Water, Cavendish, and Lavoisier

To understand biological physics, you must know the properties of water. In Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss water’s density, compressibility, viscosity, heat capacity, surface tension, thermal conductivity, dielectric constant, and index of refraction. It’s behavior is critical for osmosis, diffusion, absorption of x-rays, propagation of ultrasonic waves, and magnetic resonance imaging.

In the very first section of IPMB, about distances and sizes, we say

At the 1-nm scale and below, we reach the world of small molecules and individual atoms. Water is the most common molecule in our body. It consists of two atoms of hydrogen and one of oxygen. The distance between adjacent atoms in water is about 0.1 nm.

Every schoolchild learns that water is H₂O. But how do we know that water is made from hydrogen and oxygen? In other words, how did we first learn that water is not an element itself, but is a compound of two elements?

I’ll let Isaac Asimov tell the story. In A Short Histor of Chemistry, he writes

In 1783 [English scientist Henry] Cavendish was … working with his inflammable gas… He burned some of it and studied the consequences. He found that the vapors produced by the burning condensed to form a liquid that, on investigation, proved to be nothing more or less than water.

This was a crucially important experiment. In the first place, it was another hard blow at the Greek theory of the elements [air, water, earth, fire], for it showed that water was not a simple substance but was the sole product of the combination of two gases.

[French chemist Antoine] Lavoisier, hearing of the experiment, named Cavendish’s gas, hydrogen (“water-producer”) and pointed out that hydrogen burned by combining with oxygen and that therefore water was a hydrogen-oxygen combination.

So now you know. But these two brilliant scientists had tragic fates. You can find the stories in Asimov’s Biographical Encyclopedia of Science and Technology.

Cavendish (1731-1810):

[Cavendish] was excessively shy and absent-minded. He almost never spoke and when he did it was with a sort of stammer… He build a separate entrance to his house so he could come and leave alone… he even literally insisted on dying alone.

The eccentric had one and only one love, and that was scientific research. He spent almost sixty years in exclusive preoccupation with it. It was a pure love, too, for he did not care whether his findings were published, whether he got credit, or anything beyond the fact that he was sating his own curiosity. He wrote no books and published only twenty articles altogether. As a result, much of what he did remained unknown until years after his death…

Lavoisier (1743-1794):

In the same year that [Lavoisier’s] textbook [Elementary Treatise on Chemistry] appeared the French Revolution broke out. By 1792 the radical antimonarchists were in control…. Lavoisier… was guillotined on May 8, 1794, and buried in an unmarked grave. Two months later the radicals were overthrown. His was the most deplorable single casualty of the revolution.