<human>:  Why is the word ‘iridian’ spelled that way?
<bot>: The word iridian comes from the Greek word iridos, meaning a river. The Greek word for river, is in turn derived from the Indo-European root *reis, which means 'to flow'. The Greek word iris, meaning a rainbow, is also derived from the same root. The English word river is ultimately derived from the Latin word rīmus, meaning 'running water'.