43, 50] It may be here asked whether absolute rest in any space would not make the greatest cold, and whether there would not be rest in a place where there is absolutely no fire. But who can define the limits of cold? Where is it so intense as that it may not be still increased? It’s probable this may obtain where there is no fire, but it is impossible to find any such place.
He is more confident that there is no limit to fire (i.e., no highest temperature) because there seems to be no upper limit to expansion, which is fire’s hallmark. In essence he applies Charles’s law: [56] The expansion of the air produced by fire goes to an immense degree, which is not easy to be determined by experiment. For we find that hollow glass spheres exposed in a furnace so as to be ready to melt and then sealed hermetically and at last gradually cooled again, are not even thus left entirely destitute of air.… [U]pon breaking the hollow end of a sphere of this kind under water, though the water rush in with great force, there always remains a space in the upper part full of air, which proves that the vehement heat of the furnace had indeed rarified the air excessively but had by no means expelled it entirely. It is probable [that] a still stronger fire would rarify the air yet further. But it is no less probable that this rarefaction would never reach its utmost bounds, so that there always remains some air, even in the greatest fire.
In addition, the concentration of sunlight at the focus of a concave mirror easily reaches temperatures beyond the range of any kindled fire, and since there is no theoretical limit to the size of mirrors, there seems to be no limit to the temperature that might one day be achieved: [209] Potassium nitrate being [put in a vessel and exposed to the focus] becomes wholly volatile in a moment’s time and turns to… nitrogen dioxide, an effect more extraordinary [because] potassium nitrate [in] any other fire scarce changes at all but simply melts. To turn it to nitrogen dioxide by force of [a kindled] fire there is always required… the addition of sulfuric acid,… whereas in the above experiment the business is done without any addition at all.
whereas an ordinary fire only melts it unless sulfuric acid is added, nitrate ion being unstable in acid: 2KNO3 + H2SO4 → K2SO4 + H2O + 2NO2 + 1⁄2O2 I have attempted this decomposition outdoors with a magnifying glass on a bright day, but have not gotten it to work. Perhaps someday, with a larger glass, it will. Calorimetry Boerhaave reports in detail on many calorimetry experiments. This review of his chapter on fire concludes with a few of his results. He collected good data, but lacking a clear concept of specific heat, he had difficulty interpreting them. When equal masses of the same substance at different temperatures are mixed, the common final temperature is the average of the initial ones: [243] Taking two equal quantities of the same fluid,… and reducing these to different degrees of heat and then intimately mixing them together, they came to the same degree of heat, which was half the excess of the hotter quantity above the cooler.… [F]ire, by a close contact between the smallest particles of bodies of the same kind, immediately diffuses itself, and the excess distributes equally between the two masses.
But when unlike substances such as water and mercury are mixed the outcome is different: [244] [I]f the water were hotter than the mercury when equal bulks thereof were mixed, the degree of heat arising from the mixture was always more than the [average] which was expected. On the contrary if mercury were hotter than the water and equal bulks of each were mixed, the temperature produced by them was always less hot than the half of the difference.… [B]ut when three equal bulks of mercury are taken and two such bulks of water, it matters not whether you heat the mercury or the water, since after mixing the temperature will correspond to half the difference of the heat in each, as before in water where equal portions were mixed.
I ask my chemistry classes to determine whether “bulk” means volume or mass in this passage. If it means volume, Boerhaave’s claim is that 300 mL of mercury at 0 °C mixes with 200 mL of water at 100 °C to give a final temperature of 50 °C. Let’s see. of mercury
of water
Ti, initial temperature/K
300
0
V, bulk/mL
300
200
c, specific heat/cal g᎑1 K᎑1 d, density of mercury/g mL᎑1
0.033
1.00
13.6
1.00
Then: ᎑cHgmHg∆THg = cH2OmH2O ∆TH2O ᎑cHgVHgdHg ∆THg = cH2OVH2O dH2O ∆TH2O
These chemical observations are basically correct. In modern terms, he remarks here that the heat of focused sunlight is intense enough to decompose potassium nitrate:
(0.033)(300)(13.6)(Tf – 0) = (1.00)(200)(1.00)(100 – Tf)
2KNO3 → K2O + 2NO2 + 1⁄2O2
from which, Tf = 60 °C, which is tolerable agreement. If
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(135)Tf = 20,000 – (200)Tf
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“bulk” means mass, the claim is that 300 g of mercury at 0 °C mixes with 200 g of water at 100 °C, to give a final temperature of 50 °C: ᎑cHgmHg∆THg = cH2OmH2O ∆TH2O (0.033)(300)(Tf – 0) = (1.00)(200)(100 – Tf) (10)Tf = 20,000 – (200)Tf from which, Tf = 95 °C, which is quite far off. Evidently “bulk” means volume. His explanation of this result is not as secure as his data (italic added): [245] In this experiment… we discover a new law of nature, viz., that fire is distributed in bodies in proportion to their bulk, or extension, and not [to] their density.… T[his] is confirmed from a multitude of other experiments as we have intimated above in making mention that all kinds of bodies, when committed a considerable time to the same common degree of warmth, never showed any diversity in respect to heat other than in proportion to the space they possess; so that nothing ha[s] been found in bodies which attracts fire.
In the first italic fragment, “bulk, or extension” must mean volume, but does “fire” mean temperature or heat? Perhaps the sentence claims that energy added as heat is proportional to the volume of the samples heated, and this is true if the product of density and specific heat is constant. The second italic fragment seems to say something similar: when objects are warmed between the same two temperatures, the heat they absorb is proportional to their volumes. Again, this is true if the objects have the same product of density and specific heat. Clearly, Boerhaave’s new law lacks an important concept. If two samples are to absorb the same quantity of heat when warmed through the same temperature change, it is not volume that must be the same for the two of them, nor mass, nor density, but the product of mass and specific heat. This piece of basic thermochemical theory would not be sorted out until the work of Joseph Black, a couple of generations later. But Boerhaave, excellent chemist and teacher, broke the ice, and Black—well, Black saw further because, among other things, he stood on Boerhaave’s shoulders (6 ).
Acknowledgment I thank David O. Tanis for patiently reading this article in successive drafts and for his candid advice for its improvement. Notes 1. A search of the JCE Index Online found three articles with “Boerhaave” in the title, but none have appeared since 1956. But it must be acknowledged that the old university town of Leiden, where Boerhaave spent his career, remembers him well, with a college, a museum, a townhouse, a pharmacy, and a memorial in the Peter’s Church all carrying his name. 2. Quotations of Boerhaave in this article are from literature reference 5, in which paragraphs are numbered. Paragraph numbers are given in brackets at the head of each quotation. Most quotations have been edited to make them more accessible to students and high-school teachers alike: spelling, punctuation, capitalization, use of italic, and archaic vocabulary (including chemical nomenclature) have been modernized; anfractuous sentences have been shortened, and redundancies removed. In the original text paragraph numbers 85 through 88 were inadvertently repeated; the repeated numbers have here been changed to 85a through 88a. 3. It is of course easier to forgive his not knowing that for the next few degrees of temperature rise the liquid water continues to contract.
Literature Cited 1. Newton, I. Letter to Robert Hooke, 5 Feb, 1675 or 1676; quoted in Bartlett’s Familiar Quotations; Kaplin, J., General Editor; Little, Brown: Boston, 1992. 2. Lavoisier, A. Elements of Chemistry; Kerr, R., Translator; G. G. and J. J. Robinsons: Edinburgh, 1790. 3. Dictionary of Scientific Biography, Vol. II; Gillespie, C., Editor-in-Chief; Charles Scribner’s Sons: New York, 1973. 4. Partington, J. R. A History of Chemistry, Vol. II; MacMillan: London, 1962. 5. Boerhaave, H. A New Method of Chemistry; Shaw, P., Transl.; T. Longman: London, 1741. 6. For examples of Black’s awareness of Boerhaave’s thoughts on fire, see Partington, op. cit.; Chapter IV.
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