1766 – England

‘Three Papers Containing Experiments On Factitious Airs’

(gases made from reactions between liquids and solids)

1798 – Density of the earth
Using a torsion balance and the application of NEWTON’s theory of gravity, Cavendish concluded that the earth’s density was 5.5 times that of water.

Born of the English aristocracy and inheritor of a huge sum of money half way through his life, Cavendish is remembered for his work in chemistry.
He demonstrated that hydrogen (inflammable air) and carbon dioxide (fixed air) were gases distinct from ‘atmospheric air’. His claim to the discovery that water was not a distinct element – a view held since the time of ARISTOTLE – but a compound made from two parts hydrogen to one part oxygen, became confused with similar observations made by ANTOINE LAVOISIER.

Full length drawing of Henry Cavendish  &copy:


1871 – England

Almost all his discoveries remained unpublished until the late nineteenth century when his notes were found and JAMES CLERK MAXWELL dedicated himself to publishing Cavendish’s work, a task he completed in 1879.
By then many potential breakthroughs, significant at the time, had been surpassed by history.

In 1871 the endowment of the Cavendish Laboratory was made to Cambridge University by Cavendish’s legatees.

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1808 – France

‘Volumes of gases which combine or which are produced in chemical reactions are always in the ratio of small whole numbers’

One volume of nitrogen and three volumes of hydrogen produce two volumes of ammonia. These volumes are in the whole number ratio of 1:3:2

N2 + 3H2 ↔ 2NH3

Along with his compatriot Louis Thenard, Gay-Lussac proved LAVOISIER’s assumption, that all acids had to contain oxygen, to be wrong.

portrait of GAY-LUSSAC ©


Gay-Lussac re-examined JACQUES CHARLES’ unpublished and little known work describing the effect that the volume of a gas at constant pressure is directly proportional to temperature and ensured that Charles received due credit for his discovery.

Alongside JOHN DALTON, Gay-Lussac concluded that once pressure was kept fixed, near zero degrees Celsius all gases increased in volume by 1/273 the original value for every degree Celsius rise in temperature. At 10degrees, the volume would become 283/273 of its original value and at – 10degrees it would be 263/273 of that same original value. He extended this relation by showing that when volume was kept fixed, gas would increase or decrease the pressure exerted on the outside of the gas container by the same 1/273 factor when temperature was shifted by a degree Celsius. This did not depend upon the gas being studied and hinted at a deep connection shared by all gases. If the volume of a gas at fixed pressure decreased by 1/273 for every 1degree drop, it would reach zero volume at -273degrees Celsius. The same was true for pressure at fixed volume. That had to be the end of the scale, the lowest possible temperature one could reach. Absolute zero.

In an 1807 gas-experiment, Gay-Lussac took a large container with a removable divider down the middle and filled half with gas and made the other half a vacuüm. When the divider was suddenly removed, the gas quickly filled the whole container. According to caloric theory, temperature was a measure of the concentration of caloric fluid and removal of the divider should have led to a drop in temperature because the fluid was spread out over a greater volume without any loss of caloric fluid. (The same amount of fluid in a larger container means lower concentration).
Evidence linking heat to mechanical energy accumulated. Expenditure of the latter seemed to lead to the former.

Gay-Lussac was an experimentalist and his law was based on extensive experiments. The explanation of why gases combine in this way came from AVOGADRO.

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1811 – Italy

‘Equal volumes of all gases at the same temperature and pressure contain the same number of molecules’

In 1811, when Avogadro proposed his HYPOTHESIS, very little was known about atoms and molecules. Avogadro claimed that the same volume of any gas under identical conditions would always contain the same number of fundamental particles, or molecules. A litre of hydrogen would contain exactly the same number of molecules as a litre of oxygen or a litre of carbon dioxide.

Drawing of AVOGADRO ©

In 1814 ANDRE AMPERE was credited with discovering that if a gas consisted of a single element, its atoms could clump in pairs. The molecules of oxygen consisted of pairs of oxygen atoms, and the molecules of chlorine, pairs of chlorine atoms.
Diatomic gases possess a total of six degrees of simple freedom per molecule that are related to atomic motion.

This provides a way of comparing the weights of different molecules. It was only necessary to weigh equal volumes of different gases and compare them. This would be exactly the same as comparing the weights of the individual molecules of each gas.

Avogadro realised that GAY-LUSSAC‘s law provided a way of proving that an atom and a molecule are not the same. He suggested that the particles (molecules) of which nitrogen gas is composed consist of two atoms, thus the molecule of nitrogen is N2. When one volume (one molecule) of nitrogen combines with three volumes (three molecules) of hydrogen, two volumes (two molecules) of ammonia, NH3, are produced.

N2 + 3H2 ↔ 2NH3

However, the idea of a molecule consisting of two or more atoms bound together was not understood at that time.

Avogadro’s law was forgotten until 1860 when the Italian chemist STANISLAO CANNIZZARO (1826-1910) explained the necessity of distinguishing between atoms and molecules.

Avogadro’s constant
From Avogadro’s law it can be deduced that the same number of molecules of all gases at the same temperature and pressure should have the same volume. This number has been determined experimentally: it’s value is 6.022 1367(36) × 1023AVOGADRO’S NUMBERAvogadro's_number_in_e_notation

That at the same temperature and pressure, equal volumes of all gases have the same number of molecules allows a simple calculation for the combining ratios of all gases – by measuring their percentages by volume in any compound. This in turn facilitates simple calculation of the relative atomic masses of the elements of which it is composed.

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1787 – France

‘The volume of a given mass of gas at constant pressure is directly proportional to its absolute temperature’

In other words, if you double the temperature of a gas, you double its volume. In equation form:  V/T = constant, or  V1/T1 = V2/T2,  where  V1 is the volume of the gas at a temperature  T1 (in kelvin) and  V2 the new volume at a new temperature  T2.

This principle is now known as Charles’ Law (although sometimes named after GAY-LUSSAC because of his popularisation of it fifteen years later – Gay Lussac’s experimental proof was more accurate than Charles’).
It completed the two ‘gas laws’.

A fixed amount of any gas expands equally at the same increments in temperature, as long as it is at constant pressure.

Likewise for a decline in temperature, all gases reduce in volume at a common rate, to the point at about -273degrees C, where they would theoretically converge to zero volume. It is for this reason that the kelvin temperature scale later fixed its zero degree value at this point.

CHARLES’ Law and BOYLE‘s Law may be expressed as a single equation, pV/T = constant. If we also include AVOGADRO‘s law, the relationship becomes pV/nT = constant, where n is the number of molecules or number of moles.

The constant in this equation is called the gas constant and is shown by R
The equation – known as the ideal gas equation – is usually written as pV = nRT

Strictly, it applies to ideal gases only. An ideal gas obeys all the assumptions of the kinetic theory of gases. There are no ideal gases in nature, but under certain conditions all real gases approach ideal behaviour.

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Related sites
Poster describing the combined Gas Laws

Combined Gas Laws


1738 – Switzerland
1859 – England

‘Gases are composed of molecules which are in constant random motion and their properties depend upon this motion’

The volume of a gas is simply the space through which molecules are free to move. Collisions of the molecules with each other and the walls of a container are perfectly elastic, resulting in no decrease in kinetic energy. The average kinetic energy of a gas increases with an increase in temperature and decreases with a decrease in temperature. The theory has been extended to provide a model for two states of matter – liquids and solids.

Bernoulli had a great advantage over DEMOCRITUS. He knew that free atoms were more than simply tiny grains flying though space; they were tiny grains flying through space and obeying NEWTON’s Laws of Motion.
Bernoulli proposed a ‘bombardment theory’, which stated that a gas consisted of tiny particles in rapid, random motion like a swarm of angry bees. He realized that in the case of such a gas visualized as a host of tiny grains in perpetual frenzied motion, the atoms hammering relentlessly on the walls of any containing vessel would produce a force by bombarding the container. The effect of each individual impact would of course be vanishingly small. The effect of billions upon billions of atoms, hammering away incessantly, however, would be to push the walls back. A gas made of atoms would exert a jittery force that we would detect as a ‘pressure’.

Heating a gas would make its particles move faster.
The pressure of a gas such as steam was easy to measure using a piston in a hollow container. This was essentially a moveable wall. To deduce how the pressure of a gas would be affected by different conditions, Bernoulli first made some simplifying assumptions. He assumed the atoms were very small compared to the gulf between them. This allowed Bernoulli to ignore any force – whether of attraction or repulsion – that existed between them, as being unlikely to be ‘long range’. (This is an ‘ideal’ or ‘perfect’ gas. The behaviour of a real gas may differ from the ideal, for example at very high pressure). With the motion of each atom unaffected by its fellows, Newton’s laws dictated that it should fly at a constant speed in a straight line. The exception was when it slammed into a piston or the walls of the container. Bernoulli assumed that in such a collision a gas atom bounced off the walls of the surface without losing any speed, in the process imparting a miniscule force to the wall.

What would happen if the volume of the gas were reduced by applying an outside force to the piston? If the gas were reduced to half its original volume, the atoms would now have to fly only half as far between collisions, in any given time they would collide with the piston twice as many times and would exert twice the pressure. Similarly, if the gas were compressed to a third of its volume, its pressure would triple. This had been observed by ROBERT BOYLE in 1660 and named Boyle’s Law.

What would happen to the pressure of gas in a closed cylinder if the gas were heated while its volume remained unchanged? Exploiting the insight that the temperature of a gas was a measure of how fast on average its atoms were flying about, that when a gas was heated, its atoms speeded up, he deduced that as the atoms would be moving faster they would collide with the piston more often and create a greater force. Consequently the pressure of the gas would rise. This was observed by the French scientist JACQUES ALEXANDRE CESARE CHARLES in 1787, and christened Charles’ law.

After 120 years MAXWELL polished Bernoulli’s ideas into a rigorous mathematical theory. In Germany, LUDWIG  BOLTZMANN championed the atomic hypothesis, but was refuted by the Austrian ERNST MACH, who was convinced that science should not concern itself with any feature of the world that could not be observed directly with the senses.


At a narrow constriction in a pipe or tube, the speed of a gas or liquid is increased, but its pressure is decreased, according to Bernoulli’s principle. This effect is named the Venturi effect (and a pipe or tube with a narrow constriction the Venturi tube) after the Italian G.B. Venturi (1746-1822) who first observed it in constrictions in water channels. An atomiser works on the same principle.

‘As the velocity of a liquid or gas increases, its pressure decreases; and when the velocity decreases, its pressure increases’


The principle is expressed as a complex equation, but it can be summed up simply as the faster the flow the lower the pressure.

An aircraft wing’s curved upper surface is longer than the lower one, which ensures that air has to travel further and so faster over the top than it does below the wing. Hence the air pressure underneath is greater than on top of the wing, causing an upward force, called lift.

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BENJAMIN THOMPSON (1753-1814): known as Count Rumford

1798 – England

‘Mechanical work can be converted into heat. Heat is the energy of motion of particles’

Heat is a form of energy associated with the random motion of atoms or molecules. Temperature is a measure of the hotness of an object.

Portrait of COUNT RUMFORD ©


In the eighteenth century, scientists imagined heat as a flow of a fluid substance called ‘CALORIC‘. Each object contained a certain amount of caloric. If caloric flowed out, the object’s temperature decreased; if more caloric flowed into the object, its temperature increased.
Like phlogiston, caloric was a weightless fluid, a quality that could be transmitted from one substance to another, so that the first warmed the second up. What is being transmitted is heat energy.

One idea was that a hot object would emit ‘calorific rays’, whilst a cold one would emit ‘frigorific rays’ – an idea raised in Plutarch’s De Primo Frigido. Cold was an entity in itself, not simply the absence of heat.

It was believed that all substances contained caloric and that when a kettle was being heated over a fire, the fuel gave up its caloric to the flame, which passed it on to the metal, which passed it on to the water. Similarly, two pieces of wood rubbed together would give heat because abrasion was releasing caloric trapped within.

Working for the Elector of Bavaria, Rumford investigated the heat generated during the reaming out of the metal core when the bore of a cannon is formed. According to the caloric theory, the heat was released from the shards of metal during boring; Rumford noticed that if the tools were blunt and removed little or no metal, more heat was generated, rather than less. Rumford postulated that the heat source had to be the work done in drilling the hole. Heat was not an indestructible caloric fluid, as LAVOISIER had argued, but something that could come and go. Mechanical energy could produce heat and heat could lead to mechanical energy.

One analogy he drew was to a bell; heat was like sound, with cold being similar to low notes and hot, to high ones. Temperature was therefore just the frequency of the bell.

Rumford thought there was no separate caloric fluid and that the heat content of an object was associated with motion or internal vibrations – motion which in the case of the cannon was bolstered by the friction of the tools.
He had recognized the relationship between heat energy and the physicists’ concept of ‘work’ – the transfer of energy from a system into the surroundings, caused by the work done, results in a difference in temperature.
This transfer of energy measured as a temperature difference is called ‘heat’.

Half a century was to pass before in 1849, JAMES JOULE established the ‘mechanical equivalence of heat’ and JAMES CLERK MAXWELL launched the kinetic theory. According to Maxwell, the heat content of a body is equivalent to the sum of the individual energies of motion (kinetic energies) of its constituent atoms and molecules

US born Rumford founded the Royal Institution in London and invented the calorimeter, a device measuring heat.

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1752 – The New World

‘If you would not be forgotten when you are dead and rotten, either write things worth reading, or do things worth writing about’

Curious about how just about everything works, from governments to lightning rods, Franklin’s legacy, in addition to the many inventions such as lightning conductors, bifocal lenses and street lamps, was one of learning. He established one of the first public libraries as well as one of the first universities in America, Pennsylvania. He established the Democratic Party. Franklin was one of the five signatories of the Declaration of Independence from Great Britain in 1776 and was a later participant in the drafting of the American Constitution.

‘Benjamin Franklin’s choice for the signs of electric charges leads to electric current being positive, even though the charge carriers themselves are negative — thereby cursing electrical engineers with confusing minus signs ever since.
The sign of the charge carriers could not be determined with the technology of Franklin’s time, so this isn’t his fault. It’s just bad luck.’

Franklin was a pioneer in understanding the properties and potential of electricity. He undertook studies involving electric charge and introduced the terms ‘positive’ and ‘negative’ in explaining the way substances could be attracted to or repelled by each other according to the nature of their charge. He believed these charges ultimately cancelled each other out so that if something lost electrical charge, another substance would instantly gain the same amount.

His work on electricity climaxed with his kite flying experiment of 1752. In order to prove lightning to be a form of electricity, Franklin launched a kite into a thunderstorm on a long piece of conducting string. Tying the string to a capacitor, which became charged when struck by lightning, vindicated his theories.

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