Today In Science History – October 10 – Henry Cavendish

Henry Cavendish

Henry Cavendish (1731-1810)

October 10 is Henry Cavendish’s birthday. Cavendish was an English natural philosopher who made a meticulous study of air and calculated the density of the Earth.

In the late 18th Century, scientists used the term gas and air interchangeably. Cavendish was one of the first to notice that perhaps the air was made up of different types of ‘airs’. His most famous investigations led to the discovery of ‘inflammable air’ or hydrogen. He collected hydrogen by collecting the gas given off by the reaction of metals and strong acids and called it ‘inflammable air’. Inflammable air was almost entirely made up of phlogiston, the substance in a body that causes them to burn. He found if he combined three parts inflammable air with seven parts of common air and dropped fire into the mixture, it would make a very loud noise and produce water. He also noted that all of the inflammable air and nearly a fifth of the common air was used up in this experiment. Further investigation found if he mixed two parts of inflammable air with one part dephlogisticated air (oxygen) would produce water. We know this reaction today as:

2 H2 (g) + O2 (g) → H2O (l)

Using the data from this experiment, Cavendish also determined an accurate composition of the atmosphere where he found 79.167% was phlogisticated air (nitrogen mostly) and 20.833% dephlogisticated air (oxygen). He went on to question whether phlogisticated air was made up of many different airs. Cavendish’s theory would later be proven by Joseph Priestley.

We now know that Cavendish’s ‘common air’ is a mixture made up of mainly five things: nitrogen, oxygen, waver vapor, argon and carbon dioxide. At sea level, dry common air can contain about 78% nitrogen, 20% oxygen, 1% argon and 0.03% carbon dioxide. Depending on humidity, air can contain varied amounts of water vapor. The rest is a mixture of minute amounts of other compounds.

Another famous experiment of Cavendish’s is the attempt to ‘weigh the Earth’. His experiment was actually an attempt to calculate Newton’s universal gravitational constant, G. Newton showed the force of gravity between two objects is proportional to the two object’s masses and inversely proportional to the square of the distance between them. When expressed as a formula, the force equation looks like:

where M and m are the two masses and r is the distance between their center of mass. G is the proportionality constant Cavendish was attempting to find. Cavendish’s apparatus involved a pair of heavy lead weights suspended on a stretched wire to act as a torsion balance. These weights were brought in close proximity to another pair of weights, when these weights were moved away from the hanging weights, the attraction between them would cause the wire to twist. The rate of oscillation of the wire could be used to determine the amount of force acting between the weights. In Cavendish’s original experimental design, this force was on the order of 1×10-7 Newtons of force or about 1/1000th of the weight of a grain of salt. His measurements were so precise, his value of G was within 1% of the accepted value of 6.67×10−11 m3/kg·s2. He used this value to determine the average density of the Earth and in turn, calculated the mass of the Earth.

Henry Cavendish was supported through a stipend granted by his father, Lord Charles Cavendish. When his father died and Henry became Lord Cavendish, he was one of the wealthiest people in England. He set himself up in his private home laboratory to avoid interactions with others. One of his main contacts with other people was through is private library. He would loan books to men who were properly vouched for. He would also check books out to himself. If he needed one of his books, he would enter the loan into his ledger. This solitary life was extended to his research. He rarely published his findings. After his death, several sealed packets of his notes were discovered and edited by James Clark Maxwell and published nearly 70 years later.

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