The Black-veined White butterfly Aporia crataegi is known as le Gazé in French ('the Gossamer'). That's a much more evocative name than the matter of fact English name.
Last year they had one of their periodic boom years, and you saw them often in gardens, flowery meadows and open woodland. Sadly, they are extinct in Britain, and even in France, where they are still very commonly seen, they are in strong regression due to modern intensive farming methods, particularly the removal of hedgerows and the use of chemicals. Between 1960 and 1990 the equivalent of 440 000 hectares of hedgerow were destroyed in France, and in addition, the caterpillars are very sensitive to phytosanitary treatments used to ensure crops are pest and disease free.