Unwelcome ideas - part IX

 In 1662, seventeen centuries after Lucretius died, Benedict Spinoza wrote a long letter to Henry Oldenburg, a German natural philosopher and joint secretary of the Royal Society in London. Spinoza was giving his  critique of a book written by Robert Boyle, now best known for Boyle's Law, but also one of the first modern chemists. The first part of Spinoza’s letter in a commentary about Boyle's experiments with nitre, or potassium nitrate, a component of gunpowder.

Although Spinoza’s letter is addressed to Oldenburg, it is clear enough that it was indirectly addressed to Boyle. The two men never corresponded directly, possibly because Boyle, a devout Christian, saw Spinoza as beyond the pale, a Jew and possibly even an atheist.

Both Spinoza and Boyle were interested in the composition of nitre as might be indicated by a variety of experiments such as combustion and sublimation. They were particularly interested in the relationship between nitre and spirit of nitre (nitric acid). Behind this interest, was a deeper interest in establishing the validity of atomic theory, at least in Spinoza’s case. Boyle was no theorist and took little interest in atomic theory. Spinoza had only the haziest notion of what an atom might be, tending to take his conjectures from ancient Greek notions that an atom's propertied depend on its shape. Like Boyle, he was sure that nitre is a composite substance consisting of more than one type of atom, but shape is still felt to be important.

...if particles of nitre are put on the tongue when they are at rest, they will lie upon it on their largest surfaces, and in this way they block its pores, which causes cold...

But if these particles are placed on the tongue when they are in excited motion, they will touch it with their sharp-pointed surfaces and will penetrate its pores, and the more excited their motion, the more sharply will they prick the tongue.

Compare this with Lucretius’ conjecture seventeen centuries earlier - suggesting that colour might depend on variety in the shape of atoms.

Besides, if seeds (atoms) are colourless and yet

Endowed with varied forms, from out of which

Still various colours come and change about

According to the change of seed, and how

They’re placed, what motions they can give, or what

Receive...

Spinoza's letter and his experiments are barely even an obscure footnote in the long story of atomic theory, but interesting for all that. He and many other natural philosophers of the seventeenth century were impressed by the ancient logic of Democritus, Epicurus and Lucretius, impressed by their argument for the reality of the irreducible atom based on the idea that nothing comes from nothing and nothing is resolved into nothing. Seventeen centuries after Lucretius died, atomic theory was dusted off and subjected to experimental investigation, when it finally began to take a few faltering steps towards the light.

Only another 143 years were to elapse before John Dalton published his first table of relative atomic weights. Atomic theory had been all along, a perfectly reasonable idea - but an unwelcome one.