A Mixture of Frailties, the third volume of Robertson Davies Salterton Trilogy, is his first extended engagement with one of the great neuroses of Canadian culture: Canada's artistic relationship to Europe, and particularly to Britain. Davies begins his story with the funeral of Louisa Bridgetower, the Salterton matron whose imposing presence ranges throughout the earlier volumes of the ''Salterton'' Trilogy. The substantial income from her estate is to be used to send an unmarried young woman to Europe to pursue an education in the arts. Mrs. Bridgetower's executors end up selecting Monica Gall, an almost entirely unschooled singer whose sole experience comes from performing with the Heart and Hope Gospel Quartet, a rough outfit sponsored by a small fundamentalist group. Monica soon finds herself in England, a pupil of some of Britain's most remarkable teachers and composers, and she gradually blossoms from a Canadian rube to a cosmopolitan soprano with a unique - and tragicomic - career. ''It's a muddle'', thought Monica. ''A muddle and I can't get it straight. I wish I knew what I should do. I wish I even knew what I want to do...I want to go on in the life that has somehow or other found me and claimed me. And I want so terribly to be happy. Oh god, don't let me slip under the surface of all the heavy-hearted dullness that seems to claim so many people....'' A Mixture of Frailties is so much more than the story of Monica Gall's life in London and her education as a singer. It is an account of her education as a human being, and the result is an absorbing novel, comic in the true sense, vivid and frequently moving.