Purple-flowered magnolia (Magnolia purpurea)
"This fine species of Magnolia is a native of China and Japan [….] It is a very free growing plant; will resist the feverity of our winters, if planted in a warm situation, and is deciduous; but if kept in the conservatory, it retains its leaves the whole year, and flowers about June.
The plant from which our figure was taken is in the magnificent Conservatory of the Right Honourable the Marquis of Blandford, White Knights, near Reading, Berks; an, we believe, the finest specimen of this species of Magnolia in Britain."
This illustration and the description are from volume five of "The botanist's repository, for new and rare plants" by Henry Andrews (Published by the author, 1797-)