one gas molecule has little influence on the motion of another. This led him to believe that the pressure of a gas sample is the same whether it is the only gas in a container or it is among other gases. If gas molecules begin to interact—as may be the case at very low tempera- tures and high pressures—deviations from Dalton’s Law can occur. Also, Dalton’s Law assumes that the gases do not chemically react with one another.
Although Dalton’s Law may seem trivial, it is one of the more useful gas laws for scientists. For example, one laboratory method of gas collection involves displacing water from a bottle. Once all of the water has exited the bottle, we know that the gas occupies essentially the entire volume of the bottle, assuming that the gas solubility in the water is not significant in the experiment. In fact, you may recall from a high-school chemistry class that you used an inverted bottle filled with water as the bottle sat in a water bath. A tube from the reaction vessel transfers the gas into the upside- down bottle into which the gas bubbles to the top and displaces water. The water is forced out of the mouth of the bottle into the water bath. Note that the gas now trapped in the container is not composed entirely of the gas pumped into the bottle—rather, it is a mixture that contains a certain amount of water vapor. We are able to find the pressure of the dry gas alone, using Dalton’s Law and by subtracting the pressure of the water vapor from the total pressure:
P(dry gas)= Ptotal−P(water vapor)
We can easily substitute an actual value for the partial pressure of the water vapor, P(water vapor), because reference tables exist that show the pressure of water vapor at various temperatures. For example, the vapor pressure of water at 20◦C is 2.3 kPa, and at 40◦C the pressure is 7.4 kPa.
(kPa is a unit of pressure and stands for kilopascals.) Thus, for example, if we have a sample of hydrogen gas collected over water that is at a temperature of 20◦C and find the resultant pressure is 110 kPa, the pressure exerted by the dry hydrogen alone is 110 – 2.3 = 107.7 kPa.
Dalton’s Law is of particular interest to scuba divers, because they want their gas tanks to deliver a partial pressure of 0.20 atmospheres of oxygen to ensure proper functioning of the respiratory systems of their bodies. The total pressure must account for the external pressure expe- rienced by the diver in the water in order to prevent the lungs from collapsing. Thus, divers use a valve to equalize the pressure inside their lungs with the external pressure by adding helium gas. The valve, in effect, uses Dalton’s Law to maintain appropriate pressures and oxygen levels.
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John Dalton (1766–1844), English chemist, physicist, and meteorologist, famous for his contributions to the development of atomic theory and one of the fathers of modern physical science.
CURIOSITY FILE: Red-green color blindness has often been referred to as
“Daltonism,” after John Dalton, who suffered from and researched this affliction. • Dalton discovered butylene (a gas used today for making rubber) and determined the correct chemical formula for ether (formerly used as an anesthetic). • Dalton was exceeding versatile—in 1801, in addition to his important paper on the properties of gases, he published a book on English grammar. • In the 1990s, scientists studied one of his preserved eyes in order to understand the cause of his color blindness.
Matter, though divisible in an extreme degree, is nev- ertheless not infinitely divisible. That is, there must be some point beyond which we cannot go in the division of matter. . . . I have chosen the word “atom” to signify these ultimate particles. . . .
—John Dalton,A New System of Chemical Philosophy, 1808
Dalton transformed the atomic concept from a philo- sophical speculation into a scientific theory—framed to explain quantitative observations, suggesting new tests and experiments, and capable of being given quantita- tive form through the establishment of relative masses of atomic particles.
—Arnold Arons,Development of Concepts of Physics Chemical analysis and synthesis go no farther than to the separation of particles one from another, and to their reunion. No new creation or destruction of matter is within reach of chemical agency. We might as well attempt to introduce a new planet into the Solar System, or to annihilate one already in existence, as to create or destroy a particle of hydrogen. All the changes we can produce consist in separating particles that are in a state of cohesion or combination, and joining those that were previously at a distance.
—John Dalton,A New System of Chemical Philosophy, 1808
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John Dalton was born in Eaglesfield in northern England. He was a quiet man who attained his professional success in spite of several hardships:
He grew up in a family with little money, was a poor speaker, never had a wife to give him emotional support, was severely color blind, and was considered to be a fairly crude or simple experimentalist. Perhaps some of these challenges would have presented an insurmountable barrier to any budding chemist of his time, but Dalton persevered and made exceptional contributions to the development ofatomic theory, which states that all matter is composed of atoms of differing weights that combine in simple ratios. During his time, atomic theory also suggested that these atoms could be considered to be indestructible.
Dalton was the son of a Quaker weaver who lived in the county of Cumberland, England. Dalton’s childhood years were spent working in the fields and in his father’s shop, where cloth was made. His sister sold paper, ink, and pens. When only 12, Dalton helped run the village Quaker school at which he also taught, and two years later he taught with his brother at a boarding school in the small town of Kendal. During these early years, Dalton acknowledged that his love of science was stimulated by John Gough, a blind natural philosopher. Dalton wrote in a 1783 letter,
John Gough is . . . a perfect master of the Latin, Greek, and French tongues. . . . Under his tuition, I have since acquired a good knowl- edge of them. He knows by the touch, taste, and smell, almost every plant within twenty miles . . . he and I have been for a long time very intimate; as our pursuits are common—viz. mathematical and philosophical.
Starting in 1787, Dalton kept a daily meteorological diary, which he contin- ued to update each day until the very day of his death. This amazing com- pendium eventually contained roughly 200,000 entries on meteorological observations for the variable climate of the lake district in which he lived.
Some British scientists, such as John Frederic Daniell (1790–1845), have called Dalton “the father of meteorology.” Dalton’s obsession for note taking is also evidenced in his recreational life—even when he played the English lawn game of “bowls,” he kept meticulous records of hits, misses, and other scores.
His mathematical prowess increased while at school, and even in his early years, he had a reputation for solving yearly puzzle competitions of theLadies’ DiaryandGentleman’s Diary. His lecture topics at the Kendal school ranged from mechanics and optics to astronomy and pneumatics.
According to Arnold Thackray’s entry on Dalton in theDictionary of Sci- entific Biography, Dalton soon became restless with his exclusive focus on teaching and argued that “very few people of middling genius or capacity 176 | a r c h i m e d e s t o h a w k i n g
for other business” become teachers. He wanted to become a physician, but this was difficult for a man of his meager finances. Also, his family thought that his awkward bedside manner would preclude him from being a great physician.
During his years in Kendal, Dalton collected plants and common insects. “Some of these,” he wrote, “may be thought puerile; but noth- ing that enjoys animal life, or that vegetates is beneath the dignity of a naturalist to examine.”
In 1792, Dalton was appointed a teacher of mathematics and natural philosophy at New College in Manchester. The Presbyterians had estab- lished this college in order to give an excellent education to those who could not attend Cambridge and Oxford, which had been open only to members of the Church of England.
In 1794, Dalton was elected to the Manchester Library and Philosoph- ical Society, and within a month of his election, Dalton presented his first major paper, “Extraordinary Facts Relating to the Vision of Colours, with Observations.” In this first paper ever published on color blindness, Dalton offered a systematic study of the affliction. He had discovered his own color blindness when he realized that flowers looked different to him than to his colleagues. In particular, when Dalton looked at a flower that most people saw as pink, he regarded the flower as being blue. (For some time after he presented his paper, color blindness was even known as
“Daltonism.”)
According to legends, Dalton once bought his mother special stockings for her birthday. The mother turned to Dalton and exclaimed, “Why did you buy me scarlet stockings?” Scarlet wouldn’t have been suitable for a Quaker woman. Dalton had thought they were blue and turned to his brother to verify their suitable color. However, his brotheralsosaw blue instead of scarlet, at which point John discovered that both he and his brother were color blind.
Even though Dalton’s scientific theories on color blindness were not adequate to account for his inability to see the color red, his skill in giving a careful account of the phenomenon increased his growing prestige. In order to account for his color blindness, he wrote in “Extraordinary Facts”
that it was “almost beyond doubt that one of the humours of my eye . . . is a colored medium.” He asked that his own eye be cut open and studied after his death to confirm his hypotheses that blue fluids in his eye absorbed the color red. (The dissection indeed occurred, but did not support his theo- ries.) About his own color blindness, he noted in “Extraordinary Facts,”
that part of the image which others call red appears to me little more than a shade or defect of light. After that, the orange, yellow and green seem one colour which descends pretty uniformly from an
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intense to a rare yellow, making what I should call different shades of yellow.
In 1800, Dalton opened his private “Mathematical Academy” that offered students classes in mathematics and chemistry—and made Dalton a good living. Close to this time, Dalton began to make strides in various areas that would make him famous:
1. He discovered Charles’s Law, which described the expansion of gases at constant pressure. (As discussed under “Charles’s Gas Law” in part II, the law is named after Jacques Charles, who discovered the law independently and earlier than Dalton.) 2. He formulated the law of additive partial pressures for gases,
which was first published in Meteorological Observations.
Dalton’s Law, as it came to be called, states that every gas acts as an independent entity in a mixture of gases and that the total pressure of a mixture of gases equals the sum of the pressures of the gases in the mixture, each gas acting independently.
3. He promoted chemical atomic theory: All matter is atomic in nature. (He also calculated relative masses of atoms and elements such as hydrogen, oxygen, carbon, and nitrogen). According to Dalton, all elements are composed of tiny, indestructible parti- cles called atoms that are all alike for a particular element and have the same atomic weight.
He also formulated the Law of Multiple Proportions that stated whenever two elements can combine to form different components, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers, such as 1:1, 2:1, and 1:2. These simple ratios provided evidence that atoms were the building blocks of compounds.
Unfortunately, knowledge of such ratios was insufficient to determine the actual number of atoms in each compound. Nevertheless, Dalton’s atomic theory set the stage for great advances in decades to come, leading many to call Dalton the “father of chemistry.”
Dalton did encounter resistance to atomic theory. For example, the British chemist Sir Henry Enfield Roscoe (1833–1915) mocked Dalton in 1887, saying, “Atoms are round bits of wood invented by Mr. Dalton.”
Perhaps Roscoe was referring to the wood models that some scientists used in order to represent atoms of different sizes. Nonetheless, by 1850, the atomic theory of matter was accepted among a significant number of chemists, and most opposition disappeared.
Through the years, Dalton published essays on diverse subjects, includ- ing theories on the trade winds, dew points, heat, the aurora borealis, the solubility of gases in water, variations in barometric pressure, and 178 | a r c h i m e d e s t o h a w k i n g
evaporation. Despite prevailing contemporary views, he promoted the correct idea that the atmosphere was a physical mixture of about 80%
nitrogen and 20% oxygen instead of being a compound of elements. He published his idea that the air was not a vast chemical solvent inMeteoro- logical Observations. Neither this publication nor his forthcoming law of additive partial pressures brought much immediate scientific reaction.
In 1801, he expressed part of his famous law of partial pressures in his paper “New Theory of the Constitution of Mixed Aeriform Fluids, and Particularly of the Atmosphere” in the Journal of Natural Philosophy, Chemistry and the Arts:
When two elastic fluids, denoted by A and B, are mixed together, there is no mutual repulsion amongst their particles; that is, the particles of A do not repel those of B, as they do one another.
Consequently, the pressure or whole weight upon any one particle arises solely from those of its own kind.
Although we now know that Dalton was inaccurate when he said that only like atoms in a mixture of gases repel and unlike atoms are indifferent toward each other, Dalton’s basic ideas did point him in the correct direc- tion, causing him and his followers to reject a commonly held theory that all atoms in matter were alike. Dalton believed that atoms of different elements had different sizes and masses and that each element had its own unique and identical kind of atoms—all key points of his atomic theory.
He maintained that any two molecules of the same chemical compound are composed of the same combination of atoms. This atomic hypothesis is essential to the field of chemistry today.
He formally claimed the special status of chemical atoms when he wrote inA New System of Chemical Philosophy, “We might as well attempt to introduce a new planet into the Solar System, or to annihilate one already in existence, as to create or destroy a particle of hydrogen.” Further, he wrote, “I should apprehend there are a considerable number of what may be called elementary principles, which can never be metamorphosed, one into another, by any power we can control.”
Although the total number of atoms in the world was very large, he suggested that the number of different types of atoms is quite small. His original writings listed about twenty different elements, which he thought of as species of atoms. Today, we know of more than one hundred naturally occurring and manmade elements.
In 1816, Dalton was elected to the position of corresponding member of the French Académie des Sciences, and in 1822 he visited Paris, where he met other famous scientists of his time, such as Pierre-Simon Laplace (1749–1827), Joseph Louis Gay-Lussac (1778–1850), and André-Marie
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Ampère (1775–1836). In 1817, Dalton became president of the Manchester Literary and Philosophical Society, which he presided over for the remain- ing 27 years of his life. He was elected to the Royal Society in 1922, and received the Royal Medal in 1826 in recognition of his chemical atomic theory. In 1831, he chaired various scientific committees of the British Association for the Advancement of Science. In 1836, he became vice president-elect of the association, but his participation was cut short by two severe paralytic attacks in 1837. He was a partial invalid for the rest of his life. Thackray writes in theDictionary of Scientific Biographyof society’s growing respect for Dalton during his final years:
Dalton’s later life also illustrates the growing recognition that soci- ety was beginning to offer the man of science. Impeccable scientific credentials, a blameless personal life, and in old age a calm and equable temperament all combine to make Dalton a peculiarly suit- able recipient of civil honor.
In 1794, Dalton explained why he had never married and had no progeny:
“My head is too full of triangles, chymical processes, and electrical exper- iments, etc., to think much of marriage.” His needs were always simple, which was a reflection of his Quaker faith.
In 1844, he had another stroke. On July 26 of that year, he recorded with a shaking hand his final meteorological observation. A day later, he fell from his bed and was found dead. More than 40,000 people filed past his coffin in Manchester Town Hall. Stores and offices closed for a day as a mark of respect. Dalton’s funeral procession stretched for two miles. According to Bill Bryson’sA Short History of Nearly Everything, Dalton’s entry in Britain’s 1885Dictionary of National Biographyis one of the longest, “rivaled in length only by those of Darwin and Lyell among nineteenth-century men of science.”
As Dalton had requested, his eye was cut open, and the liquids of his eye were found to be normal. One of his eyes was preserved at the Royal Institution, and in the 1990s, cellular analysis revealed that the eye lacked the pigment that provides sensitivity to green. Today, we call this form of color blindness “deuteranope.” Roughly five out of every 100 males is deuteranomalous to at least some degree.
As another curious aside, in 2006, researchers at Cambridge University and the University of Newcastle upon Tyne in England discovered that people afflicted with red-green color blindness actually have a special sen- sitivity tootherhues. For example, the researchers found that color-blind subjects could actually distinguish between very subtly different tones of khaki, whereas people with normal vision could not. Elise Kleeman noted,
“The findings lend credence to the theory that people with red-green color 180 | a r c h i m e d e s t o h a w k i n g
blindness make good hunters or soldiers because they are not easily fooled by camouflage.” The researchers suggest that red-green color blindness could have been retained for evolutionary reasons because it helped early humans locate predators and food in forests.
Returning our attention to the legacy of Dalton, a lunar crater with a diameter of 60 kilometers was named after Dalton and approved in 1964 by the International Astronomical Union General Assembly. Dalton’s con- tribution to humankind has been considered so great by Michael H. Hart, author ofThe 100: A Ranking of the Most Influential Persons in History, that Hart ranks Dalton as the thirty-second most influential person in all of history. Hart concludes:
So convincingly did Dalton present his [atomic] theory that within twenty years it was adopted by the majority of scientists. Further- more, chemists followed the program that his book suggested: deter- mine exactly the relative atomic weights; analyze chemical com- pounds by weight; determine the exact combination of atoms which constitutes each species of molecule. The success of that program has, of course, been overwhelming. It is difficult to overstate the importance of the atomic hypotheses. It is the central notion in our understanding of chemistry.
Within just a few decades of his death, many Englishmen thought of Dalton with profound reverence. In 1874, Henry Lonsdale wrote inThe Worthies of Cumberland:
As pilgrims to the shrines of saints draw thousands of English Catholics to the Continent, there may be some persons in the British Islands sufficiently in love with science, not only to revere the mem- ory of its founders, but to wish for a description of the locality and birth-place of a great master of knowledge—John Dalton—who did more for the world’s civilization than all the reputed saints in Christendom.
In 1895, Henry E. Roscoe’s John Dalton and the Rise of Modern Chemistryforever immortalized Dalton and his fellow great scientist from Manchester, James Joule, whom I discuss in a separate entry:
In the vestibule of the Manchester Town Hall are placed two life- sized marble statues facing each other. One of these is that of John Dalton . . . the other that of James Prescott Joule. . . . Thus the hon- our is done to Manchester’s two greatest sons—to Dalton, the founder of modern Chemistry and of the Atomic Theory, and the
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