Della Porta’s salt bath

A few weeks ago I joined my friend and colleague Helen Czerski online to do ‘science’ for children. I made ice cream, cooled by a chemical trick. Having filled a jam jar with some crushed ice, I added a little water and a generous sprinkling of salt. As I stirred the mixture, the temperature plunged ever lower, settling around –11˚C.

Regardless of how many times I have done this the cooling remains magically counterintuitive. How can you add something warm to a cold mixture and cause everything to become colder? It’s a salutary reminder that the twin concepts of temperature and energy are not as straightforward as our senses would have us believe. And how to explain it? In my experience, most people will say that the salt lowers the melting point of the ice. But that is hardly an explanation. After all how does the solid ice ‘know’ that there is salt nearby? These must have been questions asked by the man who possibly first took temperatures below zero using a salt.

Giambattista (short for Giovanni Battista) Della Porta was born into a wealthy Neapolitan family, referred to as magnifici, the term for members of the wealthy nobility. His date of birth is a little uncertain, because Della Porta’s writings leave a trail of confusion, possibly deliberately. He was brought up to be the epitome of the monied young aristocrat. He was wore the most fashionable clothes and portraits show a dapper man with an intense gaze. His father was learned and eclectic, and the Della Porta household kept open house for artists, poets, philosophers and scholars of all kinds. As a result Della Porta received a broad humanistic education covering poetry and rhetoric, alchemy, medicine and mathematics. For social reasons – music was a crucial asset in high society – he and his brothers notoriously persisted with music in spite of their almost total tone-deafness.