Rev Purchas was the son of a Ross wine merchant. He was ordained in 1857. From early childhood he was interested in natural history, particularly entomology and botany.
In 1851 with Dr Bull, Messrs. Cobie and Lingwood plus a few others they met to found the Woolhope Club. His botanical work in Herefordshire was very thorough and with a band of helpers he recorded everything they observed. In 1889 in conjunction with Augustus Ley, the results were published in The Flora of Herefordshire. His name was perpetuated by its attachment to a local variety of bramble plentiful near Ross, Rubus purchasianus – a tribute to his scholarly work and study of brambles. His own collections were given to the British Museum for when he died he had gained a national reputation as a botanist.