Sir

In his otherwise excellent Concepts essay (Nature 423, 485; 2003) Gautam R. Desiraju proposes that a scientist and a layperson may have different ideas about how to define a crystal. He even mentions that Spanish speakers “have only one word (cristal) for both 'glass' and 'crystal'.”

This comment is not totally rigorous. The Spanish Real Academia dictionary (http://www.rae.es) shows two clearly distinct definitions that I have translated: “(1) glass (vidrio), (from Latin vitrum): solid, fragile material, transparent, without crystalline structure [...]; (2) crystal (cristal), (from Latin crystallus): solid material whose atoms and molecules are regularly and repeatedly distributed in space.”

However, it is true that in Spain we use the word cristal for both concepts in colloquial terms.